What is specific impulse (Isp)?

Specific impulse is the thrust produced per unit weight of propellant consumed per second. It is measured in seconds and is the standard metric for comparing rocket engine efficiency across propellants and engine types. A higher Isp means more efficient use of propellant.

What is the formula for specific impulse?

Isp = F / (ṁ × g₀), where F is thrust in Newtons, ṁ is mass flow rate in kg/s, and g₀ = 9.80665 m/s² is standard gravity. Equivalently, Isp = ve / g₀, where ve is the effective exhaust velocity in m/s.

What are typical Isp values for different engines?

Solid rockets: 230–300 s. Kerosene/LOX (e.g. Merlin): 280–350 s. Hydrogen/LOX (e.g. SSME): 380–460 s. Ion thrusters: 1,500–10,000 s. Higher Isp generally means lower thrust, so mission design requires a trade-off.

Why is Isp measured in seconds?

Isp in seconds is the same in all unit systems (SI, imperial) because it divides thrust-force by weight-force of propellant per second. This removes the mass vs. weight ambiguity and makes Isp a universal efficiency number.

How does Isp relate to the Tsiolkovsky rocket equation?

The rocket equation uses effective exhaust velocity ve = Isp × g₀. Once you have Isp, multiply by 9.80665 to get ve, then use Δv = ve × ln(m₀/mf) to compute delta-v for a given mass ratio.

Specific Impulse Calculator

Name: Specific Impulse Calculator
Availability: InStock
Author: Nham Vu

Compute rocket engine Isp from thrust and mass flow rate, or from exhaust velocity — and convert between the two.

Engine Parameters

Isp = F / (ṁ × g₀) or Isp = v_e / g₀

Thrust (F) Newtons

Merlin 1D: ~845,000 N | SSME: ~2,090,000 N

Mass Flow Rate (ṁ) kg/s

Total propellant consumption per second

Example Engines

Enter engine parameters and click Calculate to see Isp results.

Specific Impulse (Isp)

seconds

Breakdown

Exhaust velocity (v_e)
Thrust (F)
Mass flow rate (ṁ)
Specific thrust (F/ṁ)
g₀ (standard gravity): 9.80665 m/s²

Isp Context

Your Isp

0 s 500 s (chemical max)

230–300 s Solid rocket motors

310–380 s Kerosene/LOX engines

380–460 s Hydrogen/LOX engines

1,500–10,000 s Ion / electric thrusters

Summary

Compute rocket engine Isp from thrust and mass flow rate, or from exhaust velocity — and convert between the two.

How it works

Choose a calculation mode: Thrust & Mass Flow Rate, or Exhaust Velocity.
For Thrust/Flow mode: enter thrust in Newtons and mass flow rate in kg/s.
The calculator applies Isp = F / (ṁ × g₀), where g₀ = 9.80665 m/s².
For Exhaust Velocity mode: enter ve in m/s; the tool computes Isp = ve / g₀.
Results show Isp in seconds, effective exhaust velocity, and thrust per kg/s flow.

Use cases

Compare propulsion system efficiency across different rocket engines.
Convert between exhaust velocity and Isp for the Tsiolkovsky equation.
Evaluate a custom engine test by entering measured thrust and propellant consumption.
Check textbook or datasheet Isp values against known exhaust velocities.
Estimate engine Isp when only thrust and burn time/propellant mass are known.
Aerospace engineering coursework on rocket propulsion.

Frequently Asked Questions

Last updated: 2026-07-24 · Reviewed by Nham Vu