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Specific Impulse Calculator

Work out specific impulse instantly with clear inputs, formula shown and shareable results.

About Specific Impulse Calculator

Specific impulse is thrust per unit propellant flow, expressed in seconds by dividing by standard gravity. 845 kN at 287 kg/s gives an exhaust velocity of 2944 m/s and an Isp of 300 s - typical of a kerolox first stage. Hydrogen-oxygen reaches 450 s and ion thrusters exceed 3000 s, which is why they dominate deep-space propulsion.

Formulas

Effective exhaust velocity

ve = thrust / mass flow rate

Specific impulse

Isp = ve / 9.80665

Worked example

With the default values below, Specific Impulse Calculator returns Specific Impulse of 300.2. Change any input above and every figure here is recalculated in your browser.

Inputs used

Thrustminimum 0 N
845,000 N
Propellant mass flow rateminimum 0 kg/s
287 kg/s

Results

Specific Impulse
300.2 s
Effective Exhaust Velocity
2,944.3 m/s
Propellant Consumed Per Second
287.0 kg
Delta-V At A Mass Ratio Of Three
3,234.6 m/s

Step by step

  1. 1Values used
  2. 2Effective exhaust velocity
  3. 3Specific impulse
  4. 4Specific Impulse= 300.2 s
  5. 5Effective Exhaust Velocity= 2,944.3 m/s
  6. 6Propellant Consumed Per Second= 287.0 kg
  7. 7Delta-V At A Mass Ratio Of Three= 3,234.6 m/s

Assumptions and limits

  • The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why is Isp measured in seconds?

Because dividing exhaust velocity by g0 removes the unit system, so the figure is the same in metric and imperial - a historical convenience.

Does high Isp mean high thrust?

No, often the opposite. Ion engines have superb Isp but milli-newton thrust, so they take months to do what a chemical stage does in minutes.

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