What Are the Optimal Spaced Repetition Intervals?

You've probably seen recommendations to review after 1 day, 7 days, 16 days, and 35 days. But cognitive science suggests there isn't a single sequence of intervals that's optimal for everyone — or even for every piece of information.

In this article, I'll explain why the optimal spacing varies from person to person, what factors determine the best review interval, and how you can find the spacing that works best for your own learning.

The Intervals Most People Use (1, 7, 16, 35 Days)

The 1, 7, 16, 35 day sequence — and variations of it — comes from Hermann Ebbinghaus's forgetting curve, research from the 1880s showing that memory decays predictably over time. The idea is that if you review information just before you forget it, the memory strengthens and lasts longer each time.

Many spaced repetition apps hardcode intervals close to these numbers as a starting point. And they work — better than reviewing randomly or not at all. But they're averages. They assume every piece of information fades at the same rate for every person, which isn't how memory actually works.

Why These Intervals Aren't Optimal for Everyone

Every time you learn new information, your brain forms a memory trace — but not all memory traces are equally strong. The strength of that initial memory depends on how deeply you processed the information, how much attention you paid, how well it connected to your existing knowledge, and even the type of material you were learning.

Because some memories start out stronger than others, they fade at different rates. A concept you understand deeply may stay in memory for weeks, while a random fact might become difficult to recall after only a few days. A fixed sequence of intervals can't account for that variation — it treats every card the same.

What Actually Determines Your Ideal Interval

The goal of spaced repetition is to review information when recalling it is challenging but still successful — what psychologists call a desirable difficulty. Review too early, and the memory is still strong — the retrieval is too easy to do much good. Review too late, and you've forgotten too much and retrieval may fail entirely.

The ideal moment sits right in between. But there's no way to know exactly when that moment will occur. Your memory changes after every review, and every piece of information is different. That's why the optimal interval isn't a fixed number of days — it's the interval that adapts to your memory over time.

How to Find the Right Schedule for You

You could try to estimate the optimal review interval yourself, but in reality that's almost impossible. Every memory fades at a different rate, and that rate changes after every successful review.

The best spaced repetition systems solve this with an algorithm. Instead of a fixed schedule, they estimate the strength of each memory and adjust the next review based on how well you recalled it. After each review, you rate how difficult the recall was — Again, Hard, Good, or Easy — and the system uses that signal to decide when to show the card again.

Over time, every card develops its own schedule. Easy cards get longer intervals; cards you're close to forgetting get reviewed sooner. Instead of 1, 7, 16, 35 days for everything — it's the right interval for each memory, updated every time you review it.

If you want a system that works this way, Neuraloop is built around exactly this approach — available on iOS and Android.

My Experience With Various Intervals

When I first started using spaced repetition, I used fixed review intervals of 1, 4, 14, and 30 days. They worked much better than reviewing at random because they gave me a consistent review schedule.

Over time, however, I noticed a pattern. Even when I reviewed a card several days later than planned, I often still remembered it. That suggested many of my scheduled reviews were happening earlier than necessary, meaning I was spending time reviewing material I already knew well.

I later switched to Neuraloop, which uses the FSRS spaced repetition algorithm. Instead of assigning the same intervals to every card, FSRS estimates the optimal review time for each card based on my previous performance. In my experience, I needed fewer reviews while maintaining a similar level of retention.