Given one segment, find the complementary length so the ratio matches φ (long/short ≈ 1.618).
φ = (1 + √5) ÷ 2. If you enter the longer side, the shorter is long ÷ φ; if you enter the shorter side, the longer is short × φ. Input length must be positive; both results share the same unit you type.
This calculator uses plain decimal numbers. If a result looks wrong, check whether you used a comma instead of a dot (or vice versa) where the field expects one.