# FSRS simulator underestimates the number of cards memorized

**URL:** <https://forums.ankiweb.net/t/fsrs-simulator-underestimates-the-number-of-cards-memorized/67162>\
**Category:** FSRS\
**Created:** [October 25, 2025, 11:50am UTC](https://forums.ankiweb.net/t/fsrs-simulator-underestimates-the-number-of-cards-memorized/67162 "2025-10-25T11:50:57Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![ZornHadNoChoice](https://sea2.discourse-cdn.com/flex002/user_avatar/forums.ankiweb.net/zornhadnochoice/32/28874_2.png) [@ZornHadNoChoice](https://forums.ankiweb.net/u/ZornHadNoChoice)\
**Post date:** [October 25, 2025, 11:50am UTC](https://forums.ankiweb.net/t/fsrs-simulator-underestimates-the-number-of-cards-memorized/67162/1 "2025-10-25T11:50:57Z")

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The FSRS simulator seems to fail at calculating the number of cards memorized with large decks.

### Steps to reproduce

1. Generate a deck of 6000 basic notes.
2. Make a default preset for that deck (with the exception of 9999 reviews limit and turning on FSRS).
3. Run the simulator.

### Expected behavior

After ~365 days one would expect to have memorized all 6000 cards at 20 cards per day.

### Actual behavior

The simulator estimates ~5700 cards memorized after about 300 days. Counterintuitively, increasing the days to simulate to 3650 makes the estimated number decrease to around 5680.

### Debug info

```auto
AnkiDroid Version = 2.23.0alpha5 (248e715ff437c50a407d71c119b1f5b735ee2034)  
Backend Version = 0.1.62-anki25.09.2 (25.09.2 3890e12c9e48c028c3f12aa58cb64bd9f8895e30)  
Android Version = 13 (SDK 33)  
ProductFlavor = full  
Device Info = Xiaomi | POCO | mondrian | mondrian_global | 23013PC75G | qcom  
Webview User Agent = Mozilla/5.0 (Linux; Android 13; 23013PC75G Build/TKQ1.220905.001; wv) AppleWebKit/537.36 (KHTML, like Gecko) Version/4.0 Chrome/140.0.7339.209 Mobile Safari/537.36  
ACRA UUID = b9d03d9a-5af9-4e67-987e-cc15799171e2  
FSRS = 5.1.0 (Enabled: true)  
Crash Reports Enabled = true

```

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<div class="post-metadata">

**Author:** ![Keks](https://sea2.discourse-cdn.com/flex002/user_avatar/forums.ankiweb.net/keks/32/20537_2.png) [@Keks](https://forums.ankiweb.net/u/Keks)\
**Post date:** [October 25, 2025, 12:41pm UTC](https://forums.ankiweb.net/t/fsrs-simulator-underestimates-the-number-of-cards-memorized/67162/2 "2025-10-25T12:41:40Z")

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> [@ZornHadNoChoice](#):
>
> ### Actual behavior
> 
> The simulator estimates ~5700 cards memorized after about 300 days. Counterintuitively, increasing the days to simulate to 3650 makes the estimated number decrease to around 5680.

`300*20*0.95=5700`  
0.95 - Average retention with 90% Desired retention. This figure is just a rough estimate.

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<div class="post-metadata">

**Author:** ![ZornHadNoChoice](https://sea2.discourse-cdn.com/flex002/user_avatar/forums.ankiweb.net/zornhadnochoice/32/28874_2.png) [@ZornHadNoChoice](https://forums.ankiweb.net/u/ZornHadNoChoice)\
**Post date:** [October 25, 2025, 12:56pm UTC](https://forums.ankiweb.net/t/fsrs-simulator-underestimates-the-number-of-cards-memorized/67162/3 "2025-10-25T12:56:03Z")

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I guess you’re right, but it’s not intuitive.

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**Author:** ![Danika\_Dakika](https://sea2.discourse-cdn.com/flex002/user_avatar/forums.ankiweb.net/danika_dakika/32/17815_2.png) [@Danika\_Dakika](https://forums.ankiweb.net/u/Danika_Dakika)\
**Post date:** [October 25, 2025, 5:14pm UTC](https://forums.ankiweb.net/t/fsrs-simulator-underestimates-the-number-of-cards-memorized/67162/4 "2025-10-25T17:14:58Z")

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> [@ZornHadNoChoice](#):
>
> Expected behavior  
> After ~365 days one would expect to have memorized all 6000 cards at 20 cards per day.

That would work if the metric were for how many cards you had “introduced” – but introducing them does not automatically mean that every card is “memorized” or known at any particular moment. It would be misleading for the simulator to claim more than Anki/FSRS can provide.

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**Author:** ![system](https://us1.discourse-cdn.com/flex002/uploads/anki2/original/1X/8f1279ababc5879d54e4838989f606cfe55af8c7.jpeg) [@system](https://forums.ankiweb.net/u/system)\
**Post date:** [November 24, 2025, 5:15pm UTC](https://forums.ankiweb.net/t/fsrs-simulator-underestimates-the-number-of-cards-memorized/67162/5 "2025-11-24T17:15:16Z")

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