Prop Pitch Calculator
Enter any four of the five propeller variables and this calculator solves the fifth instantly. Use it to find theoretical boat speed from pitch and RPM, calculate the pitch you need to hit a target speed, fine-tune your gear ratio, or work out how much slip your propeller is generating. Supports mph, kph, and knots, with pitch in inches, centimetres, or millimetres.
What is propeller pitch?
Propeller pitch is the theoretical distance a propeller would advance through a solid, non-slipping medium in a single full revolution. Think of it like a wood screw: a screw with a 21-inch pitch would advance 21 inches into wood with one turn. For a marine propeller moving through water, the actual distance is always less than the pitch because water is not a solid - the propeller slips. The pitch is stamped on every propeller alongside the diameter, usually written as a two-number code such as "15.25 x 21", where 15.25 is the diameter in inches and 21 is the pitch in inches.
How propeller slip works
Propeller slip is the difference between the theoretical distance a prop would travel (its pitch) and the actual distance the boat moves per revolution, expressed as a percentage. At wide-open throttle a well-matched prop on a recreational outboard typically slips 10-20%. Slip is not waste: it is necessary for the propeller to generate thrust at all, just as an aircraft wing must have an angle of attack to generate lift. Too little slip can indicate the propeller is underloaded; too much suggests the prop is overloaded, cavitating, or undersized. Slip is highest during acceleration and drops as the boat reaches top speed.
The prop pitch speed formula
The formula that connects all five variables is: Speed = (RPM * Pitch_inches * (1 - slip)) / (Gear_ratio * C). The constant C depends on your speed unit: 1056 for mph, 656 for kph, and 1215.2 for knots. This formula can be rearranged algebraically to solve for any one variable when the other four are known. This calculator does that rearrangement for you. For example, to find the pitch needed to reach 50 mph at 5500 RPM with a 1.87 gear ratio and 15% slip: Pitch = (50 * 1.87 * 1056) / (5500 * 0.85), which gives about 21.2 inches.
Choosing the right pitch: lower vs higher
A lower-pitch propeller bites into the water with a shallower angle. This reduces the load on the engine at a given speed, allowing it to spin up faster and reach WOT RPM sooner - giving better acceleration and hole shot. A higher-pitch prop takes a bigger bite per revolution, which means the engine must work harder but can push the boat to a higher top speed once it is on plane. The goal is to choose a pitch that lets the engine reach the top of its rated WOT RPM band (printed on the engine tag) at full throttle. If the engine over-revs, go up in pitch; if it cannot reach the WOT range, go down. Each inch of pitch change typically shifts WOT RPM by 150-200 RPM.
Typical propeller slip by boat type
| Boat type | Typical slip at WOT | Notes |
|---|---|---|
| Race/competition hull | 2-5% | Optimised propellers, minimal resistance |
| Performance bass boat | 5-10% | High-output outboard, lightweight hull |
| Performance runabout | 8-12% | Smooth-water conditions |
| Typical recreational outboard | 10-18% | Most common for family boats |
| Pontoon / deck boat | 15-22% | Higher drag and load |
| Inboard sterndrive | 12-20% | Varies with hull form |
| Fishing / work boat (heavy) | 20-30% | Loaded hull, displacement running |
| Planing hull at hump (mid-throttle) | 25-40% | Highest slip during acceleration |
Propeller slip at wide-open throttle. Higher slip is normal at lower speeds and during acceleration.
Frequently asked questions
What is a typical propeller slip percentage?
At wide-open throttle, most recreational outboard-powered boats have 10-20% propeller slip. Performance hulls and racing boats can achieve 5-10%, while heavily loaded workboats, pontoons, or boats at hump speed (mid-throttle during acceleration) can see 25-40% or more. Higher slip during acceleration is completely normal - it decreases as the boat reaches planing speed.
How does gear ratio affect boat speed?
The gear ratio is the number of engine revolutions for every one propeller shaft revolution. A 1.87:1 ratio means the engine turns 1.87 times per prop turn. A higher gear ratio reduces prop RPM, which lets you swing a larger-diameter or higher-pitch propeller without over-revving the engine. Most outboards run between 1.75:1 and 2.5:1; inboard sterndrives vary more widely.
What is the difference between a 19-inch and a 21-inch pitch?
A 19-inch prop advances 19 inches per revolution; a 21-inch prop advances 21 inches. The 19-inch will allow the engine to reach a higher RPM at the same speed, giving better acceleration and hole shot. The 21-inch prop will reach a lower RPM for the same speed but produce a higher top speed if the engine can still reach its WOT RPM band. On most outboards, stepping from a 19 to a 21-inch prop costs about 300-400 RPM at WOT.
Why does this calculator use a constant of 1056 for mph?
The constant C = 1056 is a unit-conversion factor that makes the math work out correctly when RPM is in revolutions per minute and pitch is in inches. It combines 12 (inches per foot) and 5280 (feet per mile) divided by 60 (seconds per minute): 5280 / (60 / 12) = 1056. For knots the constant is 1215.2 (using nautical miles and feet), and for kph it is 656 (using kilometres and centimetres converted).
Can I use this calculator for drones or aircraft propellers?
The same pitch formula applies to any rotating propeller in a fluid, including air propellers on drones, RC planes, or light aircraft. You would enter air-medium speeds and the appropriate gear ratio (often 1:1 for direct-drive motors). However, the typical slip percentages in the reference table are for marine use only - air-prop slip is generally lower, around 5-15% for well-designed aircraft propellers at cruise speed.