Propeller Pitch Calculator

Predict your boat's top speed from RPM, gear ratio, pitch, and slip, or work out which pitch puts your engine in its rated wide-open-throttle range. Built on the standard prop math every prop shop uses.

How the prop math works

A propeller's pitch is the distance it would screw forward through a solid in one revolution. Theoretical speed is prop shaft RPM times pitch; real boats fall short of that by their slip percentage. The calculator divides engine RPM by your gear ratio to get prop RPM, multiplies by pitch, and discounts by slip.

  • Prop slip is the gap between theoretical and actual speed, expressed as a percent. It is not a defect; every hull has it. Light planing hulls run 10 to 15 percent, heavy or displacement hulls and pontoons run 20 to 30.
  • WOT RPM targeting. Your engine should reach the top of its rated wide-open-throttle range with a normal load. Each inch of pitch changes WOT RPM by roughly 150 to 200: under-revving (lugging) calls for less pitch, over-revving calls for more.
  • Change pitch in small steps. If the math calls for a 3 inch or larger swing, something else is usually wrong: a fouled bottom, a damaged prop, an optimistic tachometer, or a mis-entered gear ratio.

Propeller pitch FAQ

What is propeller pitch?

Pitch is how far the prop would travel forward in one full turn with zero slip, measured in inches. A 19 pitch prop moves 19 inches per revolution in theory. Higher pitch trades acceleration and RPM for top-end speed.

What is prop slip and what number should I use?

Slip is the percentage difference between theoretical speed (RPM times pitch) and what the boat actually does. Use 10 to 15 percent for a clean planing hull, and 20 to 30 percent for heavy boats, pontoons, and displacement hulls.

My engine will not reach its rated WOT RPM. What pitch should I run?

Drop about one inch of pitch for every 150 to 200 RPM you are short. An engine hitting 4,800 with a 5,400 target wants roughly 3 inches less pitch. Verify on the water and confirm the bottom and prop are clean first; fouling mimics over-propping.

Is it bad to over-rev or lug a marine engine?

Both. Exceeding rated WOT RPM accelerates wear, and lugging below the rated range loads the engine harder than it was built for at that RPM. Prop the boat so a normal load lands at the top of the maker's WOT band.

Does this calculator work for outboards and sterndrives?

Yes. The math is the same for outboards, sterndrives, and straight inboards; only the gear ratio differs. Find yours on the engine spec sheet, commonly 1.75 to 2.4 for outboards.