Specific Impulse Calculator

Calculate a rocket engine’s specific impulse (Isp), a standard measure of how efficiently it uses propellant.

Thrust

Propellant Mass Flow Rate

kg/s

What specific impulse measures

Specific impulse (Isp) measures how efficiently a rocket or jet engine converts propellant into thrust — a higher Isp means more thrust (or longer burn time) for the same amount of propellant.

The formula

Effective exhaust velocity V_e = F / ṁ, where F is thrust and ṁ is the propellant mass flow rate. Specific impulse expressed in seconds is Isp = V_e / g₀, where g₀ is standard gravitational acceleration (9.80665 m/s²), used as a conversion constant by convention even for engines operating in space.

This is the standard rocket propulsion formula for specific impulse, used across aerospace engineering references.

Last reviewed August 2026