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Specific Impulse Calculator

Enter thrust and mass flow rate to calculate the specific impulse in seconds.

How it is calculated

The tool first finds the effective exhaust velocity, v_e = F ÷ ṁ, with thrust F in newtons and propellant mass flow ṁ in kilograms per second. It then divides by standard gravity: Isp = v_e ÷ g₀, with g₀ = 9.80665 m/s² unless you enter a different value in the optional g0 field. Isp is shown in seconds and v_e in m/s, both with two decimals. Thrust must not be negative (zero gives zero), while mass flow and g₀ must be above zero; otherwise no result is shown. The result is a single steady-state figure; it does not distinguish sea-level from vacuum performance, so use the thrust that matches the conditions you care about.

Worked example

An engine produces 845,000 N of thrust while burning 280 kg of propellant per second. Effective exhaust velocity = 845,000 ÷ 280 = 3,017.86 m/s. Specific impulse = 3,017.86 ÷ 9.80665 = 307.74 s. Because Isp is thrust per unit weight of propellant flow, doubling thrust at the same mass flow doubles Isp, while burning the propellant faster for the same thrust lowers it.

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Common mistakes

  • Entering thrust in kN or tonnes-force while the field is in newtons. 845 kN must be typed as 845000.
  • Entering mass flow per minute or in pounds. The field is kilograms per second.
  • Mixing sea-level thrust with a vacuum Isp figure from a datasheet, or the reverse. The two differ for the same engine, so use matching conditions.
  • Confusing Isp in seconds with exhaust velocity in m/s. They differ by the factor g₀, about 9.81, and both are shown.
  • Changing g₀ to local gravity. Isp is defined with standard gravity 9.80665 m/s², so leave the default unless a source tells you otherwise.

About this calculator

Specific impulse (Isp) measures how efficiently a rocket engine converts propellant into thrust, expressed in seconds. It's calculated as Isp = F ÷ (ṁ × g₀), where F is thrust, ṁ is the propellant mass flow rate, and g₀ is standard gravity (9.80665 m/s²) — the same equation rearranges to give the effective exhaust velocity, vₑ = F ÷ ṁ, which is Isp multiplied by g₀.

Aerospace engineers use specific impulse as the primary metric for comparing rocket engine efficiency — a higher Isp means more thrust (or longer burn time) from the same amount of propellant, which is why ion engines (Isp in the thousands of seconds) are far more fuel-efficient than chemical rockets (Isp typically 250–450 seconds), even though chemical engines produce far more raw thrust. It's a core parameter in mission design, feeding directly into the Tsiolkovsky rocket equation used to calculate a vehicle's achievable velocity change.

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