RPM to Speed Calculator
Convert engine RPM to boat speed based on gear ratio and propeller specifications.
How to calculate boat speed from RPM
Your boat's speed at a given engine RPM is set by three things: the gear (reduction) ratio in the gearbox or lower unit, the propeller pitch, and how much the prop slips through the water. This calculator combines all three to estimate real boat speed in knots, rather than just a theoretical maximum.
The RPM-to-speed formula
First convert engine RPM to propeller RPM, then to distance travelled per minute:
Speed (knots) = (Prop RPM × Pitch in inches × (1 − slip)) ÷ 1056
where Prop RPM = Engine RPM ÷ Gear ratio. The constant 1056 converts inches per minute into nautical miles per hour. For statute miles per hour, swap 1056 for 1012.
Worked example
A 5,000 RPM outboard with a 2.0:1 gear ratio and a 19" prop: prop RPM = 2,500. Theoretical speed = (2,500 × 19) ÷ 1056 ≈ 45 knots. Apply 15% slip and the realistic speed is about 38 knots.
What is propeller slip?
Slip is the gap between the distance a prop should travel in one rotation (its pitch) and the distance it actually moves the boat — the prop never grips the water perfectly. Most recreational boats run 10–15% slip. Higher slip points to an over-pitched prop, a heavy load, a fouled hull, or the engine mounted too high. To check your own figure, use the propeller slip calculator, and to match the right ratio see the gear ratio calculator.
Frequently Asked Questions
How do you calculate boat speed from RPM?
Boat speed depends on engine RPM, gear ratio, propeller pitch and slip. First find propeller RPM (engine RPM ÷ gear ratio). Theoretical speed in knots = (prop RPM × pitch in inches) ÷ 1056. Then subtract slip (typically 10–15%) to get actual speed. This calculator combines all four inputs to estimate real-world boat speed.
What is the formula for RPM to speed in knots?
Knots = (propeller RPM × pitch in inches × (1 − slip)) ÷ 1056. The constant 1056 converts inches per minute to nautical miles per hour (12 in/ft × 60 min/hr ÷ 6076 ft/NM). For miles per hour, use 1012 instead of 1056.
Why is my actual boat speed lower than the RPM calculation?
The difference is propeller slip — the prop never grips the water perfectly. Most recreational boats see 10–15% slip; heavy or overloaded boats and incorrectly pitched props see more. A fouled hull, trim, and sea state also reduce speed below the theoretical figure.