What this calculator does
Specific Impulse works out how efficiently a rocket or jet engine converts propellant into thrust. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
FormulaIsp = F/(ṁ·g0) = ve/g0, g0 = 9.80665 m/s²
The inputs explained
| Field | What to enter |
|---|---|
| Thrust (F) (N) | A number, measured in N. Starts at 2000. |
| Propellant mass flow rate (ṁ) (kg/s) | A number, measured in kg/s. Starts at 0.68. |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the answer changes with thrust (f)
Every other input is held at the calculator’s starting values while thrust (f) varies. Select any row to load that scenario into the calculator.
How the answer changes with thrust (f)
| Thrust (F) (N) | Specific impulse (Isp) | Effective exhaust velocity (ve) | Effective exhaust velocity (ve) |
|---|---|---|---|
| 1,000 | 149.96 s | 1,470.6 m/s | 1.471 km/s |
| 1,500 | 224.94 s | 2,205.9 m/s | 2.206 km/s |
| 2,000 | 299.92 s | 2,941.2 m/s | 2.941 km/s |
| 3,000 | 449.87 s | 4,411.8 m/s | 4.412 km/s |
| 4,000 | 599.83 s | 5,882.4 m/s | 5.882 km/s |
| 6,000 | 899.75 s | 8,823.5 m/s | 8.824 km/s |