Specific Impulse Calculator
Enter your engine thrust and mass flow rate to get specific impulse (Isp) and exhaust velocity instantly. Or choose a propellant preset to load a known Isp and explore forward or reverse calculations. The Tsiolkovsky delta-v panel shows how much velocity change your rocket can achieve for any wet/dry mass ratio. Switch between SI and imperial units at any time.
What is specific impulse?
Specific impulse (Isp) is the standard measure of rocket engine efficiency. It tells you how many seconds a given weight of propellant can produce one unit of thrust equal to its own weight. A higher Isp means more thrust for each kilogram of propellant burned, so the rocket needs to carry less fuel to reach its destination. The formula is Isp = F / (m-dot x g0), where F is thrust, m-dot is the propellant mass flow rate, and g0 is standard gravity (9.80665 m/s^2). Because g0 cancels the units of weight, Isp ends up in seconds, and those seconds are identical whether you use SI or imperial units, making Isp a universal benchmark for comparing engines across all countries and eras.
How to use this calculator
Choose what you want to solve for using the "Solve for" dropdown: Isp from thrust and mass flow, thrust from Isp and flow, mass flow from Isp and thrust, or Isp directly from exhaust velocity. Select a propellant preset to load a known Isp value, or choose Custom and enter your own figures. The Tsiolkovsky delta-V section requires wet mass (rocket fully fuelled) and dry mass (empty tank and structure). The chart shows how delta-V varies across the full range of mass ratios at your current Isp, so you can see the diminishing returns of adding more propellant.
Effective exhaust velocity and the Tsiolkovsky rocket equation
Exhaust velocity (Ve) and Isp are directly related: Ve = Isp x g0. An engine with Isp 311 s has an exhaust velocity of 311 x 9.80665 = 3050 m/s. The Tsiolkovsky rocket equation connects Isp to how much velocity a rocket can gain: delta-V = Isp x g0 x ln(m0 / mf), where m0 is the initial mass and mf is the final (dry) mass. This logarithmic relationship means doubling the propellant mass only adds ln(2) = 0.69 times the base delta-V, which is why staging is so powerful: discarding empty tank mass resets the mass ratio for the next stage.
Comparing chemical, nuclear, and electric propulsion
Chemical rockets are limited to Isp values of roughly 220-451 s by the energy available in chemical bonds. Liquid hydrogen with liquid oxygen sits near the top of that range at around 450 s. Nuclear thermal engines, which heat hydrogen in a reactor rather than burning it, can reach 850-1000 s, roughly doubling chemical Isp at the cost of added complexity and mass. Electric propulsion (ion and Hall-effect thrusters) achieves 1600-10000+ s by accelerating propellant with electric fields instead of combustion, but the thrust levels are very low because the electric power available in space is limited. For missions where travel time is not critical and electric power is available, electric propulsion can be far more propellant-efficient than any chemical engine.
Typical Isp values by propulsion type
| Propulsion type | Propellant / fuel | Isp (s) | Exhaust velocity (m/s) | Category |
|---|---|---|---|---|
| Cold gas | Nitrogen (N2) | 73 | 716 | Cold gas |
| Monopropellant | Hydrazine | 220 | 2160 | Chemical |
| Solid | Standard APCP | 242 | 2374 | Chemical |
| Solid | Aluminised APCP | 268 | 2628 | Chemical |
| Liquid bipropellant | LOX / RP-1 (kerosene) | 311 | 3050 | Chemical |
| Liquid bipropellant | N2O4 / MMH | 310 | 3040 | Chemical |
| Liquid bipropellant | N2O4 / UDMH | 315 | 3090 | Chemical |
| Liquid bipropellant | LOX / Methane | 363 | 3560 | Chemical |
| Liquid bipropellant | LOX / LH2 (hydrogen) | 451 | 4423 | Chemical |
| Nuclear thermal | Liquid hydrogen | 850 | 8335 | Nuclear |
| Hall-effect thruster | Xenon | 1600 | 15690 | Electric |
| Ion thruster (NSTAR) | Xenon | 3100 | 30400 | Electric |
| VASIMR (theoretical) | Argon/hydrogen | 5000 | 49033 | Electric (theoretical) |
Approximate vacuum Isp for common propulsion systems. Chemical values are vacuum figures; sea-level Isp is 5-15% lower for atmospheric rockets.
Frequently asked questions
Why is specific impulse measured in seconds?
When you express thrust in newtons and mass flow in kilograms per second, the formula Isp = F / (m-dot x g0) gives units of (N) / (kg/s x m/s^2) = (kg-m/s^2) / (kg/s x m/s^2) = seconds. The g0 in the denominator converts weight-flow into mass-flow, so the result is the same number regardless of whether you started in SI or imperial. Seconds are also physically meaningful: an Isp of 300 s means the engine can produce thrust equal to the propellant weight-flow for 300 seconds.
What is a good specific impulse for a rocket engine?
It depends on the propulsion type. For chemical rockets, 250-310 s is typical for solids, 300-360 s for hydrocarbon/liquid-oxygen bipropellants, and up to 450 s for liquid hydrogen/oxygen. Nuclear thermal engines reach 800-1000 s. Electric thrusters range from 1500-3100 s for Hall-effect and ion thrusters. For a given mission, higher Isp means less propellant is needed, but electric thrusters produce very low thrust and may need years to complete manoeuvres that take chemical stages minutes.
What is the difference between vacuum Isp and sea-level Isp?
Atmospheric back-pressure reduces the effective thrust of a rocket nozzle at sea level because the ambient pressure opposes the exhaust expansion. Vacuum Isp is always higher than sea-level Isp by roughly 5-15% for typical engines. Upper-stage engines designed to fire only in vacuum (such as the J-2 or RL-10) are optimised with large nozzle expansion ratios that would be flow-separated at sea level. Booster engines like the Merlin or RD-170 give a lower Isp at liftoff but recover most of that loss as the vehicle climbs above the atmosphere.
What is thrust-specific fuel consumption (TSFC) and how does it relate to Isp?
TSFC = m-dot / F, the mass of propellant burned per unit of thrust per second. It is the inverse of specific impulse scaled by g0: TSFC = 1 / (Isp x g0). Aircraft engines are typically rated in TSFC rather than Isp because they breathe atmospheric air as their primary working fluid and comparisons are made per unit weight of fuel carried on-board only.
How do I calculate delta-V from specific impulse?
Use the Tsiolkovsky rocket equation: delta-V = Isp x g0 x ln(m0 / mf), where m0 is the wet (fuelled) mass and mf is the dry (empty) mass. For example, a rocket with Isp 311 s, a wet mass of 250,000 kg, and a dry mass of 20,000 kg has a mass ratio of 12.5, so delta-V = 311 x 9.80665 x ln(12.5) = 3050 x 2.526 = 7704 m/s, comfortably enough for low-Earth orbit from near sea level with some margin.