Prop Pitch Calculator (propeller pitch from speed, rpm, gear ratio & slip)

Understanding the propeller pitch calculator — concept and principle

Choosing a boat propeller starts with pitch: how far the propeller would move forward in one turn if it did not slip. This calculator finds the pitch needed to reach a target top speed at a given engine rpm, gear ratio and expected slip, rounds it to a whole-inch propeller and shows the speed that pitch would give.

The prop must advance 19.2 in per turn, allowing 10 % slip.

How it is calculated

Pitch (in) = speed (mph) × 1,056 × gear ratio ÷ (rpm × (1 − slip))

Propeller rpm = engine rpm ÷ gear ratio

Speed with a given pitch = pitch × propeller rpm ÷ 1,056 × (1 − slip)

1,056 converts mph to inches per minute: 63,360 in per mile ÷ 60 min.
Speeds in km/h or knots are converted to mph (1 mile = 1.609344 km, 1 knot = 1.852 km/h).

The principle

Pitch is like the thread of a screw: one revolution would advance the propeller by its pitch in a solid medium. Multiply pitch by propeller revolutions per minute and you get the theoretical forward travel per minute. Water is not solid, so the propeller slips; some slip is necessary to create thrust. The actual speed is the theoretical speed reduced by the slip fraction. Rearranging gives the pitch needed for a target speed.

Worked example

Default inputs: target 45 mph, 5,500 rpm, gear ratio 2.0, slip 10%.

Propeller rpm = 5,500 ÷ 2.0 = 2,750.
Pitch = 45 × 1,056 × 2.0 ÷ (5,500 × 0.90) = 95,040 ÷ 4,950 = 19.2 in = 488 mm.
Nearest whole-inch pitch = 19 in.
Speed with a 19-in prop = 19 × 2,750 ÷ 1,056 × 0.90 = 44.5 mph.
Target speed = 45 mph = 72.4 km/h.

Each point is the calculator re-run with only “Engine rpm at wide-open throttle” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Engine rpm at wide-open throttle	Propeller pitch needed (in)
1,375	76.8
2,750	38.4
4,125	25.6
5,500	19.2
6,875	15.4
8,250	12.8
11,000	9.6

How to read the result

Pitch and engine rpm work against each other: more pitch gives more speed per revolution but makes the engine work harder, lowering rpm. As a rough guide, many boaters find that each inch of pitch changes full-throttle rpm by around 150–200 rpm, but this varies. Always check that the engine still reaches the maker's recommended wide-open-throttle rpm range with the new propeller; running below it can overload the engine.

Common mistakes

Using crankshaft rpm without the gear ratio.
Assuming zero slip.
Choosing pitch for speed alone and ignoring the engine's rpm range.
Mixing mph, km/h and knots.

Keep in mind: This is a starting point. Propeller selection also depends on diameter, blade design, load and the engine maker's rpm range; consult a marine dealer before buying.

Frequently asked questions

What does a 19-inch pitch mean?

In one revolution, without slip, the propeller would move 19 inches forward.

How much slip is normal?

It varies; light, fast boats usually have less slip than heavy or slow ones.

Does more pitch always mean more speed?

Only if the engine can still reach its rated rpm; otherwise speed can drop.

Where do I find the gear ratio?

In the engine or outdrive manual, often written like 2.00:1.

Sources: Geometric pitch definition (forward travel per revolution); 1 mile = 63,360 in, 1.609344 km; 1 knot = 1.852 km/h

Checked 9 Nov 2026. Geometric pitch: forward travel per revolution (definition); 1 mile = 63,360 in, 1 mile = 1.609344 km, 1 knot = 1.852 km/h (standard values).

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