Deck board spacing: setting a metric spacer to a fractional inch gap
I bought a set of metric decking spacers for a project that specified a 3/16-inch gap, and 'close enough' spacers left the boards too tight — by midsummer they'd swollen and buckled against each other.
We write the way we argue on the bench: exact factor, one sharp trap, one worked example you can re-check.
I bought a set of metric decking spacers for a project that specified a 3/16-inch gap, and 'close enough' spacers left the boards too tight — by midsummer they'd swollen and buckled against each other.
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
Rounding a fractional inch spacing target to a nearby round millimeter number instead of the exact conversion — 3/16 in is 4.76 mm, and rounding down to 4 mm leaves too little room for board expansion.
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 (gap spacing).
- 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
Your decking manufacturer recommends a 3/16 inch gap between boards, but your spacer set is sold in millimeters.
0.1875 in (gap spacing) × 25.4 = 4.7625 mm
That's 4.7625 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 (gap spacing) | 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
Deck boards expand and contract with humidity — too tight a gap causes buckling or cupping, while too wide a gap is both a trip hazard and lets debris build up underneath.
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, not a rounded guess — 3/16 in is 4.76 mm, so reach for a 5 mm spacer if you can't find an exact match.
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 (gap spacing) 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 spacing target to a nearby round millimeter number instead of the exact conversion — 3/16 in is 4.76 mm, and rounding down to 4 mm leaves too little room for board expansion.
This article was written by The Turbo Unit Converter engineering desk (Applied units & field metrology) and last reviewed on 2026-07-18 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.