Specific Impulse Calculator
What a rocket engine’s specific impulse means in practice — its effective exhaust velocity, the propellant it burns each second for a given thrust, and how much a burn of a given length consumes.
Specific impulse is thrust per unit weight of propellant consumed per second, in seconds; multiply by standard gravity for the effective exhaust velocity, the number that goes into the rocket equation.
How the specific impulse calculator works
Specific impulse is thrust per unit weight of propellant consumed per second, in seconds; multiply by standard gravity for the effective exhaust velocity, the number that goes into the rocket equation. Thrust divided by exhaust velocity is the mass flow, and mass flow times burn time is the propellant used. Kerosene engines reach about 300 s, hydrogen 450 s, solids 250 s, ion thrusters 3,000 s or more at tiny thrust.
Formula: v_e = I_sp × g₀; ṁ = F ÷ v_e; propellant = ṁ × t
Worked examples
| Inputs | Effective exhaust velocity | Note |
|---|---|---|
| Merlin-class: 311 s, 845 kN, 162 s burn | 3,050 m/s | 3,050 m/s; 277 kg/s; 44.9 t |
| Hydrogen upper stage: 450 s | 4,413 m/s | 4,413 m/s |
| Ion thruster: 3,000 s, 0.0002 kN | 29,420 m/s | 29,420 m/s; 0.59 kg a day |
FAQFrequently asked questions
Why is specific impulse in seconds?
A historical convenience: dividing exhaust velocity by g₀ gives a figure that is the same in metric and imperial units. It is the time an engine could produce a thrust equal to the weight of its propellant.
Is higher always better?
For propellant economy, yes — but ion engines with 3,000 s give newtons of thrust, not meganewtons, so they cannot launch anything. Launch stages trade impulse for thrust; deep-space stages do the reverse.
Sea level or vacuum?
Impulse rises in vacuum because the exhaust expands fully: a kerosene first-stage engine might be 280 s at sea level and 310 s in vacuum. Use the figure for the conditions of the burn.
How does this feed the rocket equation?
Exhaust velocity is the multiplier in Δv = v_e ln(m₀ ÷ m₁). A stage with 4,400 m/s exhaust and a mass ratio of 8 delivers about 9,150 m/s — roughly orbit.
Where these figures come from
- NIST — CODATA 2018 fundamental physical constants — G and the speed of light
- IAU 2015 Resolution B3 — nominal solar and planetary conversion constants — the astronomical unit, solar mass and planetary radii
- NASA — the US space agency
Last checked: September 2026. Constants are CODATA 2018 (G, c) and IAU 2015 nominal values (solar and planetary parameters).