Free tool

Spaced Repetition Interval Calculator

Estimate your next review date using a simplified FSRS model. Adjust for your target retention and recall difficulty.

80%85%90%95%

How this calculator works

FSRS (Free Spaced Repetition Scheduler) models memory as an exponential forgetting curve. The core idea: the probability you will recall a card drops exponentially with time since your last review.

This calculator uses a simplified version of that model. It estimates your card's memory stability from your self-rated recall and the elapsed time, then computes the interval that should bring you to your chosen target retention on the day of review.

  • Again means you forgot it — the card needs review soon.
  • Hard means you recalled but with effort — interval grows slowly.
  • Good is the standard answer — interval grows at the normal rate.
  • Easy means instant recall — interval grows faster.

Study with real FSRS scheduling

Deckbase uses the full FSRS algorithm natively — not a plugin, not an approximation. It learns your personal memory patterns and optimizes review timing for maximum retention with minimum study time.

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This tool is free and ungated. Embed it or share the link anywhere.

FAQ

What is a spaced repetition calculator?

A spaced repetition calculator estimates the optimal interval before your next review based on how well you recalled a card and when you last studied it. It applies the FSRS (Free Spaced Repetition Scheduler) formula to predict the date that maximizes long-term retention.

How does the FSRS algorithm work?

FSRS models memory as a forgetting curve and adjusts the next review interval based on your recall performance. Better recall leads to longer intervals; harder cards are reviewed sooner. The algorithm balances retention against review volume.

What retention rate should I aim for?

Most users target 85–90% retention. Higher retention means more frequent reviews and a larger daily workload. Lower retention reduces workload but increases the risk of forgetting. Pick a target that fits your available study time.

How do I use this calculator?

Enter the number of days since your last review, select how well you recalled the card (Again / Hard / Good / Easy), and set your target retention rate. The calculator returns the recommended next review date.

What does recall difficulty mean?

Recall difficulty describes how hard it was to remember the answer. 'Again' means you forgot it. 'Hard' means you remembered but with effort. 'Good' means comfortable recall. 'Easy' means instant, effortless recall.

Is this calculator accurate for Anki and Deckbase?

The calculator uses a simplified FSRS model, so the results are close but not identical to Anki or Deckbase scheduling. Use it for quick estimates and planning; rely on your app’s built-in scheduler for actual study sessions.