Anki Easy bonus, Hard interval and Interval modifier explained

These are three numbers in Anki's Deck Options, under Advanced. Anki multiplies by them to decide when a card comes back. They only affect cards you've already learned, and only with Anki's default scheduler (SM-2).

• Easy bonus (default 1.30): Easy gives about 1.3 times the interval that Good would.
• Hard interval (default 1.20): Hard gives 1.2 times the current interval.
• Interval modifier (default 1.00): scales every Hard, Good and Easy interval. At 1.00 it does nothing.

For most people the defaults are right. If you've turned on FSRS, none of the three are used.

What they do, with numbers

Take a card that's due every 10 days, with the starting ease of 250%, reviewed on the day it's due:

Each of those results is then multiplied by the Interval modifier, which is 1.00 by default. Anki also adds a little random variation, so your buttons can be a day or two off.

Some limits on top of that:

• Hard always gives at least one day more than the current interval (as long as Hard interval is above 1).
• Good always gives at least one day more than Hard, and Easy at least one day more than Good.
• No interval goes past the Maximum interval, which is 36,500 days (100 years) by default.

Ease is the number in the Good row. It changes when you press Again, Hard or Easy; ease factor explained covers how.

Easy bonus

Easy bonus is how much further Easy sends a card than Good. With the default 1.30, the 10-day card above goes to about 33 days instead of 25.

Easy also adds 15 points to the card's ease, so after that the card's gaps grow faster too.

When to change it: if cards you press Easy on still feel trivial when they come back, raise it, for example to 1.50 (the 10-day card would then go to about 38 days). If you often forget cards you pressed Easy on, lower it, or press Easy less. Easy is meant for answers that were instant and certain; when to press each button has more.

Hard interval

Hard interval sets how far a Hard answer moves the card. At the default 1.20, a 10-day card goes to 12 days.

Hard also takes 15 points off the card's ease. If you use Hard for answers that were right but slow, that loss adds up. Hard vs Again covers when to press which.

When to change it: most people shouldn't. Setting it to 1.00 makes Hard keep the card at its current interval. Below 1.00, Hard shortens the interval. Above 1.20, Hard and Good get closer together.

Interval modifier

Interval modifier stretches or shrinks every gap at once. At 0.80, the 10-day card would get 20 days for Good instead of 25, and about 26 for Easy instead of 33. Lower means more reviews and fewer forgotten cards. Higher means fewer reviews and more forgotten cards.

When to change it: only after you've checked how often you actually pass mature cards (the Answer Buttons graph in Stats). Small gains cost a lot: Anki's manual works out that going from 85% to 90% needs a modifier of 0.65, so every gap gets about a third shorter. Anki interval modifier has the formula and a table. If you mainly want fewer reviews, learning fewer new cards a day is the safer fix.

Where to find them

Deck Options, Advanced section: Starting ease, Easy bonus, Interval modifier, Hard interval and New interval are listed together, along with Maximum interval. They belong to an options preset, so every deck using that preset changes too. Cards in reviews pick up the new values at their next review.

With FSRS

FSRS doesn't use any of this. It estimates when you're likely to forget each card and schedules it before then, aiming for you to remember 90% of cards (the desired retention setting). Anki hides the SM-2 options such as Easy bonus when FSRS is on, and doesn't apply Hard interval or Interval modifier. To get more or fewer reviews with FSRS, change desired retention instead. SM-2 vs FSRS compares the two.

In Decks

Decks uses SM-2 with Anki's defaults: Easy bonus 1.30, Hard interval 1.20, and no interval modifier. None of these can be changed, and Decks doesn't offer FSRS.