Spaced repetition, without the paywall

In the 1980s, Piotr Woźniak worked out that memory decays on a predictable curve — and that reviewing a fact just before you'd forget it locks it in with minimum effort. The algorithm he built, SM-2, is still the backbone of serious spaced-repetition software forty years later. The science is settled. The pricing is not.

How SM-2 actually works

Every card gets an easiness factor and an interval. Get it right, the interval grows (1 day, 6 days, then expanding). Get it wrong, it collapses back. Rate a card "hard" vs. "easy" and the algorithm adjusts. That's the whole trick: the schedule adapts to your actual memory, card by card.

The evidence base is deep — decades of research on the testing effect and spaced practice, plus the largest natural experiment in learning software: millions of Anki users, including essentially every medical student who ever lived, retaining absurd volumes of material on SM-2-derived schedules.

The algorithm is public. It was never anyone's property. Remember that when the paywall appears.

The paywall landscape, honestly

Anki is free and open-source on desktop, web, and Android; the iOS app is a one-time $24.99 that funds the entire project. It's the gold standard for the algorithm and the worst onboarding in the category — powerful, fiddly, and proud of it. If you can climb the learning curve, it's the best free option ever made.

Quizlet is the easy one: great UX, huge shared deck library. The catch is the meter — the free tier caps Learn and Test modes, and Plus runs $7.99/month or $35.99/year. Four years of college at the annual rate: $143.96. For flashcards.

The frustration students feel isn't about the dollars. It's the precedent: build your study habits on the free tier, then find the core modes behind the paywall. The algorithm didn't get more expensive to run. The business model changed.

What actually matters in a flashcard app

Strip away the marketing and there are five things:

1. The scheduler. SM-2 or a real descendant — not a streak counter wearing a lab coat.
2. Uncapped study modes. Learn and test are the product. Metering them is metering the learning.
3. Import. Your decks are your work. CSV import (at minimum) means you can leave any app, which is exactly why some apps make it hard.
4. Offline. You study on planes, in basements, in dead zones. The cards should live on your device.
5. Honest stats. Retention rate, cards due, streaks that measure consistency — data that helps you study, not gamification that helps the app retain you.

The study method that makes any app work

The app is 20% of it. The method:

• Make your own cards. The act of writing the question is half the learning. Imported decks are a starting point, not a strategy.
• One idea per card. "What are the causes of the war?" is an essay prompt. "Which treaty ended it?" is a flashcard.
• Do your reviews daily. The algorithm's intervals assume you show up. Ten minutes daily beats three hours Sunday — this is the single biggest predictor of results.
• Fail cards honestly. Marking "hard" when you guessed trains the scheduler on lies. The algorithm only works on true data.

The algorithm is free. It always was. The only question is which app respects that — and your wallet — while you use it.