Prop Pitch Calculator

Written by Great Medico

Updated September 25, 2026 •2 min read

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A prop pitch result can look precise while still being wrong if one unit or convention is off. Mechanical design calculations become much more useful when you can see the exact ideal relationship behind the headline number and where real hardware can depart from it. This page keeps the calculation focused so the number is easy to trace back to the values you entered.

What this calculator does

This calculator isolates the relationship between Boat speed, Gear ratio, Engine/crankcase speed and propeller pitch. That makes it easier to see which input is controlling the result and to distinguish the headline quantity from secondary values shown underneath.

How to use it

For a clean calculation, enter Boat speed, Gear ratio, Engine/crankcase speed, Propeller slip exactly as defined by the labels and their units. Check the headline value, then scan the supporting metrics for a relationship that should obviously increase or decrease with one input; this is a fast way to catch an entry mistake.

How the calculation works

Propeller RPM is engine RPM/gear ratio. The page rearranges the ideal pitch-speed relationship for pitch while applying the entered slip fraction, so higher boat speed or gear ratio raises required pitch, while higher engine RPM lowers it for the same target speed.

Example

Using the default example on the page (Boat speed = 30 mph; Gear ratio = 2; Engine/crankcase speed = 5000 rpm; Propeller slip = 10 %), the calculator returns propeller pitch of 14.08 in. A modest change to one driving input should move the answer in the direction predicted by the equation, which is a useful unit check.

How to interpret the result

Read the propeller pitch as an ideal mechanical estimate for the entered geometry, speed, load, ratio, or material property. Keep the displayed unit attached and use any supporting values to confirm that ratios and directions are internally consistent.

Limitations and notes

This is an ideal advance relationship corrected by one entered slip percentage. Real propeller pitch selection also depends on diameter, blade count/area, engine load curve, hull resistance, ventilation/cavitation, water conditions, and whether the stated pitch is geometric or effective.