The maths of forgetting

The maths of forgetting, and how to remember
I forget...

There is something we need to remember. It's not because this is going to be a shock. It's because, as a teacher, I do my best to help my students, but I am not sure all my students are helping themselves. I think that it is because we forget how much we forget. And if you remember how much you forget, you might feel like doing a little more review!

The forgetting function

We forget fast. In fact, we generally forget exponentially.

For maths nerds, how much you remember (R) is a function of time (t) and the strength of the memory (S):


The more time that passes, and the less powerful your memory, the less you remember.

No surprises there!

In pictures, that looks like this:

In less than one day, you have forgotten half the stuff you learned. After 5 days, it's almost all gone!

This is a very simple picture. Of course, there are other factors at play. If you are stressed out and tired, you'll forget faster. Things that are difficult are harder to remember. And memory is a complex thing. You can remember somethings very easily, and others never.

But let's assume you have a great memory. If you have an incredible memory, and it is five times better than average, you still forget more than half of the stuff you learn in only 5 days.

And after 10 days, you can only remember 10% of what you learned.

What should you do?

From this simple model, there are two things we can control: time and the strength of the memory.

The most important thing is to reduce the time between learning sessions. That means you need to review within 24 hours of your class—the sooner the better. That's also why learning one day a week is not enough. You need to take lessons through the week.

You should also make sure you are developing strong memories when you learn:

  • take your time to understand the content of your lesson;
  • use memory techniques to make things stick in your mind easier.

A good teacher will make you do this in class. They will go slowly and not rush. They will repeat things, make them interesting and help you understand and make connections between ideas. This gives you ways to make your memory strong. 

For other tips on improving your memory, check out the key to remember anything

If you want to remember what you learn, don't forget how much you are forgetting!