Spaced Repetition Intervals: How Long to Wait Between Reviews

Most explanations of spaced repetition stop at the idea: review material several times, with gaps in between. That part is covered in the main guide to the spaced repetition study method. This article is about the numbers. How long should the first gap be? Should gaps grow or stay the same? What happens to the schedule when you forget something? And how do you squeeze the whole thing into the weeks before an exam?

The short version is that there is no single perfect interval. Memory research gives clear direction on the general shape of a good schedule, and flashcard software has spent decades refining practical defaults. What follows is a working schedule you can use by hand, with the reasoning behind each choice.

A spaced repetition schedule you can start with

The table below shows a typical expanding schedule for a single item, such as a flashcard, a definition or a short process. It assumes you answer correctly each time. Treat these as starting values, not rules.

Notice that the gap roughly doubles or triples after each successful review. That growth rate is the core of an expanding schedule. The SM-2 algorithm, which many flashcard apps are based on, works the same way: after the first two fixed intervals, each new interval is the previous one multiplied by an "ease factor" that starts at 2.5 and changes depending on how hard you found the item.

Why do intervals get longer each time?

Question: If reviewing helps, why not keep reviewing every few days forever?

Answer: Because each successful retrieval makes the memory more durable, so it takes longer to fade before the next review is useful. Reviewing too soon wastes time on items you would have remembered anyway, and it makes the retrieval so easy that it does less to strengthen memory.

The aim is to review an item when it has become slightly hard to recall but is still retrievable. Early on, that point arrives quickly, often within a day. After several successful reviews, the same item might stay accessible for weeks. Expanding intervals follow that changing point instead of fighting it.

The research on spacing supports the general idea well. A 2006 review of distributed practice studies by Cepeda and colleagues found that spreading study across sessions consistently beat massing it into one session. A 2008 follow-up by the same group found that the best gap between study sessions depended on how long the information had to be kept: the further away the final test, the longer the ideal gap, although the gap did not need to grow in direct proportion to the delay.

Expanding vs fixed intervals: which is better?

Question: Do intervals actually need to expand, or would a fixed gap such as every three days work just as well?

Answer: The evidence here is less clear-cut than many apps suggest. Both approaches beat cramming. Expanding schedules are more efficient for large collections, while fixed schedules can work well for a short, defined revision window.

In a set of experiments by Karpicke and Roediger (2007), expanding retrieval gave an advantage on short-term tests, but equally spaced retrieval did as well or better on delayed tests. Their analysis pointed to the timing of the first retrieval as an important factor: a very early first review is easy and does less for long-term memory. The practical lesson is not that expanding schedules are wrong, but that the first gap should not be too short.

What happens to the interval after a wrong answer?

Question: If I forget an item on day 25, do I start the whole schedule again from day 1?

Answer: In the classic systems, yes. In more recent ones, not quite. Either way, the interval must shrink, and the item must be answered correctly again before it is allowed to stretch out.

A useful distinction is between an answer you got right on your own and one you only got with a hint or a peek. The second kind should be treated much like a miss. It shows the item is still fragile.

How to fit intervals around an exam date

Open-ended schedules assume you want to remember things indefinitely. Students usually have a fixed date. That changes the maths, because an interval that pushes the next review past the exam is useless.

These are rules of thumb consistent with the spacing research, not values taken from a specific study. The general principle is that shorter deadlines call for shorter gaps. If your exam is only days away, the 7-day exam study plan shows how to compress the schedule without losing the spacing.

What does tracking intervals by hand involve?

With twenty flashcards, a paper schedule is manageable. With three modules and several hundred items, it becomes a job in itself. Every item has its own due date. Every wrong answer changes that date. New material arrives each week, so the daily workload rises and falls in ways that are hard to predict.

• You need a record of when each item was last reviewed and what its current gap is.
• You need to decide, for each miss, whether to reset fully or step back part of the way.
• You need to cap gaps so nothing important drifts past the exam date.
• You need to keep the review itself active, with active recall rather than a quick read of the answer.

This bookkeeping is the main reason students drop spacing halfway through a term. It is also what flashcard apps were built to handle. If you are choosing one, the comparison of the best spaced repetition apps covers the main options and how their scheduling differs.

FAQ: Spaced repetition intervals