# Specific Impulse Calculator

URL: https://speedycalculator.com/calculators/physics/specific-impulse/
Last updated: 2026-10-30

> The specific impulse calculator finds an engine's specific impulse and thrust-specific fuel consumption from its thrust and mass flow rate or exhaust velocity, with typical values for well-known rocket engines and thrusters.

## Inputs

- Method
- Engine
- Thrust F (kN, N, lbf)
- Mass flow rate ṁ (kg/s, kg/h, g/s, lb/s, lb/min)
- Exhaust velocity v_e (m/s, km/h, mph, ft/s, knots, km/s)

## Results

- Specific impulse I_sp
- Effective exhaust velocity
- Thrust-specific fuel consumption

## Formula

```
I_sp = F/(ṁ·g₀) = v_e/g₀,  TSFC = 1/(I_sp·g₀)
```

## Explanation

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

FieldWhat to enterMethodChoose one: Engine preset, Thrust and mass flow rate, Exhaust velocityEngineChoose one: Merlin 1D (sea level), Merlin 1D Vacuum, RS-25 (vacuum), Raptor 2 (sea level) …Thrust FUnits: kN, N, lbfMass flow rate ṁUnits: kg/s, kg/h, g/s, lb/s, lb/minExhaust velocity v_eUnits: 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)422.5141.2558633.75211.8837845282.51151,056.25353.13941,267.5423.76731,690565.0231

## 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.

## FAQ

**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.


## Sources

- [NASA – Planetary fact sheet](https://nssdc.gsfc.nasa.gov/planetary/factsheet/)
- [NASA Glenn – Specific impulse](https://www.grc.nasa.gov/www/k-12/airplane/specimp.html)