Specific Impulse Calculator
How efficiently a rocket or jet engine turns propellant into thrust: Isp = F / (ṁ · g₀).
Inputs
Enter the engine thrust and propellant mass flow. g₀ is the standard gravity, 9.80665 m/s², so Isp in seconds is the same in metric and imperial units. Drag the handle (or focus it and press ↑/↓) to reorder. Scientific notation such as 2.8e-6 is accepted, and any unit converts automatically.
Results
Enter values and press Calculate. Your answer will appear here.
Thrust vs. mass flow
Every straight line is a constant specific impulse, because F = Isp · g₀ · ṁ. The solid line is your Isp, the dashed lines are half and double it, and the dot is your engine. Higher lines get more thrust from the same propellant flow.
What is being calculated
Reference values
| Isp | Exhaust velocity ve | Mass flow per 1 kN of thrust |
|---|
Delta-v from the rocket equation (km/s)
| Mass ratio m₀ / m_f | Isp 300 s | Isp 450 s | Isp 1,000 s |
|---|
Typical specific impulse (approximate)
| Propulsion type | Typical Isp |
|---|---|
| Cold gas thruster | 40 to 70 s |
| Hydrazine monopropellant | 200 to 235 s |
| Solid rocket motor | 250 to 290 s |
| Kerosene and liquid oxygen | 300 to 350 s |
| Liquid hydrogen and liquid oxygen | 420 to 460 s (vacuum) |
| Hall and ion thrusters | 1,500 to 5,000 s and more |
| Air-breathing turbofan | several thousand s (counts only fuel mass) |
Values vary by engine and by altitude, so treat these as ballpark figures.
Unit conversions
| Quantity | Conversion |
|---|---|
| Thrust | 1 kN = 1,000 N · 1 lbf = 4.44822 N · 1 kgf = 9.80665 N |
| Mass flow | 1 kg/s = 1,000 g/s · 1 lb/s = 0.453592 kg/s · 1 kg/min = 0.016667 kg/s |
| Isp and velocity | 1 s of Isp = 9.80665 m/s of exhaust velocity · 1 N·s/kg = 1 m/s |
| Mass | 1 t = 1,000 kg · 1 lb = 0.453592 kg |
| Fuel use | TSFC = ṁ / F · 1 g/(kN·s) = 10⁻⁶ kg/(N·s) |
Formula
- Isp: specific impulse (s)
- F: thrust (N)
- ṁ: propellant mass flow rate (kg/s)
- g₀ = 9.80665 m/s²: standard gravity (a fixed constant, not the local gravity)
Example: An engine makes F = 110 kN using ṁ = 24.1 kg/s. Isp = 110,000 ÷ (24.1 × 9.80665) ≈ 465.4 s, which is an effective exhaust velocity of about 4,564 m/s.
Rearranged: F = Isp · g₀ · ṁ and ṁ = F / (Isp · g₀). The effective exhaust velocity is ve = Isp · g₀ = F / ṁ. With an initial mass m₀ and final mass m_f, the rocket equation gives Δv = ve · ln(m₀ / m_f), the burn time is (m₀ − m_f) / ṁ, the total impulse is F × burn time, and the thrust-to-weight ratio at ignition is F / (m₀ · g₀). Isp changes with altitude because the nozzle pressure thrust changes, so use vacuum or sea-level values consistently. The Advanced vehicle results assume constant thrust and mass flow and ignore gravity and drag losses.
References
- Wikipedia, Specific impulse: definition, units and typical values
- NASA Glenn, Specific Impulse: Isp for rockets and jet engines
- Wikipedia, Tsiolkovsky rocket equation: Δv from exhaust velocity and mass ratio
- Wikipedia, Standard gravity: the value of g₀
How to Use This Tool
- Pick a preset, or enter the thrust F and the mass flow ṁ with their units.
- In Advanced, you can instead solve for thrust or mass flow from a known Isp.
- In Advanced, add the initial and final mass to get Δv, burn time, total impulse and thrust-to-weight ratio.
- Click Calculate.
- Read Isp in the highlighted result, then check the table and the chart.
What This Tool Offers
- Basic and Advanced modes on one calculation engine
- Thrust in N, kN, MN, lbf and kgf, and mass flow from mg/s to t/s, with automatic conversion
- Specific impulse, effective exhaust velocity and fuel use per unit thrust
- Solve for thrust or mass flow, plus Δv, burn time, total impulse and thrust-to-weight (Advanced)
- Constant-Isp thrust chart and a labelled engine diagram
- Reorderable inputs, presets and a mobile-friendly layout
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| I_sp | Specific impulse | s |
| F | Thrust | N |
| ṁ | Mass flow rate | kg/s |
| g₀ | Standard gravity | m/s² |
What it means
Specific impulse (I_sp) is a key performance metric for rockets and jet engines, indicating how effectively the propellant is converted into thrust. It is defined as the thrust per unit weight flow rate of propellant, with units of seconds. The higher the I_sp, the less propellant is needed to achieve a given Δv, which is critical for spacecraft mass fraction. For chemical rockets, I_sp typically ranges from 250‑450 s; for ion thrusters, it can exceed 3000 s. The formula shows that I_sp is proportional to the effective exhaust velocity (v_e/g₀). It is used in the Tsiolkovsky rocket equation to determine mass ratios and in mission planning to compare propulsive options. In aircraft, it relates to thrust specific fuel consumption (TSFC). Understanding I_sp is essential for aerospace engineers to optimise propulsion system design and mission feasibility.
Worked example
Specific Impulse – Two Examples
Real‑World| Parameter | Value |
|---|---|
| F | 250,000 N |
| ṁ | 85 kg/s |
| Parameter | Value |
|---|---|
| F | 450,000 N |
| ṁ | 140 kg/s |
Common mistakes
- Specific impulse I_sp: Usually in seconds – I_sp = F / (ṁ·g₀).
- g₀: Standard gravity (9.80665 m/s²) – not local g.
- Units: F in N, ṁ in kg/s → I_sp in seconds.
- Higher I_sp: Indicates better propellant efficiency; typical values: 250‑450 s for chemical rockets.
- Effective exhaust velocity: v_e = I_sp·g₀ – often used in rocket equations.
Applications
Specific impulse, I_sp = F / (ṁ·g₀), is a measure of rocket engine efficiency, representing the thrust per unit weight flow of propellant (with units of seconds). Higher I_sp indicates better fuel efficiency, allowing rockets to achieve higher delta‑v for a given propellant mass. This figure of merit is essential for comparing propellant combinations (e.g., hydrazine vs. hydrogen‑oxygen) and for preliminary design of launch vehicles and upper stages. Engineers use I_sp to compute propellant mass fractions, to size fuel tanks, and to optimise engine parameters. In electric propulsion (ion thrusters), I_sp can be very high, enabling long‑duration missions. By understanding specific impulse, aerospace engineers can make informed choices about propulsion systems, balancing performance, cost, and safety.
- Propellant selection and performance comparison
- Launch vehicle preliminary design and mass budgeting
- Engine cycle trade‑off studies (chemical, electric, nuclear)
- Propulsion system optimisation for interplanetary missions
- Education and standardisation of rocket engine metrics
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