Propeller Pitch Speed Calculator
Enter propeller pitch (in inches), engine wide-open-throttle RPM and gear ratio to find the theoretical maximum boat speed and prop shaft RPM. Compare the result with your GPS speed using the prop-slip calculator to assess propeller efficiency.
RPM
in
Assumes zero prop slip — actual speed is typically 10–25% lower
- 1
Prop shaft RPM = engine RPM ÷ gear ratio
4,500 ÷ 1.85 = 2,432.4 - 2
Speed (mph) = prop RPM × pitch ÷ 1056
2,432.4 × 21 ÷ 1056 = 48.41056 = 5280 ft/mi × 12 in/ft ÷ 60 min/h converts in·RPM to mph.
How does this calculator work?
Theoretical boat speed (mph) = Engine RPM ÷ Gear Ratio × Pitch (in) ÷ 1056. A 21-inch prop at 4500 RPM through a 1.85 gear gives prop shaft RPM of 2432 and theoretical speed ≈ 48 mph. Actual GPS speed will be 10–25% lower due to prop slip.
Formula
How this is calculated
A marine propeller converts engine rotation into forward thrust. The "pitch" number stamped on the hub is the theoretical distance in inches the propeller would advance through the water in one complete revolution if there were no slipping — like a screw advancing into a solid material.
The gear ratio relates engine shaft speed to propeller shaft speed. A ratio of 1.85:1 means the engine turns 1.85 times for every single propeller turn, so prop RPM = engine RPM ÷ gear ratio. Multiplying prop RPM by pitch in inches gives theoretical inches per minute; dividing by 1056 — which equals 5280 ft/mile × 12 in/ft ÷ 60 min/h — converts the result to miles per hour. Knots follow by dividing by 1.151; km/h by multiplying by 1.609.
The result is a theoretical upper limit assuming zero slip. Real-world propeller slip reduces actual speed by 10–25% for typical planing hulls, and 30–35% for pontoons and displacement hulls. The curve on the chart shows how theoretical speed scales linearly with RPM, letting you read off the theoretical speed for any RPM in the operating range.
Frequently asked questions
Pitch is the theoretical distance a propeller would advance in one revolution with no slippage — like a screw in a solid. A 21-inch pitch prop theoretically moves the boat 21 inches per revolution. Higher pitch means more distance per turn but demands more engine torque to reach rated RPM.
Your engine should reach its maximum rated RPM at wide-open throttle with a full load. If RPM falls below the rated range the pitch is too high; if RPM exceeds the range the pitch is too low. Adjust in 2-inch increments and re-test with a tachometer until WOT RPM lands in the middle of the rated band.
The gear ratio appears in the engine or outboard service manual and is often labelled on a plate on the lower unit. Common outboard ratios run from 1.85:1 to 2.33:1; inboard and sterndrive ratios vary more widely. Check the manufacturer spec sheet for your exact model and model year.
Also known as
TG we-Calculate Editorial Team. (2026). Propeller Pitch Speed Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/prop-pitch-calculator
TG we-Calculate Editorial Team. "Propeller Pitch Speed Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/prop-pitch-calculator.
TG we-Calculate Editorial Team, "Propeller Pitch Speed Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/prop-pitch-calculator
@misc{wecalculate_prop_pitch_calculator, title = {Propeller Pitch Speed Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/prop-pitch-calculator}}, year = {2026}, note = {TG we-Calculate} }
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