Specific impulse is a rocket engine’s “fuel economy”: how many seconds one kilogram-weight of propellant can produce one kilogram-force of thrust. Chemical rockets reach 250–465 s; ion thrusters several thousand – but with tiny thrust.
How to use the Specific Impulse Calculator
- Method
- Engine
- Thrust F
- Mass flow rate ṁ
- Exhaust velocity v_e
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Choose an engine preset, or calculate from thrust and mass flow, or from exhaust velocity.
Inputs explained
| Field | What to enter |
|---|---|
| Method | Choose one: Engine preset, Thrust and mass flow rate, Exhaust velocity |
| Engine | Choose one: Merlin 1D (sea level), Merlin 1D Vacuum, RS-25 (vacuum), Raptor 2 (sea level) … |
| Thrust F | Units: kN, N, lbf |
| Mass flow rate ṁ | Units: kg/s, kg/h, g/s, lb/s, lb/min |
| Exhaust velocity v_e | Units: m/s, km/h, mph, ft/s, knots, km/s |
What each result means
- Effective exhaust velocity = [Specific impulse I_sp]×9.80665
- Thrust-specific fuel consumption = 3600÷([Specific impulse I_sp]×9.80665)
Example values
How the main result changes when Thrust F changes and everything else stays at the example values above:
| Thrust F (kN) | Specific impulse I_sp (s) |
|---|---|
| 422.5 | 141.2558 |
| 633.75 | 211.8837 |
| 845 | 282.5115 |
| 1,056.25 | 353.1394 |
| 1,267.5 | 423.7673 |
| 1,690 | 565.0231 |
Formula
I_sp = F/(ṁ·g₀) = v_e/g₀, TSFC = 1/(I_sp·g₀)
Worked example
Inputs — Method: Thrust and mass flow rate; Thrust F: 845 kN; Mass flow rate ṁ: 305 kg/s.
Results — Specific impulse I_sp = 282.5115 s; Effective exhaust velocity = 2.77049 km/s; Thrust-specific fuel consumption = 1.29941 kg/(N·h).
Key terms
- TSFC
- Propellant mass flow per unit thrust.
Frequently asked questions
Why is specific impulse measured in seconds?
Dividing by g₀ makes it the same in metric and imperial units.
Which propellant has the highest I_sp?
Liquid hydrogen with liquid oxygen (~450 s) among common chemical rockets.
Why is sea-level I_sp lower?
Atmospheric pressure reduces the effective exhaust velocity.
Why aren't ion engines used for launch?
Their thrust is far too small to lift off.