Grain Size Calculator
Enter the grain diameter in millimetres, micrometres or inches. The calculator gives the Wentworth grain class, the phi value, the class range and the rock that grains of that size form.
- Grain class
- Phi
- Rock type
Phi is minus the base 2 logarithm of the diameter in mm.
Sediment is graded by grain diameter. A 0.3 mm grain is medium sand with a phi value of 1.74. Grains finer than 1/16 mm are silt, finer than 1/256 mm clay, and over 2 mm gravel.
Wentworth grain size chart
Udden-Wentworth. Search for a class.
| Class | Size (mm) | Phi |
|---|---|---|
| Boulder | Over 256 | Under −8 |
| Cobble | 64 to 256 | −6 to −8 |
| Pebble | 4 to 64 | −2 to −6 |
| Granule | 2 to 4 | −1 to −2 |
| Very coarse sand | 1 to 2 | 0 to −1 |
| Coarse sand | 1/2 to 1 | 1 to 0 |
| Medium sand | 1/4 to 1/2 | 2 to 1 |
| Fine sand | 1/8 to 1/4 | 3 to 2 |
| Very fine sand | 1/16 to 1/8 | 4 to 3 |
| Silt | 1/256 to 1/16 | 8 to 4 |
| Clay | Under 1/256 | Over 8 |
Millimetres.
Phi and class
Wentworth scale. Slide the phi value.
The arrow moves along the scale.
How to classify a grain
Size to class. Step through it.
Measure
Diameter.
Sieve coarse sediment and use settling for silt and clay.
Sand, silt or clay
Fine sediment. Compare them.
1/16 to 2 mm.
1/256 to 1/16 mm.
Under 1/256 mm.
Rub a pinch between your fingers: sand is gritty, silt smooth, clay sticky.
Before you sieve
Sample checks. Tick each item.
Tick each item as you go.
Dry the sample before sieving for accurate weights.
What you get
Grain sizing. Each tile explains one result.
Wentworth.
Value.
mm.
Formed.
Field test.
Each Wentworth class is twice the size of the one below.
Frequently Asked Questions
No question matches that search.
What grain size is sand?
Between one sixteenth of a millimetre and two millimetres.
What is the phi scale?
Minus the base two logarithm of the grain diameter in millimetres.
What size is silt?
From one two hundred and fifty sixth to one sixteenth of a millimetre.
How big is a pebble?
From four to sixty four millimetres.
What is the Wentworth scale?
The standard set of sediment grain size classes.
What rock does sand make?
Sandstone.
How do I measure grain size?
Sieve coarse grains, use settling or laser methods for fine ones.
Name every grain
Enter a size and see where it sits from clay to boulder, with its phi value and the rock it makes.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.