How this instrument works
Propeller pitch describes a helix the same way a screw's thread does: it is the linear distance a blade would advance through a solid medium in one full revolution, if it behaved like a rigid thread instead of a rotating airfoil. A pitch of 20 inches means one turn should, in principle, carry it 20 inches forward. Multiplying that per-revolution distance by how many revolutions happen each second produces a speed — exactly what this formula does, dividing RPM by 60 to turn revolutions per minute into revolutions per second first.
The math is pure geometry, not an empirical curve fit: distance per revolution times revolutions per unit time equals a rate, the same relationship that governs a lead screw or a drill bit advancing into wood. Pitch is still specified in inches on most catalogs and hobby packaging, even for aircraft and boats built to metric drawings everywhere else, because the propeller industry's inch-based convention predates most of those metric airframes. This calculator's own unit menu switches to centimetres for the sheets that do quote metric.
Treat the result as a ceiling, not a forecast. Real propellers never reach it, because they work by accelerating a fluid rather than gripping a solid — some of each revolution's motion pushes air or water backward instead of carrying the craft forward, a shortfall called slip. The most extreme case makes the gap obvious: an engine at full static run-up, propeller spinning, aircraft not moving at all, still returns this same nonzero theoretical figure even though the true forward speed is zero.
- Enter Propeller pitch — the per-revolution advance distance stamped on the blade or hub, such as the second number in an 18x10 prop spec.
- Enter Propeller RPM — the actual shaft speed itself, not a throttle percentage or a redline figure.
- Read Theoretical (100% efficient) speed — the zero-slip ceiling for that pitch and RPM combination.
- Switch the result's unit menu between mph and km/h to match how your source data is quoted.
Worked example — a 20-inch-pitch prop at 2,500 RPM
Enter a Propeller pitch of 20 inches (0.508 m) and a Propeller RPM of 2,500. The instrument converts pitch to metres, divides RPM by 60 to get 41.667 revolutions per second, and multiplies: 0.508 m × 41.667 rev/s = 21.1667 m/s. Switched to the result field's default mph, that reads 47.3 mph; switched to km/h it comes out to an exact 76.2 km/h, since 21.1667 m/s × 3.6 is precisely 76.2.
That number is a ceiling, not a promise. A well-matched propeller in real air typically achieves somewhere between 70 and 85 percent of it once slip is accounted for, so an actual cruise speed nearer 35 to 40 mph would be unremarkable for this pitch-and-RPM pairing — the shortfall is the propeller's slip, not an error in this calculation.
Questions
What does '100% efficient' mean in the result label?
It marks the result as a theoretical ceiling: zero slip, as if the propeller were a rigid screw threading through a solid rather than spinning fluid. No real propeller reaches it, because turning air or water backward to generate thrust always costs some of each revolution's forward advance. Use the number to compare propellers on paper, not to predict an actual top speed.
How much slower will a real propeller actually go?
Commonly 15 to 30 percent slower for a well-matched aircraft propeller near its efficient RPM range, and 30 to 50 percent slower for a heavily loaded or mismatched boat propeller. That shortfall is called slip. The extreme case is a static engine run-up: RPM is real and this formula still returns a nonzero theoretical speed, yet the aircraft's actual forward speed is zero.
Why is propeller pitch usually given in inches?
Convention, not physics — English-unit manufacturing dominated the early propeller industry, and most catalogs, marine dealers, and RC hobby suppliers still stamp pitch and diameter in inches, as in an 18x10 propeller (18-inch diameter, 10-inch pitch), even on airframes and hulls built to metric drawings elsewhere. Switch the field's unit menu to centimetres if your source sheet uses metric.
Does propeller diameter or blade count change this result?
No. Diameter and blade count govern how much air or water a propeller can move per revolution, and therefore its thrust and torque demand, but the theoretical speed here depends only on pitch and RPM. Two propellers with different diameters and blade counts but the same two values return an identical theoretical speed.
Where does dividing by 60 come from in the formula?
RPM counts revolutions per minute, but a speed needs revolutions per second to pair correctly with a per-revolution distance. Dividing RPM by 60 makes that conversion; the calculator then multiplies revolutions per second by the pitch distance and resolves the result internally in metres per second before converting to whichever output unit the result field is set to.
If I double the pitch, does the propeller go twice as fast?
Only in this theoretical figure, and only at the same RPM — pitch and speed are directly proportional here, so doubling one doubles the other. In practice, a coarser setting also demands more torque to turn at that RPM, so swapping to a higher-pitch propeller on a fixed engine usually pulls the RPM down rather than delivering the full doubled speed; matching it to available power is a trade-off this formula does not capture.