Specific Impulse Calculator
Enter a thrust and a propellant mass flow rate to get the specific impulse in seconds — the standard measure of how efficiently a rocket engine turns propellant into thrust.
Efficiency in one number
Enter the thrust in newtons and the mass flow rate in kilograms per second and the calculator returns the specific impulse (Isp) in seconds.
Use SI units
Thrust in newtons and mass flow rate in kilograms per second give Isp in seconds, using standard gravity g0 = 9.80665 m/s².
What is specific impulse?
The fuel economy of a rocket
This specific impulse calculator answers a single question: how efficiently does a rocket engine turn propellant into thrust? Specific impulse, written Isp, is the rocket equivalent of miles per gallon. It tells you how much thrust an engine produces for each unit of propellant weight it burns every second, and it is expressed in seconds so that engineers using metric or imperial units arrive at the same figure. The calculator takes two measurements — the thrust in newtons and the propellant mass flow rate in kilograms per second — and returns the Isp using standard gravity g0 = 9.80665 m/s².
Enter a thrust in newtons and a mass flow rate in kilograms per second to get the specific impulse in seconds instantly.
Specific impulse is the thrust divided by the weight flow rate of propellant — the mass flow rate multiplied by standard gravity.
Isp = F / (ṁ × g0)Here F is the thrust in newtons, ṁ is the propellant mass flow rate in kilograms per second, and g0 is standard gravity, 9.80665 m/s². Dividing by g0 converts the mass flow into a weight flow, which cancels the units of thrust and leaves an answer in seconds. A higher number means the engine extracts more thrust from each kilogram of propellant burned.
Suppose an engine produces 2,000,000 N (2 MN) of thrust while burning 700 kg of propellant every second.
Find the weight flow rate
700 × 9.80665 = 6,864.66 N/s — the propellant weight burned each second.
Divide thrust by weight flow
2,000,000 ÷ 6,864.66 = 291.35 — thrust per unit of weight flow.
Read the result
The specific impulse is about 291.35 s, typical of a kerosene–oxygen engine.
A higher specific impulse means a more efficient engine: it produces the same thrust while burning less propellant each second, which is exactly what you want when every kilogram you carry to orbit is expensive. The figure also sorts engines neatly by type. Solid rocket boosters sit around 250 s, kerosene–oxygen engines land near 300 s, and hydrogen–oxygen upper stages reach roughly 450 s — the practical ceiling for chemical propulsion. Electric and ion thrusters are in a different league entirely, with specific impulses above 3,000 s, because they accelerate a tiny mass of propellant to enormous speed; they trade thrust for efficiency, producing only gentle pushes over long durations. So the number you get tells you not just how good an engine is, but what kind of engine it is and what missions it suits — a high-thrust, modest-Isp engine for lifting off a planet, or a low-thrust, high-Isp thruster for the patient work of deep space.
The formula is exact, but a couple of practical points are worth keeping in mind.
Standard gravity and steady-state operation
This calculator uses the conventional standard gravity g0 = 9.80665 m/s² to express Isp in seconds, which is the universal convention and does not depend on the local gravity where the engine actually fires. It also assumes steady-state operation — a constant thrust and a constant mass flow rate. Real engines vary with throttle setting and ambient pressure, so vacuum and sea-level specific impulse differ for the same engine; quote the value that matches the conditions you care about.