What this converter does
This converter turns US ASTM inch sieve sizes into millimetres and back, so an inch-based grading spec reads straight across to the ISO metric series. A sieve aperture is a physical width, so the match is exact. Type a size and read it as you type.
One inch equals 25.4 mm exactly. For the hardened mix instead of the aggregate, see the concrete strength converter or the density converter.
The units it covers
Sieve apertures are quoted in millimetres almost everywhere, and in inches (or numbered mesh) under US ASTM practice — the two scales convert by a fixed 25.4.
View all units & their values
| Unit | Symbol | Value | Mainly used |
|---|---|---|---|
| Millimetre | mm | 1 | ISO/metric sieve designation |
| Inch | in | 25.4 | US ASTM coarse sieves |
| Micrometre | µm | 0.001 | Fine sieves, filler grading |
| Centimetre | cm | 10 | Occasional coarse sizing |
The formula
Every sieve size converts through the millimetre with a fixed factor:
result = value × factor_from ÷ factor_toWhere:
- value = the sieve size you typed
- factor_from = the “from” unit’s value in millimetres
- factor_to = the “to” unit’s value in millimetres
Worked example
Convert a 3/4 in sieve (0.75 in) to millimetres.
1 in = 25.4 mm0.75 × 25.4 = 19.05 mmSo the 3/4 in sieve is 19 mm — a standard coarse-aggregate size.
The units in this example
The US customary sieve size for coarse aggregate. Below about 4.75 mm, ASTM switches to numbered mesh rather than inch fractions.
- 1 in = 25.4 mm
- 3/8 in = 9.5 mm
- 3/4 in = 19.0 mm
- 1 in = 25,400 µm
The metric sieve aperture, used in ISO and most national aggregate standards. Fine fillers are quoted in micrometres (1 mm = 1000 µm).
- 1 mm = 0.03937 in
- 4.75 mm = No. 4 sieve
- 0.075 mm = 75 µm (No. 200)
- 1 mm = 1,000 µm