Invar, also known generically as FeNi36 (64FeNi in the US), is a nickel–iron alloy notable for its uniquely low coefficient of thermal expansion (CTE or α) Like other nickel/iron compositions, Invar is a solid solution; that is, it is a single-phase alloy. In one commercial version it consists of approximately 36% nickel and 64% iron. Common grades of Invar have a coefficient of thermal expansion (denoted α, and measured between 20 °C and 100 °C) of about 1.2 × 10−6 K−1 (1.2 ppm/°C), while ordinary steels have values of around 11–15 ppm/°C. Extra-pure grades (<0.1% Co) can readily produce values as low as 0.62–0.65 ppm/°C.

Invar is used where high dimensional stability is required, such as precision instruments, clocks, seismic creep gauges, television shadow-mask frames, valves in engines and large aerostructure molds. One of its first applications was in watch balance wheels and pendulum rods for precision regulator clocks. At the time it was invented the pendulum clock was the world’s most precise timekeeper, and the limit to timekeeping accuracy was due to thermal variations in length of clock pendulums.