Propeller Size Calculator
Enter the top speed you want, the engine's maximum rpm, the gearbox ratio and an estimate of prop slip. The calculator gives the pitch you need, the nearest stock pitch and the speed it should reach.
- Pitch from speed
- Gear ratio and slip
- Stock pitch match
Gear ratio is on the outboard's spec sheet, such as 2.0:1. Typical slip is 10% for fast boats and 15% to 20% for heavier ones.
Pitch is how far a propeller would move forward in one turn. A 19 inch pitch prop would travel 19 inches per revolution if the water were solid. In real water it slips a little. Matching pitch to the engine's rated rpm lets the motor reach full power without over-revving.
Propeller pitch chart
2.0 : 1 gear, 10% slip. Search for a speed.
| Speed (mph) | 5,000 rpm | 5,500 rpm | 6,000 rpm |
|---|---|---|---|
| 20 | 9.4 | 8.5 | 7.8 |
| 30 | 14.1 | 12.8 | 11.7 |
| 40 | 18.8 | 17.1 | 15.6 |
| 50 | 23.5 | 21.3 | 19.6 |
| 60 | 28.2 | 25.6 | 23.5 |
Pitch in inches. Speed is roughly in proportion to pitch when the engine still reaches the same rpm.
Speed and pitch
2.0 : 1 gear, 10% slip. Slide the target speed.
The wave on the right shows one turn of the blade tip. Its length is the pitch.
How to work out pitch
Speed, rpm and gear. Step through it.
Target speed
In mph.
1,056 converts miles per hour to inches per minute: 63,360 inches per mile ÷ 60.
Aluminium or stainless prop
Propeller materials. Compare them.
Cheap and bends on impact, protecting the gearbox.
Thinner, stiffer blades for more speed.
Light, with replaceable blades.
Three blades suit most boats. Four blades hold better in turns and rough water but cost a little top speed.
Testing a prop
On the water. Tick each item.
Tick each item as you go.
Test with your usual load. A prop that is right with one person may be too much pitch with a full boat.
What you get
Prop selection. Each tile explains one result.
Inches per turn.
Nearest size.
With that prop.
After the gearbox.
Used in the sum.
Each inch of pitch usually changes full-throttle rpm by about 150 to 200.
Frequently Asked Questions
No question matches that search.
How do I calculate propeller pitch?
Pitch = mph × 1,056 × gear ratio ÷ (rpm × (1 − slip)).
What pitch for 40 mph at 5,500 rpm?
About 17 inches with a 2.0:1 gear and 10% slip.
What is prop slip?
The difference between theoretical and real distance travelled per turn, usually 5% to 20%.
Does more pitch mean more speed?
Only if the engine can still reach its rated rpm.
What happens if pitch is too high?
The engine cannot reach full rpm and works too hard.
How much does one inch of pitch change rpm?
About 150 to 200 rpm.
What size prop diameter do I need?
It is set mostly by the engine. Use the maker's chart for the pitch you choose.
A prop matched to your boat
Work out the pitch for your target speed, pick the nearest stock size and test at full throttle. When the engine hits its rated rpm, you have the right prop.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.