Hinge mortise depth: cutting a fractional inch spec with a metric router jig
I set my metric depth-stop router jig to '2 mm' for a hinge mortise that actually called for 3/32 inch, assumed they were close, and ended up with a door that wouldn't sit flush in the frame.
We write the way we argue on the bench: exact factor, one sharp trap, one worked example you can re-check.
I set my metric depth-stop router jig to '2 mm' for a hinge mortise that actually called for 3/32 inch, assumed they were close, and ended up with a door that wouldn't sit flush in the frame.
The factor below is the one that ends the argument — write it once, then stop improvising mid-job.
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
Eyeballing a fractional inch mortise depth as 'close to' a round metric number instead of converting the exact fraction — 3/32 in is 2.38 mm, not 2 mm, and that 0.4 mm gap shows up as a visible door gap.
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 (mortise depth).
- 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 hinge spec calls for a 3/32 inch mortise depth, and your router's depth stop is calibrated in millimeters.
0.09375 in (mortise depth) × 25.4 = 2.38125 mm
That's 2.38125 mm — round only to the precision your tool or spec actually needs.
Flip the factor once as a sanity check. If you do not land near the starting number, you mixed definitions or inverted the direction.
Quick reference table
| in (mortise depth) | 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
Even a fraction of a millimeter of extra or missing mortise depth throws off how flush a door or cabinet face sits, which is very visible once installed.
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
Convert the exact fraction to a decimal first, then multiply by 25.4 — 3/32 in becomes 2.38 mm, not a rounded 2 or 2.5.
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 (mortise depth) 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?+
Eyeballing a fractional inch mortise depth as 'close to' a round metric number instead of converting the exact fraction — 3/32 in is 2.38 mm, not 2 mm, and that 0.4 mm gap shows up as a visible door gap.
This article was written by The Turbo Unit Converter engineering desk (Applied units & field metrology) and last reviewed on 2026-07-27 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.