ASTM Grain Size Calculator
ASTM E112 defines grain size number G using the relationship N = 2^(G-1), where N is the number of grains per square inch at 100x magnification. This calculator works in both directions: enter G to find the number of grains per in^2 at 100x and the estimated mean grain diameter, or enter the measured grain count to find G. For example, ASTM G = 7 gives N = 2^6 = 64 grains per in^2 at 100x, corresponding to a mean grain diameter of approximately 44 micrometers.
ASTM E112 grain size formula
N = 2^(G - 1) (grains per in^2 at 100x)
G = 1 + log2(N)
d (micrometers) = 25,400 / (sqrt(N) * 100)
N is the number of grains counted per square inch at 100x magnification. G is the ASTM grain size number. The grain diameter formula converts N to actual grain diameter in micrometers by accounting for the 100x magnification factor and converting inches to micrometers (1 in = 25,400 micrometers).
Reference values table
G = 1: 1 grain/in^2, ~635 micrometer diameter. G = 3: 4 grains/in^2, ~317 micrometer. G = 5: 16 grains/in^2, ~159 micrometer. G = 7: 64 grains/in^2, ~79 micrometer. G = 9: 256 grains/in^2, ~40 micrometer. G = 11: 1,024 grains/in^2, ~20 micrometer. G = 13: 4,096 grains/in^2, ~10 micrometer. Most structural steels fall in the G = 5 to 9 range; fine-grained steels targeted for toughness are often G = 7 to 11.
ASTM grain size: frequently asked questions
What is the ASTM grain size number?
The ASTM grain size number G (per ASTM E112) defines the number of grains per square inch at 100x magnification as N = 2^(G-1). Larger G numbers mean smaller, more numerous grains. G = 1 means about 1 grain per in^2 at 100x; G = 10 means about 512 grains per in^2.
How does grain size affect mechanical properties?
Finer grains (higher G number) generally produce higher yield strength, better toughness, and more uniform properties due to the Hall-Petch relationship (yield strength increases as 1/sqrt(grain diameter)). Coarser grains may improve high-temperature creep resistance and reduce fatigue initiation at boundaries.
How is ASTM grain size measured?
Per ASTM E112, grain size is measured on a polished, etched metallographic cross section. Methods include comparison to standard chart images, planimetric counting (counting grains in a known area), and the intercept method (counting grain boundary intersections along lines). The intercept method is most precise.
What is the relationship between ASTM G number and grain diameter?
The mean grain diameter d (in mm) is related to G by: d = 1 / sqrt(N * (1/645.16)) where N is grains per mm^2 at actual size (not 100x). Alternatively, for a quick estimate: d (micrometers) is approximately 2^((14-G)/2) micrometers, which gives about 225 micrometers for G = 5 and about 22 micrometers for G = 10.
Why is grain size important for heat treatment specifications?
Most heat treatment and welding procedure specifications include a required ASTM grain size range. Austenitic grain size of steel before quenching affects the final martensite packet size and therefore toughness. Coarsening of grains during annealing above a critical temperature (grain coarsening temperature) is a key process control parameter.
Official sources
- ASTM E112, "Standard Test Methods for Determining Average Grain Size": astm.org.
- ASM International, "ASM Handbook Vol. 9: Metallography and Microstructures": asminternational.org.
- NIST, Materials Measurement Laboratory: nist.gov/mml.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.