Free Prop Pitch Calculator

What Propeller Pitch and Diameter Mean

Boat propellers are specified by two main dimensions: diameter and pitch. The diameter is the full width of the circle traced by the blade tips; a larger diameter displaces more water and is often chosen for heavy loads or low‑speed handling. The pitch is the theoretical distance the propeller would advance in one complete revolution if it were moving through a solid medium. For example, a propeller marked “13″×16″” has a diameter of 13 inches and a pitch of 16 inches, meaning it would theoretically travel 16 inches per revolution.

High‑pitch propellers cover more ground per turn and therefore increase top‑end speed. Low‑pitch propellers travel less distance per revolution but deliver stronger acceleration and a better “hole shot.” There is also a trade‑off with diameter: larger diameters generally pair with smaller pitches to favour low‑end power, whereas smaller diameters allow larger pitches for higher top speeds. When you use a boat propeller pitch calculator, understanding this relationship helps you pick the right propeller for your needs.

The Role of Propeller Slip

In real water, the propeller never reaches its full theoretical advance because the blades slip through the fluid. This propeller slip is expressed as a percentage of the theoretical travel. At wide‑open throttle, slip typically falls between 10 % and 20 %; it is higher at lower speeds. Slip must be factored into any conversion between pitch and speed, and the prop pitch to speed calculator accounts for it automatically.

Core Formulas for Pitch and Speed

Let:

• $P$ = propeller pitch (inches)
• $V$ = boat speed (in the chosen unit)
• $R$ = engine crankshaft RPM
• $G$ = gear ratio (number of engine revolutions per propeller revolution; e.g., 2:1 is written as 2)
• $S$ = slip (as a decimal, e.g., 0.10 for 10 %)
• $C$ = conversion constant that depends on the speed unit (see table below)

The fundamental relationship is:

$$V = \frac{P \times R \times (1 - S)}{G \times C}$$

When speed is known and pitch is unknown, the equation can be rearranged:

$$P = \frac{V \times G \times C}{R \times (1 - S)}$$

Speed unit	Conversion constant $C$
mph	1056
kph	656
knots	1215.2

These constants keep the pitch result in inches while using the corresponding speed unit. Whether you are planning a new setup or checking an existing one, the marine propeller calculator applies these formulas instantly.

Step‑by‑Step Procedures

To estimate boat speed from a known prop pitch

1. Multiply the pitch by engine RPM.
2. Multiply that result by $(1 - S)$, where $S$ is the decimal slip.
3. Divide by the gear ratio.
4. Divide by the appropriate conversion constant $C$ to obtain speed in mph, kph, or knots.

To determine the prop pitch needed for a target speed

1. Multiply the target speed by the gear ratio and then by $C$.
2. Divide that value by engine RPM.
3. Divide by $(1 - S)$.

These steps are handled behind the scenes by the boat speed calculator inside the tool, but knowing them helps you verify the output and understand the effect of each variable.

Using the Propeller Pitch Calculator

This tool is designed for bidirectional use. You can enter any combination of speed, gear ratio, RPM, and slip, and it will return the missing variable. For example, if you have an existing propeller and want to know its effective pitch, input the measured speed, gear ratio, RPM, and slip; the calculator outputs the pitch. Conversely, if you are evaluating a new propeller and want to predict the speed it will deliver, input the pitch instead of speed. All values can be entered in the preferred unit (mph, kph, or knots), and the correct conversion constant is applied internally.

The marine propeller calculator eliminates manual formula work and provides rapid answers for both planning new engine‑prop combinations and troubleshooting current performance.