Okun's Law
Empirical link between unemployment and the output gap.
Each 1 percentage-point rise in unemployment above its natural rate is associated with roughly a 2% fall in real GDP below potential. It is a rule of thumb, not an exact law, but useful for translating labour-market data into growth estimates.
Formula / theory
(Y − Y*)/Y* ≈ −c · (u − u*) with c ≈ 2