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Thermal Expansion Correction for Precision Measurement

Guide · Updated October 28, 2026

Every dimension on a drawing applies at 20 °C (68 °F), the reference temperature set by ISO 1. Parts, gages and machines all grow as they warm, and different materials grow at different rates. When the shop, the part or your hands are warmer than 68 °F, the reading drifts. For tolerances of a few tenths, that drift is often larger than the tolerance.

Soak parts and blocks together

Put the part, gage blocks and gage on the same plate to reach the same temperature. The free Gage Block Calculator builds the gage block stack for any size you need to set.

Step by step

Measure the temperature: Read the part temperature with a contact thermometer, not the room thermostat.
Find both coefficients: Look up α for the part and for the gage. Steel gages are about 6.4 µin/in/°F.
Calculate the difference: Correction = (α part − α gage) × length × (T − 68 °F).
Apply the sign: If the part expands more than the gage, subtract the correction from the reading to get the 68 °F size.
Or wait: Let the part soak until it is near 68 °F and measure again; this is better than correcting when possible.
Record it: Write the temperature next to critical readings so later checks can be compared.

Typical expansion coefficients

Approximate values near room temperature: carbon and alloy steel about 6.4 µin/in/°F (11.5 µm/m/°C); 6061 aluminum about 13 µin/in/°F (23.4 µm/m/°C); brass about 10.4 to 11.4 µin/in/°F; cast iron about 5.9 to 6.0 µin/in/°F; 300-series stainless about 9.6 µin/in/°F. Exact values vary by alloy, so use the material data sheet for critical work.

Worked example: aluminum part with a steel micrometer

A 4.0000 in 6061 aluminum part is measured at 80 °F with a steel micrometer that is also at 80 °F, and it reads 4.0004 in. The difference in growth rate is 13 − 6.4 = 6.6 µin/in/°F. Correction = 6.6 × 4 × 12 = 317 µin, about 0.0003 in. The part size at 68 °F is 4.0004 − 0.0003 = 4.0001 in. If the drawing allowed +0.0002 in, the uncorrected reading would have rejected a good part.

Same material, same temperature

If a steel part is measured with a steel gage and both are at the same temperature, the growth cancels and no correction is needed, even if both are at 80 °F. Trouble starts when the part is warm from machining and the gage is cool, or when the materials differ. A steel part fresh off the lathe at 95 °F, measured with a 68 °F gage, reads 6.4 × 2 × 27 ≈ 346 µin large on a 2 in diameter.

Handling heat

Holding a micrometer frame or a gage block in a bare hand warms it several degrees in minutes. Use the insulated pads, hold gage blocks with gloves or tongs, and set gages on the plate between readings. For large parts and tight tolerances, an hour or more of soak per inch of thickness is common practice.

Tips

Gage blocks: gage block thermal soak.
Mics: check a micrometer.
Shrink fits: shrink fitting uses the same coefficients.

FAQ

What is the reference temperature for measurement?

20 °C (68 °F), set by ISO 1.

How much does steel grow?

About 6.4 millionths of an inch per inch per °F.

How much does aluminum grow?

About 13 millionths per inch per °F, roughly twice steel.

Do I need to correct a steel part measured with a steel mic?

Not if both are at the same temperature; the growth cancels.

How do I correct a reading?

Subtract (α part − α gage) × length × (T − 68 °F).

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