Flashcard Retention Decay Calculator
Flashcards are powerful, but their impact depends on when you review them. Wait too long and you forget; review too soon and you waste time on cards you already know. This calculator uses a simple exponential decay model to estimate how much you will remember after a certain number of days, given an initial retention level and a daily forgetting rate. The goal is not to predict your memory with perfect precision, but to give you a practical way to plan review intervals and tune spaced repetition settings.
How the flashcard retention decay model works
The calculator assumes that, after a study session, your probability of recalling a card declines smoothly over time. Psychologists often approximate this with an exponential decay curve: the steeper the curve, the faster you forget. This is similar to the classic Ebbinghaus forgetting curve, but adapted for everyday study planning.
We start with four main quantities:
- Initial retention – how much you remember right after studying (as a percentage).
- Days until next review – how long you wait before seeing the card again.
- Daily forgetting rate – how quickly your memory fades each day (as a percentage).
- Desired retention – the minimum recall level you want to have at review time.
The underlying model assumes that the rate of forgetting is proportional to your current level of memory. Mathematically, if R(t) is your retention at time t (in days), we can write:
dR/dt = -fR
where f is the forgetting rate per day (expressed as a decimal). Solving this differential equation gives an exponential curve.
In more formal notation:
Here,
Using percentages in the calculator
To keep the inputs intuitive, the calculator works directly with percentages. You can think of it in two steps:
- Convert your initial retention and forgetting rate to decimals.
- Apply the exponential decay, then convert back to a percentage.
If your initial retention is Initial percent and your daily forgetting rate is Rate percent, the calculator estimates your retention after Days as:
Retention_after_Days = Initial × (1 − Rate/100) ^ Days
This is a discrete version of the continuous formula above. For small daily forgetting rates, it closely matches the exponential curve and is easier to interpret: each day you keep a constant fraction of what you remembered the day before.
Interpreting the calculator results
Once you enter your values and run the calculation, you will see an estimated retention level on the review day. Here is how to use that number in practice:
- Retention above your target (e.g., you aim for 80%, and the calculator predicts 90%) suggests that you could safely increase the interval a bit. You are reviewing earlier than necessary.
- Retention near your target (e.g., target 80%, prediction 78–82%) means your chosen interval is well aligned with your goal. You are using your study time efficiently.
- Retention well below your target (e.g., target 80%, prediction 50%) indicates that the gap between reviews is too long for that card or topic. Shorten the interval and try again.
You can also work backward from your desired retention. If the predicted value is too low, experiment by reducing the number of days until review, or by adjusting the daily forgetting rate to better match your experience.
Worked example: planning a review interval
Suppose you finish a study session where you feel fairly confident about a set of vocabulary flashcards. You estimate:
- Initial retention: 90%
- Daily forgetting rate: 15%
- Days until next review: 3
- Desired retention: 80%
Using the discrete percentage formula:
Retention_after_3_days = 90 × (1 − 0.15) ^ 3
= 90 × 0.85 ^ 3
≈ 90 × 0.614
≈ 55.3%
A predicted retention of about 55% is well below your desired 80%. This tells you that, at a 15% daily forgetting rate, a 3-day gap is too long for this material. You might instead test a 1- or 2-day interval:
-
1 day:
90 × 0.85 ^ 1 ≈ 76.5%(close to your goal) -
2 days:
90 × 0.85 ^ 2 ≈ 65.0%(still too low)
In this example, a 1-day review interval better matches your target retention, while a 3-day interval leads to too much forgetting.
Comparing review strategies
The decay-based approach in this calculator is one way to schedule reviews. The table below compares it with two common alternatives.
| Approach | How it works | Pros | Cons | When it fits best |
|---|---|---|---|---|
| Fixed schedule | Same interval for all cards (e.g., review everything every 2 days). | Simple; easy to remember; no parameters to tune. | Ignores difficulty differences; often over-reviews easy cards and under-reviews hard ones. | Short-term cramming, small decks, or low-stakes material. |
| Generic spaced repetition | Predefined intervals (e.g., 1, 3, 7, 14 days) that grow with successful recalls. | Efficient for most users; minimal setup; supported by many apps. | Intervals may not match your personal forgetting rate or specific goals. | General study, large decks, and learners who prefer automation. |
| Decay-based planning (this calculator) | Uses an assumed forgetting rate to estimate recall on any future day. | Helps you align intervals with a target retention; encourages reflection on how fast you forget. | Requires estimating your own forgetting rate; still only an approximation. | Optimizing schedules, fine-tuning app settings, or planning reviews for high-stakes exams. |
Estimating your daily forgetting rate
The daily forgetting rate is personal and can vary across subjects. You can approximate it by:
- Tracking a small test deck: Study 20–30 cards once, avoid reviewing them, and test yourself after 1, 2, and 3 days. Note how your score changes.
- Comparing with your app logs: Many flashcard apps show how often you mark cards as forgotten after certain intervals. Use these patterns to guess how fast your recall drops.
- Starting with a rough default: For relatively easy material, you might start around 10–15% per day; for harder or more abstract content, you might try 20–30% or higher, then refine based on experience.
You do not need a perfect value. The main benefit comes from testing different rates, seeing how the predicted retention compares to your real performance, and adjusting accordingly.
Model assumptions and limitations
This calculator is intentionally simple. To use it responsibly, keep these assumptions and limits in mind:
- Single, constant forgetting rate: The model assumes one daily forgetting rate for the entire time period. In reality, forgetting is faster right after learning and then slows down, and it can change after each successful review.
- Same rate for all cards: The calculation treats all cards in a deck as equally difficult. Actual decks often mix very easy and very hard items that decay at very different speeds.
- No context or interference effects: Factors like stress, sleep, prior knowledge, and interference from similar material are not included, even though they clearly affect memory.
- Approximate, not predictive: The numbers you see are rough planning aids, not guarantees. You should treat them as a starting point and refine your intervals based on observed performance.
- Best for medium-term planning: The model is most helpful for day- to week-scale intervals. For very long-term memory (months to years), additional factors come into play that this simple curve does not capture well.
Used with these caveats in mind, the retention decay calculator can guide you toward more deliberate, efficient review schedules, while leaving room for your own judgment and experience.