Specific Impulse Calculator (Isp)
Specific impulse (Isp) is the rocket-propulsion equivalent of fuel economy: the seconds one newton of thrust is produced per newton-weight of propellant per second. Enter thrust and mass-flow rate to get Isp, effective exhaust velocity, and the Δv achievable with a 2:1 mass ratio.
N
kg/s
Thrust per unit weight-flow of propellant — the "fuel economy" of a rocket engine
- 1
Propellant weight-flow rate
275 × 9.80665 = 2,696.83 Ng₀ = 9.80665 m/s² converts mass flow to weight flow. - 2
Specific impulse Isp = F ÷ (ṁ × g₀)
934,000 ÷ 2,696.83 = 346.3 s
How does this calculator work?
Specific impulse Isp = F / (ṁ × g₀) in seconds, g₀ = 9.80665 m/s². Effective exhaust velocity ve = Isp × g₀. Delta-v follows the Tsiolkovsky equation Δv = ve × ln(m₀/mf). Higher Isp means more velocity change per kilogram of propellant — kerosene/LOX ~340 s, hydrogen/LOX ~450 s.
Formula
How this is calculated
Specific impulse is defined as Isp = F / (ṁ × g₀), where F is thrust (N), ṁ is the propellant mass flow rate (kg/s), and g₀ = 9.80665 m/s² is standard gravity. The result is in seconds and is the same regardless of unit system, making it straightforward to compare engines that use very different propellant flows. A higher Isp means the engine extracts more momentum per kilogram of propellant burned.
The effective exhaust velocity ve = Isp × g₀ (m/s) links directly to the Tsiolkovsky rocket equation: Δv = ve × ln(m₀/mf), where m₀ is the fully-loaded wet mass and mf is the dry burnout mass. The calculator shows the Δv for a 2:1 mass ratio (50 % propellant load) as a practical reference. To reach low Earth orbit from the surface requires roughly 9,400 m/s of Δv; the Moon requires a further ~3,200 m/s.
Typical Isp values: liquid-hydrogen/LOX engines (RS-25, J-2) achieve 420–450 s in vacuum; kerosene/LOX (Merlin 1D vacuum) ≈ 348 s; solid boosters ≈ 240–280 s; hydrazine monopropellant ≈ 220 s; ion engines 1,500–10,000 s at micro-newton thrust. These are vacuum figures; sea-level Isp is lower because ambient back-pressure reduces effective thrust.
Frequently asked questions
Dividing thrust (N) by mass-flow rate (kg/s) gives the exhaust velocity in m/s, which varies by unit system. Dividing by g₀ (m/s²) converts it to seconds — a unit-independent number. An Isp of 300 s means each kilogram of propellant delivers 300 × 9.81 ≈ 2,943 N·s of total impulse.
LOX/LH2 (RS-25 SSME): ~453 s vacuum. LOX/kerosene Merlin 1D vacuum: ~348 s. LOX/kerosene Raptor vacuum: ~380 s. Solid boosters (SRB): ~250 s. Monopropellant hydrazine: ~220 s. Ion engines: 1,500–10,000 s at very low thrust.
The Tsiolkovsky equation shows that Δv scales logarithmically with mass ratio. Doubling propellant adds only ve × ln(2) ≈ 0.69 ve of Δv. Going further demands higher Isp or staging. LEO requires ~9.4 km/s from the surface; geostationary transfer orbit an additional ~2.5 km/s.
Also known as
TG we-Calculate Editorial Team. (2026). Specific Impulse Calculator (Isp) [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/specific-impulse-calculator
TG we-Calculate Editorial Team. "Specific Impulse Calculator (Isp)." TG we-Calculate. 2026. https://we-calculate.com/calculator/specific-impulse-calculator.
TG we-Calculate Editorial Team, "Specific Impulse Calculator (Isp)," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/specific-impulse-calculator
@misc{wecalculate_specific_impulse_calculator, title = {Specific Impulse Calculator (Isp)}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/specific-impulse-calculator}}, year = {2026}, note = {TG we-Calculate} }
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