Coefficient of thermal expansion units — ppm/°C, ppm/°F and 10⁻⁶/K
Steel's CTE is ~12 × 10⁻⁶/K. Invar's is 1.2 × 10⁻⁶/K — ten times lower. Precision optics need ppb-level control across a ±5 °C window.
Coefficient of thermal expansion controls how much a part grows per degree. The units mix SI and imperial degree sizes freely, and the ratio matters when you copy a spec from a European datasheet to a US drawing.
The exact conversions
1 ppm/°C = 1 × 10⁻⁶/K = 1 µm/(m·°C) > 1 ppm/°F = 5/9 ppm/°C ≈ 0.556 ppm/°C > 1 ppm/°C = 9/5 ppm/°F = 1.8 ppm/°F
Common values
Invar 36: 1.2 ppm/°C. Fused silica: 0.55 ppm/°C. Borosilicate glass: 3.3 ppm/°C. Steel: 12 ppm/°C. Aluminium: 23 ppm/°C. Polycarbonate: 65 ppm/°C.
Worked example — telescope mirror mount
A 300 mm steel mount holding a Zerodur mirror (0.05 ppm/°C) contracts 3.6 µm across a 1 °C swing. Zerodur contracts 0.015 µm — a 240× mismatch that flexes the mirror surface out of λ/10 in a single night's temperature drop.
Recap
Multiply ppm/°F by 1.8 to get ppm/°C. Match materials by CTE, not by strength.
Try the converters mentioned in this article
FAQ
Is 'in/in/°F' the same as ppm/°F?+
Yes — in/in/°F is a strain-per-degree ratio, dimensionally identical to ppm/°F.
Does CTE change with temperature?+
Yes — datasheets quote average CTE over a stated range (often 20–100 °C). At cryogenic or high temperatures the value can shift by 30–50%.
This article was written by The Turbo Unit Converter engineering desk (Applied metrology & engineering units) and last reviewed on 2026-07-07 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.