Thermal Expansion Calculator
Calculate how much a length of steel, aluminium, concrete, glass or plastic grows or shrinks with a temperature change.
How much a beam, rail or pipe grows when it warms up.
How the thermal expansion calculator works
Solids expand almost linearly with temperature: ΔL = α × L × ΔT, where α is the coefficient of linear expansion. Steel grows about 12 millionths of its length per degree, so a 100 m bridge span moves 36 mm over a 30 °C swing — which is why expansion joints exist. Aluminium expands about twice as much, concrete about the same as steel (which is why reinforced concrete works), PVC four times more.
Areas expand at roughly 2α and volumes at 3α.
Formula: ΔL = α L ΔT
Worked examples
| Inputs | Change in length | Note |
|---|---|---|
| 10 m of steel, +30 °C | 3.6 mm | 3.6 mm |
| 10 m of aluminium, +30 °C | 6.9 mm | nearly twice as much |
| 100 m concrete bridge, +40 °C | 48 mm | 48 mm — hence expansion joints |
FAQFrequently asked questions
Why do bridges have expansion joints?
A 100 m steel or concrete span moves several centimeters between winter and summer. Without a joint the stress would crack the deck or buckle the rails.
Why does reinforced concrete not crack from heat?
Steel and concrete have nearly the same coefficient (about 12 × 10⁻⁶ /K), so they grow together.
How does area or volume expansion work?
For small changes, area grows by 2α and volume by 3α per degree. A liter of water expands differently — liquids have their own volumetric coefficients.
Why does a tight jar lid loosen under hot water?
The metal lid expands more than the glass jar (19 vs 9 × 10⁻⁶ /K), so it grows away from the thread.
Where these figures come from
- NIST — CODATA 2018 fundamental physical constants — G, g₀, R, c
- NIST Special Publication 811 — Guide for the use of the International System of Units — unit conversions
- The Engineering ToolBox — material properties — specific heats, expansion coefficients, densities
- National Institute of Standards and Technology — the US measurement authority
Last checked: September 2026. Constants are the CODATA 2018 values; formulas are the standard textbook forms.