Prop Pitch Calculator - Solve Pitch or Boat Speed

A prop pitch calculator converts between theoretical propeller pitch in inches and the boat speed the propeller will produce once you also enter engine RPM, gear ratio, and the typical propeller slip for the hull. The pitch printed on a propeller, such as 13" x 16", is the distance the blades would advance per revolution in a solid medium, and the formula below tells you which pitch matches a given top speed or which speed a known pitch will deliver.

• • Sizing a new propeller: Match a target top speed in mph, kph, or knots to a propeller pitch in inches before buying.
• • Inverting for boat speed: Estimate the boat speed that a currently installed prop will deliver at wide-open throttle.
• • Diagnosing engine loading: Compare the calculated pitch against the rated RPM range to see whether the prop is over- or under-pitched.
• • Comparing pitches for hulls: Compare a 19-inch pitch against a 21-inch pitch to weigh acceleration against top speed.

Boat propellers are stamped with diameter and pitch. Diameter controls how much water the blades move per revolution, and pitch controls how far the blades would ideally travel if the medium were solid. Because water gives way, the real advance per revolution is shorter than the rated pitch. That difference is the propeller slip percentage, which the prop pitch calculator uses to bridge the printed pitch and the actual boat speed on a GPS.

If you already know the boat speed you want and the engine RPM you plan to run, the calculator returns the pitch in inches you should ask for at the marine shop. If you already own a prop, the same calculator inverts the relation and shows the answer in mph, kph, or knots.

Gear ratio is one of the four inputs here, so Gear Ratio & RPM Calculator helps confirm whether the gearbox reduction you assumed is realistic for your hull.

The prop pitch formula relates five quantities. Knowing four lets you solve for the fifth.

• prop_pitch (in): Theoretical distance a propeller advances per revolution in inches.
• boat_speed: Boat speed through water in the unit you select (mph, kph, or knots).
• gear_ratio: Engine revolutions divided by propeller revolutions. 2.0 means the engine turns twice per propeller turn.
• C: Unit conversion constant: 1056 for mph, 656 for kph, or 1215.2 for knots.
• engine_rpm: Engine crankshaft revolutions per minute at wide-open throttle.
• slip: Decimal fraction of theoretical advance lost to slippage through water (10% means 0.10).

The constant C ties pitch in inches to boat speed. One mile is 63360 inches and one hour is 60 minutes, so C equals 1056 for mph. The same division gives 656 for kph (one kilometer is 39370.0787 inches) and 1215.2 for knots (one nautical mile is 72913.3858 inches).

To use the relation in the other direction, swap numerator and denominator. Boat speed equals pitch times RPM times one minus slip, divided by gear ratio times C. The Solve For dropdown exposes both directions.

According to Omni Calculator, prop pitch equals boat speed times gear ratio times C divided by engine RPM times one minus slip, with C equal to 1056 for mph, 656 for kph, and 1215.2 for knots.

For a hull-speed check from waterline length and horsepower, Boat Speed Calculator gives a separate estimate you can compare with the result here.

Four concepts come up every time the prop pitch calculator sizes a propeller for a hull.

These four concepts feed the same five inputs in the prop pitch calculator. Pitch and slip come from the propeller itself, engine RPM from your wide-open-throttle reading, and gear ratio from the owner's manual.

If the result is far outside the engine's recommended RPM band, the pitch is mismatched. Drop the pitch if RPM peaks above the rated maximum, and add pitch if RPM never reaches the bottom of the band.

Boat propeller pitch behaves like fastener thread pitch, so Thread Pitch Calculator is a useful adjacent reference for the same concept on screws and bolts.

The calculator has two solve-for modes that share the same inputs. Pick the mode that matches your goal.

1. 1 Pick a solve direction: Choose 'Prop Pitch' if you know your top speed and want the pitch in inches. Choose 'Boat Speed' if you own a propeller and want to know its speed.
2. 2 Select the speed unit: Use mph for US ratings, kph for European ratings, or knots for marine charts. The unit sets constant C inside the formula.
3. 3 Enter the four known values: Fill in boat speed (or prop pitch), gear ratio, engine RPM, and propeller slip. Leave the variable you are solving for at zero.
4. 4 Read the primary result: Propeller pitch shows in inches to two decimals. In Speed mode the boat speed uses your unit and shows two decimals.
5. 5 Check the secondary outputs: Theoretical Advance (in/min) shows what the prop would cover in a solid medium. Slip Loss (in/min) shows how many inches per minute the slip percentage removes.

For a pontoon that needs 28 mph at 5200 RPM through a 2.20:1 gearcase at 12% slip, set Solve For to Prop Pitch, Speed Unit to mph, enter 28 in Boat Speed, 2.20 in Gear Ratio, 5200 in Engine RPM, and 12 in Propeller Slip. The calculator returns about 15.4 in of pitch.

After sizing the propeller, Torque, Power & Speed Calculator converts the resulting RPM into torque and horsepower to check if the drivetrain sustains the target top speed.

Using the prop pitch calculator before buying a propeller saves time on the water and protects the engine.

• • Pick a pitch before you order: Convert a target top speed into the exact pitch in inches the marine parts desk needs.
• • Compare two pitches: Run the same RPM, gear, and slip twice with different pitches. A 19-inch prop accelerates faster, while a 21-inch prop reaches a higher top speed.
• • Diagnose engine loading: If wide-open-throttle RPM is below the rated band, the pitch is too high. If above, the pitch is too low. The calculator shows the right pitch.
• • Use the unit you read: mph, kph, and knots are all first-class options. Constant C is applied automatically so the answer is in inches no matter which unit you start with.
• • Plan around realistic slip: Slip is the most common source of miscalculated top speed. The calculator carries it through the formula so you can adjust it when conditions change.
• • Keep the math consistent: Switching to Speed mode is just an algebraic rearrangement of the same relation, so the math never drifts between the two sides.

The biggest payoff is during repower season. A new engine with a higher or lower rated RPM band is reason to recheck the prop pitch.

A correctly pitched propeller also burns less fuel at cruise, so pairing this with Boat Fuel Consumption Calculator shows the full operating cost of a propeller choice.

Real-world factors bend the propeller-pitch relation away from the textbook number. Adjust these before you trust the prop pitch calculator output.

• • The formula treats water as a uniform solid with a fixed slip percentage, so it cannot detect ventilation, cavitation, or partial submersion at high RPM.
• • Slip is an input you supply. If you enter an unrealistic slip the calculator will produce an unrealistic pitch; treat slip as a calibration knob.

Run the calculator with the slip your boat type typically shows, then re-run after a sea trial if RPM does not land in the engine's rated band.

According to Wikipedia – Marine propeller, propeller pitch is the theoretical distance a propeller would advance in one revolution in a solid medium, and typical slip at top speed is roughly 10 to 20 percent.

Hull drag, fouling, and ventilation push propeller slip away from the textbook 10 to 20 percent, and Drag Equation Calculator shows the same v² scaling so you can estimate how a fouled bottom raises slip on a given hull.