When supply is unitary elastic , it means that the percentage change in quantity supplied is exactly equal to the percentage change in price.

In symbols, the price elasticity of supply (PES) equals 1.

So if price rises by 10%, the quantity supplied rises by 10%; if price falls by 5%, quantity supplied falls by 5%.

Economists describe this as supply being “proportionately responsive” to price: not more responsive (elastic), not less responsive (inelastic), but in equal proportion.