How Long Should You Wait Between Reviews? Choosing Spacing Intervals

The idea: match the gap to how long you need to remember

Two terms help. The gap is the time between two study sessions. The delay is the time until the final test. A meta-analysis of verbal recall tasks (word lists and facts, mostly in laboratories) found that the gap giving the best final-test retention increased as the delay increased (Cepeda et al., 2006).

A later experiment looked at this directly. More than 1,350 people learned a set of facts, reviewed them after gaps of up to 3.5 months, and were tested up to a year later. Final-test performance first rose and then gradually fell as the gap grew, and the best gap grew with the delay. Measured as a share of the delay, the best gap was about 20 to 40% for a one-week delay and about 5 to 10% for a one-year delay (Cepeda et al., 2008).

Two things stand out. A gap can be too short or too long, and the best one sits in between. And the best share of the delay gets smaller as the delay gets longer. These figures describe one study with one kind of material. They are not a personal schedule.

Starting gaps by test date (rules of thumb)

The table turns the study's percentages into days where it can and fills the rest with our own rules of thumb. None of it is a finding. We read only the abstract of that study, so we cannot say how it applies to a long series of reviews. Use these as first gaps, then adjust.

With a test two or three days away, spaced reviews are squeezed. The comparison in spaced practice versus cramming covers that case honestly.

Expanding gaps or even gaps?

Many spacing schedules make each wait longer than the last. That feels logical, but the evidence is not decisive. A meta-analysis of 29 studies found a strong benefit of spaced over massed retrieval practice, yet no significant difference between expanding and uniform schedules (Latimier et al., 2021). The authors say the results do not support the widely held belief that intervals should progressively increase. Twenty-nine studies is a modest evidence base, so this is not the last word either.

Practice uses growing waits of 1, 2, 4, 8 and 16 days. That is a simple model inspired by the Leitner system. It has not been shown to be optimal. How the five boxes work walks through it step by step.

Adjusting after your first reviews

1. Count the days to your test and pick a starting gap from the table.
2. Do the first review and note your score. If you answered nearly everything correctly, try a longer next gap. If you missed many cards, shorten it. This is practical advice, not a tested rule.
3. Put every review date on a calendar when you add the cards, so the plan does not depend on memory.
4. Finish your last review before the test date, not after it.

For a week-by-week version, see a four-week retention plan before an exam. You can also review key spacing terms in Practice.

Common mistakes

• Using the same gap for every test date. The best gap tended to grow with the delay until the test (Cepeda et al., 2006). Pick a shorter gap for a closer test.
• Waiting so long that almost everything is forgotten. In one large study, performance first rose and then gradually fell as the gap grew (Cepeda et al., 2008). If many cards come back wrong, shorten the next gap.
• Reading the percentage figures as your personal schedule. They come from one study with one kind of material. Use them as a starting point and let your own results guide you.
• Believing that gaps must keep growing. The meta-analysis found no significant difference between expanding and uniform schedules (Latimier et al., 2021). Choose a rhythm you will keep.

Frequently asked questions

What is the best interval for spaced repetition?

No single interval is best for everyone. In one large study the best gap grew with the delay until the test: about 20 to 40% of the delay for one week and about 5 to 10% for one year (Cepeda et al., 2008). Use that idea, then adjust to your own results.

Should intervals get longer each time?

They can, but you do not have to. A meta-analysis of 29 studies found no significant difference between expanding and uniform schedules (Latimier et al., 2021). Pick the pattern that is easiest for you to follow.

Are 1, 2, 4, 8 and 16 days the optimal intervals?

We do not claim that. It is a simple model inspired by the Leitner system, used in Practice. Change the waits if your test date or your results call for it.

What if I do not know my test date?

Choose a horizon you care about, such as the end of the term, and treat it as the delay. This is practical advice, not a finding. Start with gaps of a few days and lengthen them while your answers stay correct.

Sources

We opened each source below before citing it. See Methodology and Sources for how we check them.

1. Cepeda, N. J., Vul, E., Rohrer, D., Wixted, J. T., & Pashler, H. (2008). Spacing Effects in Learning: A Temporal Ridgeline of Optimal Retention. Psychological Science, 19(11), 1095-1102. Open the source
2. Cepeda, N. J., Pashler, H., Vul, E., Wixted, J. T., & Rohrer, D. (2006). Distributed practice in verbal recall tasks: A review and quantitative synthesis. Psychological Bulletin, 132(3), 354-380. Open the source
3. Latimier, A., Peyre, H., & Ramus, F. (2021). A Meta-Analytic Review of the Benefit of Spacing out Retrieval Practice Episodes on Retention. Educational Psychology Review, 33(3), 959-987. Open the source