Is There an optimal Spacing Formula for Spaced Repetition? Debunking Common Myths

Is There an optimal Spacing Formula for Spaced Repetition? Debunking Common Myths

Dominic Zijlstra
Dominic Zijlstra
Publish date
Mar 25, 2023
If you keep hoping that you’ll remember what you learned, I’ll help you to reduce the struggle with this article today.
You might be familiar with the concept of spaced repetition and already serious about improving your studying. After having mastered 3000 Chinese characters (and more different stuff) using spaced repetition, one of the questions I get most often is: what are the optimal intervals to review flashcards?
In this article, we’ll explore both the power and the limitations of spaced repetition, and will find out if there is an optimal review schedule. If that’s the case, we’ll see how to incorporate it into our study.
Let's dive in and discover how spaced repetition can take your learning to the next level.

What is spaced repetition and how does it work?

Spaced repetition is a learning technique that involves strategically spacing out your study sessions to enhance long-term memory retention. Instead of cramming large volumes of information in a short period, this method breaks down your learning into smaller, manageable chunks that are revisited at gradually increasing intervals.
Spaced repetition promises an automated, science-backed system that lets you remember anything you want.
This study method has steadily become more popular over the past years, with over 67% of students using flashcards right now - according to this study
The rise of spaced repetition: interest in “Spaced repetition” on Google Trends over the past 5 years
The rise of spaced repetition: interest in “Spaced repetition” on Google Trends over the past 5 years
Personally, I first came across spaced repetition way after college when learning Chinese characters. Suddenly, I had a system for my review practice. And it showed me what I needed to learn at the optimal time!
Spaced repetition capitalizes on your brain's natural memory processes, making it an invaluable tool for people like medical students who are grappling with vast amounts of complex material.
But how does it work exactly?

The forgetting curve (and the myth of Hermann Ebbinghaus)

At the core of spaced repetition lies the concept of the "forgetting curve", which describes the exponential decline in retention after first learning new information.
Unlike what many articles will tell you, neither spaced repetition nor the forgetting curve were discovered by German psychologist Hermann Ebbinghaus. In fact, his original research paper describes how he memorized meaningless series of syllables with hundreds of repetitions - which is counter to the principle of spaced repetition, which is all about remembering useful information with the smallest number of repetitions.
As you can read in my Guide to using Spaced Repetition to Master not Memorize, spaced repetition, and the forgetting curve as we know it, were invented by Piotr Wozniak in 1985, exactly one hundred years after Ebbinghaus’ research.
Piotr Wozniak’s forgetting curve: the forgetting curve as we usually know it
Piotr Wozniak’s forgetting curve: the forgetting curve as we usually know it
The forgetting curve illustrates that memory retention declines rapidly in the initial days after learning, but the rate of decline slows down as time goes on.
Storage Strength vs Retrieval Strength Fun fact: forgetting can actually improve your learning. The "Forget to Learn" theory suggests that our brains store memories with two strengths: storage strength, which remains steady, and retrieval strength, which fades over time. Interestingly, when retrieval strength drops, learning increases when we access the memory again. This idea, called desirable difficulty, is a key reason why spaced repetition works so well. It helps us learn more effectively by spreading out our study sessions, making the most of our brain's natural "forgetting" process! In this case, the brain works similar to how your muscles work: Let’s say you go to the gym religiously for about six months. After that, you have stocked up on enough muscle and decide to take it easy. Once you’ve lost the muscle again, you decide to train again - and you’ll be pleased to see that your muscles grow twice as fast because of their “muscle memory.”
Now, let’s see how spaced repetition can ‘reset’ and slow down the forgetting curve:

The forgetting curve with spaced repetition

To remember things for a long time, revisit the material just before you forget it. This resets the forgetting curve and makes your memory stronger. Each review makes the information last longer in your memory.
By repeating the information after it has faded to about 80% retention, you can not only reset the forgetting curve to 100%, but also slow down the decay. This means that the time interval for the next repetition is increased. Repeating this multiple times following the right schedule makes the forgetting curve flat, which means you can remember the information for a long time.
Wozniak’s hypothesis of resetting the forgetting curve with spaced repetitions
Wozniak’s hypothesis of resetting the forgetting curve with spaced repetitions

Is spaced repetition effective?

So how effective is spaced repetition really? Well, we need to realize that spaced repetition flashcards rely on two key principles from cognitive science:

1. The testing effect

Also called ‘active recall’, the testing effect states that it’s more effective to actively test yourself on a piece of knowledge by giving your brain a prompt, rather than passively re-reading that same piece of knowledge.
A recent meta-analysis of 118 studies in cognitive science showed that the testing effect is well-established, being on average 51% more effective compared to passive re-reading, and 93% more effective compared to ‘doing nothing’. The results hold both in the lab and in the classroom.

2. The spacing effect

Despite the similarity, the spacing effect is not the same as spaced repetition. The spacing effect merely states that it is more effective to space out your studying over multiple sessions (spaced practice), compared to cramming everything at once (massed practice). A meta-analysis of 29 studies showed that the spacing effect is also well-established, with spaced practice being 74% more effective compared to massed practice (cramming).

The benefits of learning with a spaced repetition system

So, both active recall and the spacing out of study sessions have been scientifically proven to be effective study methods, and are much more effective than passive methods like re-reading.
But spaced repetition is more than just active recall and spaced practice. Spaced repetition says that by choosing optimal, increasing time intervals, we can get even bigger learning improvements.
Let’s look into the optimal time intervals for spaced repetition:

The best spaced repetition time intervals

To find the optimal time intervals, let’s look at the research again.

The myth (?) of the optimal spaced repetition schedule

Since the first spaced repetition software, SuperMemo, was built before any actual research into the forgetting curve and repetitions (apart from Ebbinghaus’ research dissected above). Most ideas on the forgetting curve and the spaced repetitions come from the engineers of SuperMemo, mostly Piotr Wozniak.
Since it sounded compelling, research soon started.
Initial research, like this study, found spaced repetition to be effective for learning simple language vocabulary, suggesting a schedule of "reviews 5-10 minutes after the end of the study period, 24 hours later, 1 week later, 1 month later, and finally 6 months later”.
And yet, lots of recent studies show that increasing repetition intervals like above are only slightly superior to fixed time intervals (example: reviewing flashcards once a week for six weeks). Especially when students encoded the information before, there was barely any difference in effectiveness whatsoever: On average, spaced repetition schedules were 3% more effective than fixed repetition.
Effectiveness of retrieval practices: The real benefit comes from active testing, spaced out over multiple sessions. An optimal spaced repetition schedule is the icing on the cake
Effectiveness of retrieval practices: The real benefit comes from active testing, spaced out over multiple sessions. An optimal spaced repetition schedule is the icing on the cake
Seems like the gains from finding the optimal schedule for spaced repetitions are very marginal.
Nevertheless, there are still clear benefits to using spaced repetition, because it generates a schedule in the first place, removing the boring work of planning and calendaring so you can spend more time learning the actual content. Besides, a nice side effect of a schedule of increasing intervals is that you’ll always have a good mix of new and old learning material.

A schedule that works for most cases

Obsessing about the optimal schedule could be a misplaced effort for you, knowing that we can expect benefits at an order of just around 3%.
By looking at the most popular spaced repetitions systems and apps (explored in more detail below), we can find a schedule that will do great in most cases. You can use this schedule as your template for spaced repetition:
  • Day 0: Initial learning
  • Day 1: First repetition within 24 hours
  • Day 6: Second repetition in about one week
  • Day 14: Third repetition in about two weeks
  • Day 30: Fourth repetition in about a month
  • Day 66: Fifth repetition in about two months
  • Day 150: Sixth repetition in about five months
  • Day 360: Seventh repetition in about a year
7 repetitions usually suffice to remember information for life. Also notice that after the second repetition, the next interval can be calculated by multiplying the previous interval with a factor of around 2.2. This number is called the ease factor, and depending on your implementation, it is usually set between 2 and 2.5.
But how can you tailor your schedule to constraints like exams or papers?

Adapting the spaced repetition schedule for your needs

Customizing a Study Schedule for an Upcoming Exam

To reduce your anxiety and maximize your performance around exams, here are some handy tricks:
  1. Prioritize high-yield material: When preparing for an exam, it's essential to focus on the most critical topics and concepts that are likely to appear on the test. Identify high-yield material and prioritize it in your spaced repetition schedule. This approach ensures that you dedicate more time and effort to the material that will have the most significant impact on your exam performance.
  1. Adjust intervals for shorter timeframes: With an exam on the horizon, you'll need to modify the intervals between your study sessions to account for the limited time available. While maintaining the principle of gradually increasing intervals, you may need to shorten the time between reviews to ensure you can cover all the essential material. For example, you might start with intervals of a few hours, then progress to daily reviews and finally, every few days as the exam approaches.
  1. Incorporate practice questions: As you adjust your spaced repetition schedule for an upcoming exam, make sure to simulate practice questions in your study sessions. They will help reinforce your understanding of the material and allow you to apply your knowledge in exam-like conditions. I also recommend you use these questions independent of exams, as this is a great tool to uncover gaps in your knowledge.
  1. Factor in time for rest and recovery: Though exam preparation can be intense, it's essential to recognize the importance of rest and recovery in your spaced repetition schedule. Overworking yourself can lead to burnout and diminished performance, so make sure to include regular breaks and rest days in your study plan (which, by the way, includes breaks from electronic devices). Maintaining a healthy work-life balance will not only help you retain information more effectively but also reduce stress and anxiety during exam season.
  1. Monitor progress and adjust as needed: As you prepare for your exam using spaced repetition, continuously assess your progress and adjust your study plan accordingly. You may need to revisit certain topics more frequently or allocate additional time to practice questions based on your performance. Stay flexible and responsive to your learning needs, and remember that it's okay to tweak your spaced repetition schedule as you go.
By adapting your spaced repetition schedule to suit the demands of an upcoming exam, you'll be better equipped to excel in your tests. With a tailored approach to learning, you can confidently tackle exam season and stay on track to achieve your goals as an academic top-performer.

Prior knowledge and familiarity with the subject

Familiarity with a subject can significantly impact the effectiveness of your sptudy schedule. Adapting your study plan to this facet will help you optimize your learning experience. Here’s how exactly:
  1. Assess your baseline knowledge: Before diving into a new subject, take some time to evaluate your current understanding of the material. This self-assessment can help you identify areas where you already have a strong foundation and those that require more attention.
  1. Adjust the intervals for familiar topics: For topics you're already familiar with, you can afford to start with longer intervals between study sessions. Since you already have a basic understanding of the material, your brain will require less frequent reviews to reinforce the knowledge. However, continue to monitor your retention and adjust intervals as needed to maintain your grasp of the subject matter.
  1. Focus on new or challenging material: Allocate more time and shorter intervals to new or challenging concepts in your spaced repetition schedule. Since these topics are less familiar, your brain will require more frequent reviews to solidify the information in your long-term memory. Gradually increase the intervals between study sessions as you become more comfortable with the material.
  1. Interleave related topics: As you progress through your study plan, consider interleaving related topics to strengthen connections between different areas of knowledge. Mixing familiar and new material in your spaced repetition schedule can help reinforce your understanding and improve your ability to apply the information in various contexts.
By adapting your spaced repetition schedule based on your prior knowledge, you can create a more personalized and efficient learning experience. Approaching learning like this will enable you to focus on the areas where you need the most improvement while reinforcing your existing knowledge, setting you up for success in your medical education journey.

Balancing spaced repetition with other evidence-based learning strategies

The most important thing when studying using spaced repetition is to not over-rely on it. As shown in the research above, obsessing over your spaced repetition schedule is not the best use of study time.
Instead, one thing that is easily overlooked by many students using spaced repetition is initial learning. Here, you put in the effort to make logical sense of the material: Improving the way you store material in your long-term memory can significantly reduce the number of repetitions needed to master the material. And not only that - it gives you a significant advantage over your peers: whereas flashcards are used by over 67% of students, so-called ‘encoding techniques’ to get a better grasp of the material are used by only 12%, even though the expected increase in retention is even larger.
You can read more about incorporating encoding and other evidence-based learning strategies in your spaced repetition practice in our Guide to use Spaced Repetition to Master not Memorize.
Effectiveness of retrieval and encoding practice, compared to actual use by students.
Effectiveness of retrieval and encoding practice, compared to actual use by students.

The best spaced repetition systems and algorithms

Ready to supercharge your learning with spaced repetition? In this section, we'll explore the best systems and algorithms for both analog and digital learners. First, we'll dive into the classic Leitner System, which uses physical flashcards to effectively boost memory retention.
We'll also cover various digital apps and algorithms that offer advanced features and customization options. Whether you prefer physical flashcards or digital tools, this list has got something for everyone. Let's start with a time-tested approach you can use both digitally or in the analog world:

The Leitner System: A Time-Tested Approach to Spaced Repetition

The Leitner System is a popular and straightforward spaced repetition technique developed by German scientist Sebastian Leitner in the 1970s. This system uses flashcards and a series of labeled boxes to help learners efficiently review and retain information. Here's a closer look at how the Leitner System works:
  1. Setting up the Leitner System:
    1. Create flashcards with questions or concepts on one side and answers or explanations on the other.
    2. Label a series of boxes (or use a flashcard app with a built-in Leitner System algorithm) with numbers representing review intervals (e.g., Box 1 for daily review, Box 2 for three-day intervals, Box 3 for weekly intervals, etc.).
  1. How to use the Leitner System:
    1. Start by placing all flashcards in Box 1.
    2. Review the cards in Box 1. When you answer a card correctly, move it to the next box (e.g., from Box 1 to Box 2). If you answer a card incorrectly, it moves back to Box 1 (alternatively, you can just move it one box lower).
    3. Continue reviewing cards in each box according to their designated intervals. The cards in higher-numbered boxes will be reviewed less frequently, while those in lower-numbered boxes will require more regular reviews.
    4. Regularly assess your progress and adjust the intervals or number of boxes as needed to suit your learning preferences and goals.
The Leitner System: an analog implementation of spaced repetition (image source:
The Leitner System: an analog implementation of spaced repetition (image source:
The benefit of the Leitner System is that it provides a structured, easy-to-understand approach to implementing spaced repetition, making it an excellent starting point for students new to this learning technique.
But its main limitation is the fact that it’s hard to connect it with your other study materials and methods, which nowadays mostly live digitally. So let’s also look at some digital apps you can use for your spaced repetition practice.

The best spaced repetition apps

In this section, we'll introduce you to the best spaced repetition apps that utilize modern algorithms to optimize your study sessions. This is the field pioneered by SuperMemo, who created the popular SM-2 algorithm that has set the standard for many other spaced repetition apps.


SuperMemo, a pioneering spaced repetition software, now struggles with an outdated user interface and limited device compatibility. Developed in 1987 by Piotr Wozniak, it introduced the popular SM-2 algorithm.
Nowadays, te app's design feels archaic, making navigation frustrating and time-consuming. Additionally, SuperMemo lacks seamless integration with other devices, hindering its adaptability to various study environments.
Despite its historical significance, the steep learning curve and overwhelming options deter potential users. Today, learners have numerous user-friendly alternatives to choose from, leaving SuperMemo as a reminder of the early days of spaced repetition software.
SuperMemo (image source:
SuperMemo (image source:


Anki is a widely popular spaced repetition app that has become a standard for many learners. It uses its own adaptation of the SM-2 algorithm. However, some students over-rely on Anki as a "complete study solution," which can be dangnerous. While Anki is an excellent tool for memorization, it should not be the only method of studying. Relying solely on Anki can lead to a superficial understanding of the material, as well as burnout from constant repetition. It's essential to supplement Anki with other learning techniques, such as effective encoding techniques like mind mapping. Remember, Anki is a tool to help you learn, not a substitute for active engagement with the material. Used correctly, Anki can be a powerful addition to your study routine, but be wary of overreliance.


RemNote is a spaced repetition app that allows users to add notes with context, making it a useful tool for retaining information. One limitation is that its text-first approach can make it challenging to deeply grasp complex topics. The lack of visualization and the constraints of linear notes can hinder understanding and make it harder to connect the dots between concepts.
It's important to supplement RemNote with other methods that cater to different learning styles. Visual aids, interactive resources, and practice questions can help learners build a deeper understanding of the material.
In summary, RemNote is a decent spaced repetition app with good note-taking capabilities, but its text-heavy approach is not ideal for all learners. It's essential to supplement RemNote with other learning methods to build a comprehensive understanding of the material.


We created Traverse to offer learners a complete, science-based study method in a single app. It includes user-friendly spaced repetition flashcards, note-taking features, and a visually organized mind map that allows for vibrant and colorful expression of ideas. If you're using Anki, decks can be easily imported, including scheduling information, media, and image occlusions. Also, the default scheduling algorithm used in Traverse is similar to Anki’s algorithm, but you can fully customize it according to your needs as mentioned above.
Traverse is currently the only app on the market that provides a comprehensive evidence-based study process. Try out Traverse and see how it can revolutionize your learning experience.
notion image


In conclusion, spaced repetition is a valuable tool to optimize your learning process, but it's not a complete solution on its own. Prioritizing understanding and optimizing for memory are key elements in achieving success with spaced repetition.
By utilizing active recall, spaced repetition, and varied retrieval practice, you can optimize your learning sessions. The optimal spaced repetition schedule may not matter as much as the effort you put into understanding and encoding the information properly.
If you're ready to take your studying to the next level, give Traverse a try by going here. You’ll have everything you need to encode what you’re learning and use spaced repetition along with it, in the same app. Especially if you don’t believe that learning can be fun, let Traverse change your mind.

Read More

Don't forget to share this article:
10x your learning
Improve your memory and thinking skills with our science-based method
Try Traverse Today
Try Traverse Today