Drywall screw length: matching an inch spec to a metric box on the shelf
I moved to a country that sells everything in metric and stood in the fastener aisle for ten minutes trying to figure out which box matched the '1-1/4 inch' screws my American drywall guide called for.
We write the way we argue on the bench: exact factor, one sharp trap, one worked example you can re-check.
I moved to a country that sells everything in metric and stood in the fastener aisle for ten minutes trying to figure out which box matched the '1-1/4 inch' screws my American drywall guide called for.
Here is the conversion I wish I had taped to the fridge before that went sideways.
The factor that settles arguments
This is the relationship our converters use. Screenshot it if you have to — just stop re-deriving it from a half-remembered blog post.
1 in = 25.4 mm
Write that line once at the top of your notes. Every later arithmetic step should point back to it instead of inventing a friendlier number mid-stream.
The mistake that keeps showing up
Rounding a fractional inch screw length to the nearest whole number in mm instead of converting the actual fraction — 1-1/4 in is 31.75 mm, not a round 30 or 32.
I still write the unit next to every intermediate value. It looks pedantic until it catches the one swap that would have shipped wrong.
- Write down the known value in in.
- Confirm the target unit is really mm, not a similar-looking one.
- Apply the factor: 1 in = 25.4 mm.
- Round only at the end, to the precision your tool or spec supports.
Worked example
A drywall install guide calls for 1-1/4 inch screws, but the hardware store only stocks metric sizes.
1.25 in × 25.4 = 31.75 mm
That's 31.75 mm — round only to the precision your tool or spec actually needs.
I usually convert once, write the answer with units, then convert back. That two-step habit has saved more weekends than any fancy app.
Quick reference table
| in | Calculation | mm |
|---|---|---|
| 1 | 1 × 25.4 | 25.4 |
| 2 | 2 × 25.4 | 50.8 |
| 5 | 5 × 25.4 | 127 |
| 10 | 10 × 25.4 | 254 |
Use the table for speed once you trust the factor. Do not use it as a substitute for understanding which definition of the unit you are on.
Why the exact number matters
Screw length determines how deep you bite into the stud behind the drywall — too short and the board isn't secured, too long and you risk hitting wiring or plumbing.
Precision is not about showing off decimals. It is about making sure the shopping list, the machine setting, and the acceptance check are all describing the same physical quantity.
Pocket recap
Multiply the inch length by 25.4 to get mm, and always convert the actual fraction, not a rounded version of it.
Keep the converter link nearby for the next time the same debate shows up in a group chat. Re-typing the factor from memory is how the wrong cousin of a unit sneaks back in.
Try the converters mentioned in this article
FAQ
What is the exact in to mm factor here?+
1 in = 25.4 mm
Should I round partway through the calculation?+
No — carry full precision through the multiplication and round only the final answer, to the precision your tool, gauge, or spec actually supports.
What's the single biggest mistake people make with this conversion?+
Rounding a fractional inch screw length to the nearest whole number in mm instead of converting the actual fraction — 1-1/4 in is 31.75 mm, not a round 30 or 32.
This article was written by The Turbo Unit Converter engineering desk (Applied units & field metrology) and last reviewed on 2026-07-22 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.