What this converter does
This converter turns aggregate sieve apertures in millimetres into inches and back. It lines up ISO metric sieve designations with the US ASTM inch series so you can read a grading curve or spec in either system. Type a size and read the match as you type.
A sieve size is a physical aperture, so the conversion is exact: one inch is 25.4 mm. For the finished mix side, 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 4.75 mm sieve (the classic No. 4) to inches.
1 in = 25.4 mm4.75 ÷ 25.4 = 0.187 inSo the 4.75 mm sieve is about 0.187 in — the sand/gravel split point.
The units in this example
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
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