Spaced Repetition: A Practical Guide

The short version

You forget things you've learned, quickly at first and then more slowly. Each time you successfully recall something, you forget it more slowly afterwards. Spaced repetition takes advantage of that: review a card soon after you learn it, then after a longer gap, then a longer one, and so on. Over time, each card needs very little attention.

Doing this by hand for hundreds of cards would be tedious, so software keeps track of when each card is due. Your job is to show up and review whatever's due today.

Where the idea comes from

In the 1880s, the German psychologist Hermann Ebbinghaus memorised lists of nonsense syllables and tested himself at different delays. He found that memory falls off steeply in the hours after learning and then levels out — what's now called the forgetting curve. He also found that relearning was faster each time, and that spreading practice out was more efficient than massing it together.

That spacing effect has since been replicated many times, with many kinds of material. A large 2006 review by Cepeda and colleagues combined results from hundreds of experiments and found a consistent advantage for spaced over massed practice. A 2008 study by the same group looked at how the best gap depends on how long you need to remember something: in general, the longer you need to retain information, the longer the gaps between reviews should be.

Spacing pairs naturally with the testing effect (see our active recall guide): each spaced review is also a retrieval attempt, which strengthens the memory more than re-reading.

How a scheduler decides when to show a card

The first widely used computer scheduler was SuperMemo's SM-2, from the late 1980s, which Anki was originally built on. It works roughly like this: each card has an interval (days until the next review) and an ease factor. Answer correctly and the interval is multiplied by the ease; answer wrongly and the card starts over.

Newer schedulers model memory more directly. FSRS (Free Spaced Repetition Scheduler), which modern Anki versions offer and which our review mode uses, estimates for each card:

• Stability – roughly, how long the memory will last before your chance of recalling it drops to a target level.
• Difficulty – how hard this particular card is for you.
• Retrievability – the current probability you'd recall it right now.

It schedules the next review for when retrievability is expected to fall to your target retention (commonly around 90%). Its parameters can be fitted to review history, so the schedule adapts to how you actually remember.

You don't need to understand the maths to use it. The practical upshot: be honest about whether you recalled a card, and the scheduler takes care of the timing.

How to grade your reviews

After you reveal the answer, you tell the scheduler how it went. Anki-style schedulers, including FSRS, use four grades: Again, Hard, Good, and Easy:

• Again – you didn't recall it, or recalled it wrong. The card comes back soon.
• Hard – you got it, but with real effort or hesitation.
• Good – you recalled it correctly with normal effort. This should be your most common answer.
• Easy – you knew it instantly and it felt trivial.

The biggest mistake is pressing Good when you half-knew it. That pushes the card further out than it should go, and you'll forget it before it comes back. When in doubt, choose Again.

Building a daily habit

Spaced repetition works best a little at a time, every day. Some habits that help:

• Do your reviews before adding new cards. Reviews are what keep old material alive.
• Limit new cards. Each new card generates several reviews over the following weeks. If daily reviews start to feel like a chore, add fewer new cards for a while rather than skipping days.
• Keep sessions short. Ten to twenty minutes is plenty for many people. You can split it across the day.
• Watch your streak and forecast. The stats page shows your streak, a calendar heatmap, retention, and how many reviews are coming up, so you can see a heavy week before it arrives.

If you miss a few days, the cards don't disappear — they're just overdue. Work through the backlog over several days instead of all at once, and add no new cards until it's cleared.

What makes cards easy to schedule

The scheduler can only work with the cards you give it. Cards that test one clear fact get consistent grades, so their intervals grow steadily. Cards that bundle several facts get inconsistent grades and keep bouncing back. If a card keeps failing, don't just keep pressing Again: rewrite it, split it, or add an image or example. See how to make flashcards for wording tips.

Spaced repetition before an exam

Spacing pays off most when you start early. If an exam is three weeks away, starting now gives each card several spaced reviews. If it's tomorrow, the scheduler can't do much; use Learn and Test modes to drill everything once or twice, and plan to start earlier next time.

For a cumulative final, keep reviewing earlier chapters' cards alongside new ones. That's exactly what the scheduler is for, and it's much less work than re-learning everything in the last week.

Using spaced repetition here

Open any deck and start a spaced repetition review. You'll see the cards that are due today, along with new cards you haven't studied yet. Everything — your review history and the schedule — is stored in your browser on your device, so there's nothing to sign up for. If you switch computers, export a backup and import it on the other one.

Prefer Anki? You can make cards here, including cloze and image cards, and export an .apkg file to study in Anki instead.