You study for an exam, you know it cold on the night, and three weeks later most of it is gone. That isn't a personal failing. It's how memory works, and spaced repetition is the most reliable way to work with it instead of against it.
Why you forget: the forgetting curve
In the 1880s the German psychologist Hermann Ebbinghaus memorised lists of nonsense syllables and tested himself at different delays. He found that forgetting is fast at first and then slows down: much of what you learn fades within the first day or two, and what survives fades more slowly after that. Plotted on a graph, this is the forgetting curve.
Ebbinghaus also found the fix. Each time he relearned a list, it took less effort, and it was forgotten more slowly afterwards. Reviewing doesn't just top memory back up; it makes the next decline gentler.
What spaced repetition is
Spaced repetition turns that finding into a schedule. Instead of reviewing everything every day, you review each piece of information at growing intervals, timed to land just before you'd forget it:
- a few minutes after you first learn it,
- then the next day,
- then a few days later,
- then a week or two later,
- then a month, and so on.
Each successful review pushes the next one further out. Material you find hard comes back often; material you know well barely takes any time.
This is known as the spacing effect, and it's one of the most consistently replicated findings in learning research. A large 2006 review by Cepeda and colleagues found that spreading study sessions out beat cramming the same amount of study into one session. A 2013 review of ten common study techniques by Dunlosky and colleagues rated distributed practice as one of only two "high utility" techniques. The other was practice testing, which is exactly what reviewing a flashcard is.
How a spaced repetition algorithm decides
Working out the right gap for hundreds of cards by hand is impossible, so apps do it for you. Most are built on SM-2, the algorithm Piotr Woźniak created for SuperMemo in the late 1980s, which Anki later adapted. It works roughly like this:
- Every card has an "ease". It starts at 2.5.
- You grade each review. For example: Again, Hard, Good or Easy.
- A good answer multiplies the interval by the ease. If the last gap was 3 days and the ease is 2.5, the next gap is about 8 days.
- Hard answers shrink the ease; easy ones grow it. Cards you struggle with stay on shorter intervals.
- Forgetting resets the card. It goes back into short relearning steps, and its interval shrinks.
In StudyPix's spaced repetition, new cards go through 1-minute and 10-minute steps before their first 1-day interval. If you answer Good each time after that, a card comes back after roughly 1, 3, 8 and 20 days. A forgotten card gets a 10-minute relearning step, then restarts at 20% of its old interval.
How to build a spaced repetition habit
Review every day, briefly
Spaced repetition works best as a daily habit of 10 to 20 minutes. Skip a week and the reviews pile up. Many students tie it to something they already do every day, like breakfast or their first Pomodoro block.
Cap new cards
Adding 200 new cards on Sunday means a mountain of reviews by Wednesday. A limit of around 20 new cards a day per subject keeps the daily load steady. StudyPix applies that limit for you.
Keep cards small
One fact per card. A card that asks for a list of seven things will always be graded Hard, and you'll never know which part you're missing. Our guide to writing flashcards that work goes into detail.
Start early
Spacing needs time. Starting four weeks before an exam gives your cards room to reach longer intervals. Starting two days before is cramming with extra steps. If you're short on time, review more often, but don't expect long-term retention.
Spaced repetition beyond flashcards
The principle isn't limited to term-and-definition cards. Quiz questions work too: a right answer counts as a successful review and a wrong one sends the question back into relearning. Some subjects, like maths, need practice problems more than cards. Spacing those out across weeks, rather than doing one topic all at once, helps in the same way.
The bottom line
You will forget most of what you learn unless you review it, and reviewing everything all the time is impossible. Spaced repetition solves both problems: a few minutes a day, on exactly the cards that need it. Make your cards by hand, or generate a first draft from your notes with AI, and let the schedule do the rest.