Coefficient of linear thermal expansion for metals, polymers and ceramics — with the formula for calculating actual growth over a temperature change.
Data verified 2026-09-29 · based on n/a — physical property data, no governing revision
Quick Answer
The coefficient of thermal expansion is how much a material grows per degree, in µm/m·°C (numerically identical to the older µin/in·°F × 0.556). Aluminium at 23.6 expands twice as much as steel at 11.7 — which is why aluminium pistons need more clearance than cast iron ones.
Values are average coefficients over roughly 20–100 °C. The coefficient itself varies with temperature, so for a wide temperature span the instantaneous value differs from the average. Polymers show the largest values and the strongest temperature dependence.
The Formula
ΔL = α × L × ΔT
where ΔL is the change in length, α is the coefficient, L is the original length and ΔT is the temperature change.
Worked examples
1 m aluminium bar, +50 °C: 23.6 × 10⁻⁶ × 1000 mm × 50 = 1.18 mm
1 m steel bar, +50 °C: 11.7 × 10⁻⁶ × 1000 × 50 = 0.59 mm
1 m PVC pipe, +50 °C: 52 × 10⁻⁶ × 1000 × 50 = 2.60 mm
The unit µm/m·°C is convenient because it is numerically the same as parts per million per degree — so 23.6 means 23.6 mm of growth per kilometre per degree.
Why the Differences Matter
Differences in expansion, not the absolute values, cause most engineering problems:
Dissimilar materials bolted together — an aluminium housing with a steel shaft will change fit substantially over a temperature swing. At 50 °C the aluminium grows twice as far, which can close a clearance to zero.
Long runs — a 30 m steel pipe run at 80 °C grows about 25 mm, which is why pipework has expansion loops. PVC grows more than four times as much over the same span.
Precision at temperature — a measuring instrument accurate at 20 °C is not accurate at 40 °C unless its expansion is corrected for.
Low-expansion alloys — Invar 36 at 1.2 µm/m·°C expands twenty times less than steel, and Zerodur is lower still. They exist specifically to solve this class of problem.
Polymers are the outlier: their coefficients are several times those of metals and depend strongly on temperature and on moisture content, so a single figure is a rough guide.
Frequently Asked Questions
What is the coefficient of thermal expansion of steel?
About 11.7 µm/m·°C for plain carbon steel, roughly 6.5 µin/in·°F. Stainless steels are higher — about 17.3 for 304 — which is one reason welding stainless can distort more than welding carbon steel.
What is the thermal expansion of aluminium?
About 23.6 µm/m·°C for 6061 and 23.1 for pure aluminium — roughly twice that of steel. This is why aluminium components usually need greater running clearance and why mixed aluminium-steel assemblies change fit substantially with temperature.
How much does steel expand per degree?
About 11.7 µm per metre per degree Celsius. A 1 m bar grows roughly 0.012 mm per degree rise, so a 50 °C rise gives about 0.59 mm.
Which material has the lowest thermal expansion?
Zerodur, a glass-ceramic, at about 0.05 µm/m·°C — practically zero over normal ranges, which is why it is used for telescope mirrors and precision optics. Invar 36, at 1.2, is the low-expansion metal.
How do I calculate thermal expansion?
Multiply the coefficient by the original length and by the temperature change: ΔL = α × L × ΔT. Keep the units consistent — with α in µm/m·°C, express the length in metres and you get the answer in micrometres.
Does thermal expansion affect threaded fasteners?
Yes. A steel bolt in an aluminium housing loses preload as the assembly heats, because the aluminium expands more than the bolt and the joint grows away from it. Where temperature swings are large, that loss has to be accounted for in the preload specification.
Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.
#
Source
Type
Revision / method
[1]
Imperial conversion
derived
µm/m·°C × 0.5556 = µin/in·°F.
[2]
Standard engineering handbook values (ASM and comparable sources)
standard
n/a — compiled physical property data, not a revisioned specification — source
Data Sources
Standard
Revision
What it covers on this page
Compiled engineering handbook values
n/a — physical property data, no governing revision
coefficient of linear thermal expansion for all materials listed
Cross-checked against:
Values cross-checked against two independent handbook sources
Imperial column verified at build time against the metric column
Derived values — the following values on this page are calculated, not taken directly from the standard:
Value
How it is derived
Imperial CTE column
µm/m·°C × 0.5556.
Average coefficients over roughly 20–100 °C. The instantaneous coefficient varies with temperature, and polymer values depend strongly on temperature and moisture.
Accuracy and use. The values on this page are compiled from the published standards and cross-checked sources listed above. Where values are derived, the derivation is stated. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.
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