Specific Impulse Calculator

About this calculator

Specific impulse (Isp) measures how efficiently a rocket engine converts propellant into thrust, expressed in seconds. It's calculated as Isp = F ÷ (ṁ × g₀), where F is thrust, ṁ is the propellant mass flow rate, and g₀ is standard gravity (9.80665 m/s²) — the same equation rearranges to give the effective exhaust velocity, vₑ = F ÷ ṁ, which is Isp multiplied by g₀.

Aerospace engineers use specific impulse as the primary metric for comparing rocket engine efficiency — a higher Isp means more thrust (or longer burn time) from the same amount of propellant, which is why ion engines (Isp in the thousands of seconds) are far more fuel-efficient than chemical rockets (Isp typically 250–450 seconds), even though chemical engines produce far more raw thrust. It's a core parameter in mission design, feeding directly into the Tsiolkovsky rocket equation used to calculate a vehicle's achievable velocity change.