PoE2 Quality Rounding: Round Down, Not Nearest
PoE2 quality rounding goes down, not to the nearest whole number. On real items, "+3 to Level of all Spell Skills" shows exactly +4 at 34% Quality and stays +3 at 33%. ExileDesk once switched this to normal rounding, found it didn't match the game, and switched back.
Zundamon
Do Quality-boosted numbers round to the nearest whole number?
Metan
We thought so too, and switched to that once. Honestly, this is a story about our own mistake.
Zundamon
You got it wrong!? What happened?
Metan
The owner lined up real in-game items, and one number was off. Something we said would be +4 at 33% Quality was still +3.
Does Quality Round to Nearest or Down?
Down. Quality multiplies the mod's base value by (1 + Quality) and drops the fraction when the value is displayed.
Zundamon
So what happens to the fraction after multiplying?
Metan
It's always dropped. Here's "+3 to Level of all Spell Skills" with Quality (Caster Modifiers).
| Quality | Multiplied | Round down (the game) | Round to nearest (ExileDesk v0.1.526) | What happens |
|---|---|---|---|---|
| 20% | 3 × 1.2 = 3.6 | +3 | +4 | Nearest rounding already jumps to +4 |
| 33% | 3 × 1.33 = 3.99 | +3 | +4 | Just short of 4, so the game stays at +3 |
| 34% | 3 × 1.34 = 4.02 | +4 | +4 | Past 4, so both show +4 |
Zundamon
Still +3 at 3.99! That's basically 4.
Metan
Right. Real items showed +3 at 33% and +4 at 34%. Nearest rounding gives +4 at both 20% and 33%, which doesn't match.
Metan
Only rounding down matches the game.
Why Did We Switch to Rounding in the First Place?
Here's the history, version by version.
Zundamon
Was it rounding to nearest from the start?
Metan
No. It was right at first, and we went out of our way to change it.
| Version | Rounding | What we did |
|---|---|---|
| v0.1.474 | Down | Added a Quality type selector (Caster and others) for Jewellery Quality. The note said "Caster at 34% turns +3 Spell Skills into +4" |
| v0.1.526 | To nearest | While checking against CoE, changed quality-boosted values to round to nearest. The note said "previously truncated, same as Craft of Exile" |
| v0.1.567 | Down | Didn't match the owner's items (+4 at 34%, +3 at 33%), so we went back to rounding down |
Zundamon
Why switch to rounding to nearest?
Metan
CoE rounds to nearest. We matched an outside source, so we assumed it had to be right.
How Did We Catch It?
Items right at the threshold settled it.
Zundamon
Where do you look to tell which rounding it is?
Metan
With rounding down, a +3 mod first reaches +4 at 34%. With rounding to nearest, it gets there at a lower Quality.
Metan
So checking whether 33% shows +3 or +4 gives a clear answer.
Zundamon
And when the owner checked, 33% was +3.
Metan
Yes, and exactly +4 at 34%. It was rounding down.
Zundamon
So we can't trust CoE anymore?
Metan
We still use it to check probabilities. The lesson is that its fine display rounding isn't automatically right.
Metan
Of the rules from our CoE comparison on 10-16, only quality rounding was overridden by the owner's check on 10-18.
Not yet verified (1)
- The in-game check covers only a +3 skill level mod on jewellery at 33% and 34%. We haven't compared other mods, or mods with larger numbers, against real items one by one. The engine assumes they round down the same way and treats them all alike.
How Is This Different from Sanctified Rounding?
Sanctified items are the confusing part. There are two roundings, and they go in different directions.
Zundamon
Wait, doesn't Sanctified round to nearest? I'm getting mixed up.
Metan
Sanctified multiplies the base value by 0.78 to 1.22 and rounds to nearest. In the calculation, Quality applies later, at display, and rounds down.
| Boost | Applied to | Rounding | Order |
|---|---|---|---|
| Sanctified | Base value | To nearest | First |
| Quality | At display | Down | After |
Metan
That combination is why a 50% Quality Absent Amulet never shows "+5 to Level of all Spell Skills."
Metan
The table for each multiplier is in Sanctify vs Quality Order. The rounding down here is the same rule used there.
Zundamon
How much Quality should I aim for?
Metan
For small numbers, missing the threshold by even 1% means the display won't go up. To turn +3 into +4, you need 34%, not 33%.
What Changed in the Emulator?
Since v0.1.567, ExileDesk's emulator rounds down like the game.
| Quality | v0.1.526 to 566 (to nearest) | From v0.1.567 (down) | What changed |
|---|---|---|---|
| 17 to 33% | +4 | +3 | It showed one too high; now it stays +3 |
| 20% (normal cap) | +4 | +3 | Even at the normal cap, a +3 skill level doesn't move |
| 34% | +4 | +4 | No change |
Zundamon
Did the display for Sanctified items change too?
Metan
Yes. With rounding down, it now matches the tables in Sanctify vs Quality Order.
Quality-boosted numbers round down, so +3 first becomes +4 at 34%. When you line up with an outside tool, checking real items at the threshold is the fastest way to be sure.
Where the Numbers Come From
Show sources (4)
- +4 at 34%, +3 at 33%: checked by the owner on real in-game items (game).
- The rounding history: src/data/changelog-notes.json, v0.1.474 (added, rounding down), v0.1.526 (to nearest, same as CoE), v0.1.567 (back to rounding down).
- CoE rounding to nearest (Math.round): our 10-16 CoE comparison record (poe2db / CoE).
- The table's multiplications (3.6, 3.99, 4.02) apply the rule above directly (the same formula as the engine's quality.ts).
Try It Yourself
On the crafting bench, set Quality (Caster Modifiers) to 33% and then 34% and watch "+3 to Level of all Spell Skills" go from +3 to +4.
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VOICEVOX: Zundamon / Shikoku Metan
Character art: Shikoku Metan / Zundamon standing art by Sakamoto Ahiru