Spaced Repetition Schedule: How Intervals Work and When to Review

"Space your reviews out" is solid advice, right up until you sit down to study and realise nobody told you when.

Tomorrow? In three days? Next week? And when you inevitably miss a day, does the whole thing fall apart?

A spaced repetition schedule answers that question with intervals: the gaps between your review sessions. Get them right and a 15-minute daily habit can lock knowledge in for months. Get them wrong and you either waste time reviewing things you already know or forget things you studied last week. (If you are new to the method itself, start with our complete guide to spaced repetition, then come back here for the schedule.)

Here is how intervals work, what the research says about spacing them, and a 30-day worked example showing exactly when reviews land.

The Short Version

The standard spaced repetition schedule is to review new material after 1 day, then 3 days, 7 days, 14 days, 30 days, and 60 days, roughly doubling the gap after each successful recall. If you use an app with an adaptive algorithm such as SM-2, let it set the intervals instead: it starts each card at 1 day, then 6 days, then multiplies the gap by about 2.5 after every successful review, shrinking it whenever you struggle. The research rule of thumb from Cepeda and colleagues (2008) is that your first gap should be about 20 to 40 percent of the time until you need the material if that is a week away, and 5 to 10 percent if it is a year away.

That is the whole system. The rest of this guide explains why those numbers work, what a real month looks like on this schedule, and what to do when life gets in the way. (Studying for a specific test date instead? Our spaced repetition plan for exams covers counting backwards from exam day, including the 2357 method.)

Why Intervals Matter More Than Total Study Time

Most students think about studying in terms of hours: "I studied for three hours last night." But memory research consistently shows that when you study matters more than how long you study.

Consider two students who each spend 60 minutes reviewing the same 50 flashcards:

• Student A reviews all 50 cards three times in a single hour-long session.
• Student B reviews all 50 cards once today (20 minutes), once in three days (20 minutes), and once in ten days (20 minutes).

Same total time. Same cards. But Student B will remember substantially more on a test four weeks later. This is the spacing effect, one of the most replicated findings in cognitive science: a review of 317 experiments by Cepeda and colleagues (2006) found that spaced study beat back-to-back repetition in almost every condition tested. The gap between sessions forces your brain to reconstruct the memory each time, which strengthens it far more than repetition within a single session.

The interval is not dead time. It is when the actual learning happens.

How Expanding Intervals Work

The core mechanic of every spaced repetition schedule is expanding intervals: after each successful review, the gap before your next review gets longer.

The logic is straightforward:

1. You learn something new. You will forget it fast, within hours if you do nothing.
2. You review it 1 day later. The act of recalling it strengthens the memory. Now it will last a few days before fading.
3. You review it again after 3 to 6 days. The memory is stronger now. It will hold for a week or two.
4. You review after 2 weeks. Then a month. Then two months. Each time, the memory becomes more durable and the required interval grows.

This expansion tracks how your brain physically stores information. Each successful retrieval strengthens the connections behind the memory, increasing what researchers call memory stability. A more stable memory decays more slowly, so it can survive a longer gap before the next review.

How long should the gaps be? The best evidence comes from Cepeda, Vul, Rohrer, Wixted and Pashler (2008), who taught facts to more than 1,350 people, varied the gap before a review session, then tested retention up to a year later. Published in Psychological Science as "Spacing Effects in Learning: A Temporal Ridgeline of Optimal Retention", the study found the optimal gap depends on how long you need to remember: about 20 to 40 percent of a one-week retention goal (so 1 to 2 days), falling to 5 to 10 percent of a one-year goal (so roughly 3 to 5 weeks). Longer horizons need proportionally wider spacing, which is exactly what expanding intervals give you.

The forgetting curve maps how fast memories fade between those reviews. Expanding intervals exploit that curve by catching each memory right before it drops below a usable threshold, then boosting it back up. Over multiple cycles, the decay rate slows and the gaps can safely grow.

Three Approaches to Scheduling Intervals

There is more than one way to schedule reviews. Here are three approaches, ranked from simplest to most effective.

1. Fixed Schedule (Manual)

The simplest approach: pick a set of intervals and apply them to every card.

A common fixed schedule:

• Day 1: first review
• Day 3: second review
• Day 7: third review
• Day 14: fourth review
• Day 30: fifth review
• Day 60: sixth review

Pros: Easy to understand and implement with nothing more than a calendar. Cons: Treats every card identically. An easy vocabulary word and a complex biochemistry pathway get the same intervals, which wastes time on easy cards and under-reviews hard ones.

This approach works in a pinch. It is far better than cramming, but it leaves real efficiency on the table.

2. The Leitner System (Box-Based)

The Leitner system adds a layer of adaptivity using physical boxes. Cards you get right move to a higher box with a longer interval. Cards you get wrong drop back to Box 1.

A typical Leitner setup:

• Box 1: review every day
• Box 2: review every 3 days
• Box 3: review every week
• Box 4: review every 2 weeks
• Box 5: review every month

This is smarter than a fixed schedule because difficult cards stay in the early boxes and get reviewed more often, while easy cards graduate to longer intervals. It is a solid system for paper flashcards.

3. Algorithm-Based (SM-2 and FSRS)

Modern flashcard apps calculate a unique interval for every single card based on your history with it. The most widely used algorithm is SM-2, published by Piotr Wozniak in 1987 for the original SuperMemo and used today by Sticky, Anki's classic scheduler, and many other apps.

SM-2 tracks three values per card:

• Ease factor: how easy the card is for you. It starts at 2.5 and never drops below 1.3.
• Repetition count: how many times you have recalled it successfully in a row.
• Current interval: the number of days until the next review.

The schedule it produces is concrete: the first successful review earns a 1-day interval, the second a 6-day interval, and every review after that multiplies the current interval by the card's ease factor. Rate a card "Easy" and the ease factor rises, so intervals grow faster. Rate it "Hard" and the factor falls, so the card comes back sooner. Fail it and the interval resets to 1 day while the ease factor takes a penalty.

Newer algorithms refine this further. FSRS (Free Spaced Repetition Scheduler), an open-source algorithm developed by Jarrett Ye and the open-spaced-repetition project, models each card's difficulty, stability, and current recall probability separately, and was fitted against hundreds of millions of real reviews. Anki has shipped it as a scheduling option since late 2023. Its practical advantage is a desired retention dial: tell it you want to remember 90 percent of your cards and it spaces reviews to hit that target with as few sessions as possible.

The result of any adaptive algorithm is that two cards in the same deck can have wildly different schedules. A card you find intuitive might be scheduled 90 days out, while a card you keep confusing appears every 3 days. See how Sticky implements this.

A 30-Day Worked Example

Abstract intervals are hard to picture, so here is a real month. Say you create 15 Spanish vocabulary cards on Day 1 and study them in an SM-2 app, rating every card "Good" each time it appears. The default schedule (1 day, 6 days, then multiply by 2.5) plays out like this:

Four touches in the first month, then the batch disappears until Day 60. Total review time for all 15 cards across the entire month: roughly 10 to 12 minutes. That is the trade the schedule makes on your behalf: a few short, precisely timed sessions instead of rereading the same list every night.

Now add a difficult card. Suppose one card (say, the difference between ser and estar) fails at the Day 8 review. Its interval resets to 1 day, so it returns on Day 9, then Day 15, then around Day 30. By month's end your 14 easy cards have had 4 reviews each while the hard card has had 6, and its ease factor has dropped so its future intervals will grow more slowly. Nobody planned that. The algorithm noticed where your attention was needed and spent it there.

In practice you will not learn one batch and stop. Add 10 to 15 new cards a day and these per-batch waves overlap: yesterday's cards at their 1-day review, last week's at their 6-day review, last month's trickling back at 15 or 37 days. That overlap is why steady-state review counts settle around 8 to 10 times your new-card rate, and it is why the same maths works whether your deck is AP Biology or a language you plan to study for years.

Building a Practical Review Schedule

What does that overlap look like week by week? Here is a concrete plan for someone adding 10 new cards per day.

Week 1: The Ramp

• Add 10 new cards per day
• Review yesterday's cards (10 cards on Day 2, 20 on Day 3, etc.)
• Daily review time: 5 to 15 minutes, growing as cards accumulate
• Focus: getting the habit right, not volume

Week 2: First Wave Returns

• Cards from Day 1 start coming back for their second review (3 to 6 day intervals)
• Daily review count: 60 to 80 cards (new cards plus returning cards)
• Daily review time: 12 to 18 minutes
• Focus: rating cards honestly. Do not tap "Easy" on cards you hesitated on

Weeks 3 to 4: Steady State

• Review count stabilises around 80 to 100 cards per day (the 8 to 10x multiplier in action)
• Daily review time: 15 to 20 minutes
• Some cards are already on 2-week intervals; others are still showing up every few days
• Focus: consistency over perfection. Six days per week beats seven days for three weeks then quitting

Month 2 and Beyond: Compound Growth

• Your earliest cards are now on month-long intervals
• New cards continue at your sustainable rate
• Total known cards: 200+ with high retention
• Daily review time: still 15 to 20 minutes (mature cards take less time per review)

The key insight: your daily time stays roughly constant even as your total knowledge grows. Mature cards need infrequent reviews, so they barely add to your daily load. This is why spaced repetition scales where cramming does not.

One boundary worth naming: this is a maintenance schedule, designed for material you want to keep indefinitely. If you are working backwards from a fixed exam date, the shape changes: you front-load card creation, stop adding new cards in the final week, and accept shorter intervals. Our week-by-week exam plan covers that case, and our spaced repetition vs cramming comparison shows why even a compressed schedule beats an all-nighter.

What Affects Your Optimal Intervals

Your optimal intervals will not match someone else's. Here is what shifts them:

Material Difficulty

Simple paired associations (word and translation) tolerate longer intervals than complex conceptual cards. If your cards test multi-step reasoning, expect shorter intervals and slower graduation to long-term.

Your Prior Knowledge

If you already know biology well and are adding advanced anatomy cards, your intervals will be longer because existing knowledge provides scaffolding. A beginner in the same subject will need shorter intervals for the same material.

Sleep and Stress

Sleep is when your brain consolidates memories. Poor sleep or chronic stress shortens the effective life of each review, meaning you need shorter intervals to maintain the same retention. During exam season, this is a real factor, so plan accordingly.

Rating Accuracy

The algorithm only works if your self-ratings are honest. Tapping "Easy" on a card you actually hesitated on gives the algorithm bad data, leading to intervals that are too long and surprise failures weeks later. When in doubt, rate conservatively.

Active Recall Quality

Genuinely retrieving the answer from memory before flipping the card produces a stronger memory trace than passively recognising it. If you find yourself glancing at the answer before committing to your response, the interval the algorithm assigns will be longer than your actual retention warrants.

What to Do When You Miss a Day (or a Week)

Missed reviews are the number one reason people abandon a spaced repetition schedule, and they almost never need to be. The memories do not vanish when a review date passes; they decay a little further down the forgetting curve, and the fix is a shorter next interval, not starting over. Here is the recovery playbook:

1. After one missed day: Just do your due reviews as normal. A single day of extra decay is trivial for any card past its first interval, and recalling a slightly overdue card successfully is a stronger workout for the memory, which the algorithm rewards with a longer next interval.
2. After several missed days: Pause new cards completely and clear the backlog first. Overdue cards represent knowledge that is actively fading; new cards just make the pile deeper.
3. Work through the backlog in normal-length sessions: Two or three days of 20 to 30 minute sessions beats one heroic 2-hour purge that burns you out. Expect to fail more cards than usual. That is the algorithm recalibrating, not evidence the system broke.
4. After weeks away: Consider trimming before grinding. Suspend or delete cards that no longer matter, then clear what remains. A 500-card backlog of which 200 are still relevant is a 200-card problem.
5. Never reset the whole deck: Even badly overdue cards retain most of their history. Relearning from their existing state is far faster than starting from zero.

If your backlog came from overload rather than a holiday, the deeper fix is reducing your new-card rate; our guide on what to do when you have too many flashcard reviews covers that in detail.

Common Scheduling Mistakes

Starting with Too Many New Cards

Adding 30 new cards on Day 1 feels great. By Day 10, you owe 300+ reviews and the whole thing collapses. Start with 10 or fewer and ramp up gradually.

Reviewing at Random Times

Consistency matters. Reviewing at roughly the same time each day builds the habit and gives the algorithm predictable data. Morning reviews work well for most people because you are alert and the reviews are done before the day gets busy.

Overriding the Algorithm

If the app says a card is not due for 18 days, trust it. Reviewing cards early gives minimal benefit and crowds out cards that actually need attention. The algorithm has seen your entire history with the card; your intuition has not.

Treating a Missed Day as Failure

The schedule is a tool, not a streak to protect. People who quit after breaking a streak lose months of accumulated intervals over a two-day gap. Follow the recovery steps above and keep going.

How to Start Today

You do not need to understand the maths. Here is the fastest way to start:

1. Pick your tool. An app like Sticky handles all interval calculations automatically. If you prefer paper, use the Leitner system with five boxes.
2. Create 10 cards. Focus on the material that matters most: upcoming exams, core vocabulary, key concepts. Follow the principles in our guide on creating effective flashcards.
3. Review tomorrow. Then the next day. Then the next. The algorithm handles the scheduling. Your only job is to show up and recall honestly.
4. Trust the gaps. When a card disappears for two weeks, that is the system working. It will come back right when you need it.
5. Adjust your pace. If reviews feel overwhelming, add fewer new cards. If they feel too easy, add more. The right pace is the one you can sustain for months, not the one that feels impressive for a week.

Properly spaced intervals are the most efficient path from "I just learned this" to "I will remember this next year." The schedule does the heavy lifting. You just need to show up. For the science behind the schedule, browse the rest of our learning guides.