Spaced repetition: how it works and how to start

Spaced repetition means reviewing something at growing intervals, a little before you would forget it. You review a new fact the next day, then a few days later, then a few weeks later, and each time you remember it the next gap gets longer. It works because reviews spread over time beat reviews crammed together, and because each review is a small test of your memory. You can follow a schedule by hand, use a box of index cards, or let an app like Anki pick the gaps for you.

Why spaced repetition works

Two well-studied effects are behind it.

The first is the spacing effect. A 2006 review of 317 experiments by Nicholas Cepeda and colleagues found that the same amount of study time leads to better memory when it's spread out than when it's done in one block.

The second is the testing effect. In spaced repetition each review asks you to recall the answer before you see it, and pulling something out of memory strengthens it more than reading it again. There's more on this in active recall.

Put together, each review lifts your memory back up and makes the next drop slower, which is why the gaps can keep growing. The forgetting curve post shows what that drop looks like.

Is spaced repetition effective?

Yes. In 2013 a team led by John Dunlosky reviewed ten common study techniques and gave the top rating to two of them: spreading practice out over time and practice testing. Spaced repetition combines both.

How long should the gaps be?

It depends on how long you want to remember the material. In a 2008 study, Cepeda and colleagues taught more than 1,350 people a set of facts, gave them one review after a gap of up to three and a half months, and tested them up to a year later. The best gap grew with the test delay, but as a share of it, the best gap shrank: about 20% to 40% of the delay when the test was a week away, and about 5% to 10% when it was a year away.

In practice:

• To remember something for a week, review it after a day or two.
• To remember something for a year, a review three to five weeks after you learn it works better than one the next day.

Spaced repetition uses several reviews with growing gaps, so you don't have to guess the one best gap.

Spaced repetition schedules you can follow by hand

The 2-7-30 rule

Review 2 days, 7 days and 30 days after you learn something. It isn't from one specific study, but it follows the spacing research and it's easy to remember. How to remember what you read uses a version of it for articles.

The 2357 method

This schedule is popular with students and works back from an exam. Birmingham City University describes it as a session the day before the exam, one 2 days before that, one 3 days before that, one 5 days before that and the first one 7 days before that, so you start 18 days out.

The Leitner box

In the 1970s the German science writer Sebastian Leitner described a box of index cards split into sections. Every card starts in the first section. When you answer a card correctly it moves to the next section, and when you get it wrong it goes back to the first. You review the first section often and each later one less often, for example daily, weekly, every two weeks and monthly. The cards you know drift to the back, and the ones you don't keep coming up.

How spaced repetition apps pick the gaps

Apps keep a schedule for every card, so you only see the cards that are due.

Anki's default scheduler and many other apps are based on SM-2, an algorithm Piotr Woźniak wrote for SuperMemo in 1987. It shows a new card after 1 day, then after 6 days, then multiplies the gap by an ease factor that starts at 2.5, and a card you forget goes back to the start.

FSRS, the Free Spaced Repetition Scheduler, is a newer approach created by Jarrett Ye. It keeps three numbers for each card: how difficult it is, how stable your memory of it is, and how likely you are to recall it today. It schedules the next review for when that likelihood is about to fall below a target you choose, 90% by default, and a card you forget doesn't lose all its progress. Anki added FSRS in version 23.10 in 2023 as an option you can switch on. Domenic Denicola's spaced repetition recap explains both algorithms in more detail, and my FSRS simulator shows the gaps FSRS picks as you answer a card.

How to use spaced repetition on what you read

Spaced repetition is best known for vocabulary and exam facts, but it works for articles and books too. A few things make it easier:

• Make cards from ideas, not from highlights. After an article, write two or three questions about the ideas you want to keep, each with a short answer.
• Keep the daily queue small. A few new cards a day adds up over months, while a hundred new cards a day turns reviews into a long chore.
• Delete cards you no longer care about. Spaced repetition is for what's worth remembering, not for everything you read.
• Review a little every day. The schedule only works if you see cards around the day they're due.
• Use the same gaps for lists you keep in a memory palace. Walk the route again a day later, a week later and a month later.

I made Bundle to do this for articles. When you finish an article, a Quick check asks you a few questions about it, and you keep the good ones as cards. Bundle schedules the cards with FSRS and sends one reminder a day with the ones that are due, which usually takes about five minutes.

Bundle is on the App Store for iPhone, and you can try it free for 14 days.

Questions about spaced repetition

What's the difference between spaced repetition and active recall?

Active recall is how you review: you pull the answer from memory before you look. Spaced repetition is when you review: at growing gaps, timed to come a little before you'd forget. They work best together.

Do I need an app for spaced repetition?

No. A Leitner box or a calendar with the 2-7-30 rule works. An app helps once you have more than a few dozen cards, because it tracks each card's schedule for you.