Blade Pitch Calculator: Set Your Prop Angle

A blade pitch calculator works out the theoretical forward travel of a propeller or rotor per revolution, based on the blade angle and diameter you enter. You input the blade pitch angle in degrees and the propeller diameter, and it returns the pitch distance in inches or millimeters, plus the estimated pitch speed at a given RPM. This is the same geometry used by model aircraft pilots, drone builders, and boat propellers to match a prop to a motor and expected speed.

Formula

The basic blade pitch formula comes from the geometry of a helix. If a propeller blade is set at a constant angle, each revolution advances the prop forward by a distance equal to the circumference of the circle it sweeps, multiplied by the tangent of the blade angle.

Pitch = π × D × tan(θ)

• Pitch – the theoretical forward distance per revolution (inches, mm, or meters)
• D – propeller diameter (same unit as pitch)
• θ – blade pitch angle measured at a reference point, usually 75% of the blade radius from the hub (degrees)

For a boat or aircraft propeller, the blade angle varies along the length, so manufacturers specify the angle at a standard station (often 75% of the radius). The calculator above assumes a constant angle, which is a simplification that works for basic sizing and for many small fixed-pitch props.

Worked Example

Using the default values from the calculator: blade angle = 30°, diameter = 10 inches, RPM = 10,000.

1. Convert the angle to radians: 30° × (π / 180) = 0.5236 radians
2. Find the tangent: tan(0.5236) = 0.5774
3. Multiply by π and diameter: π × 10 × 0.5774 = 18.14 inches
4. Theoretical pitch speed: (18.14 / 12) × 10,000 × 60 / 5280 = 171.7 mph

The calculator shows a pitch of 18.14 inches and a pitch speed of 171.7 mph for those inputs.

Why Blade Pitch Matters

Blade pitch is the single most important factor in matching a propeller to a motor and a vehicle. Too low a pitch gives good acceleration but low top speed; too high a pitch gives high top speed but poor acceleration and may overload the motor. For aircraft, the pitch also determines the angle of attack of the blade, which affects efficiency and stall behavior. For boats, pitch is measured in inches of travel per revolution, and a prop with too much pitch can cause the engine to lug or overheat.

In variable-pitch systems (like helicopter rotors or constant-speed aircraft props), the pilot or flight controller adjusts the blade angle to keep the engine at an efficient RPM. The calculator helps you understand the relationship between angle and travel, even if the actual blade has twist.

How to Choose a Propeller Pitch

Choosing a pitch is a balance between speed and thrust. Here are the practical steps most hobbyists and engineers use:

• Determine the desired top speed and the motor’s maximum RPM under load (usually 70–80% of no-load RPM).
• Calculate the pitch needed using the formula: Pitch (inches) = (Speed in mph × 1052) / RPM. The constant 1052 converts mph to inches per minute and accounts for slip.
• For multirotor drones, start with a pitch that is 50–70% of the propeller diameter; a 10-inch prop often uses 4.5–5.5 inches of pitch.
• For boats, use a pitch that is 1.5 to 2 times the propeller diameter for a typical recreational prop, but always check the engine manufacturer’s recommended range.
• Test with a watt meter or current logger to ensure the motor is not drawing more current than its rating.

Common Mistakes and Misconceptions

One common error is mixing units. If you measure diameter in inches but want pitch in millimeters, you must convert first. Another is using the angle at the tip instead of the reference station; most props have significant twist, so the angle at 75% radius is the standard. Many people also forget that pitch speed is theoretical – real props have slip (air or water) of 10–30%, so actual speed is lower. Finally, a higher pitch does not always mean more speed; if the motor cannot spin the prop fast enough, the vehicle may be slower.

Pitch vs. Diameter: What’s the Difference?

Diameter and pitch are independent but related. Diameter affects the amount of air or water the prop can move (thrust), while pitch affects how far it moves per revolution (speed). A large diameter with low pitch gives strong thrust for heavy loads, like a tugboat or a cargo drone. A small diameter with high pitch gives high speed but less thrust, like a racing plane. The calculator above treats them separately, but in practice they are chosen together to match the power available.

Safety and Limits of the Formula

The formula assumes a flat blade with no twist and no slip. Real propellers are twisted so that each section operates at an efficient angle of attack. The result is a theoretical maximum, not a guarantee. When testing a prop, always check the motor temperature and current draw – an oversized prop can burn out a motor or ESC. For aircraft, never fly with a prop that is not rated for the RPM you plan to run; propeller failure at high speed is dangerous. For boats, ensure the prop is clear of people and debris before starting the engine. Always follow the motor and vehicle manufacturer’s recommendations for prop size and pitch.

Alternatives to a Pitch Calculator

If you don’t have the blade angle, you can measure the pitch directly with a pitch gauge, which is a tool that fits over the blade and reads the angle at a specific radius. Many online calculators also let you input the pitch in inches directly, without the angle. For model aircraft, you can often find the pitch printed on the prop (e.g., “10×4.5� means 10 inches diameter and 4.5 inches pitch). For custom props, you can use a 3D scanner or a protractor to measure the angle. The formula above is the basis for all these methods.

Frequently Asked Questions

What is the difference between pitch and pitch speed?

Pitch is the theoretical distance the prop advances in one revolution, measured in inches or millimeters. Pitch speed is the forward speed that distance would produce at a given RPM, usually expressed in mph or km/h. Pitch speed is a theoretical maximum; real speed is lower due to slip.

How do I measure the blade angle of a prop?

Measure the angle at a point 75% of the blade radius from the hub. Use a protractor or a pitch gauge. Place the prop flat on a table, set the gauge at the reference point, and read the angle. For twisted blades, this is the standard reference because it represents the average effective angle.

Can I use this calculator for boat props?

Yes, the same geometry applies. Boat props are often specified by pitch in inches, and the formula works the same. However, water is denser than air, so slip is lower, but the theoretical pitch speed is still a good starting point for estimating top speed.

Why does my actual speed not match the calculated pitch speed?

Slip is the main reason. Air and water are not solid, so the prop does not advance the full theoretical distance. Typical slip is 10–20% for aircraft and 15–30% for boats. Other factors include drag, hull or fuselage shape, and motor efficiency.

What happens if I use a prop with too much pitch?

Too much pitch can overload the motor, causing excessive current draw, overheating, and possible failure. It also reduces acceleration and may cause the motor to stall or the boat to cavitate. Always check the motor’s maximum current and temperature during testing.

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in a | Blade angle | degrees | 30 | 0
in d | Propeller diameter | inches | 10 | 0
in rpm | Rotational speed | RPM | 10000 | 0
out pitch | Pitch distance | inches | d * 3.14159265 * tan(a * 3.14159265 / 180) | 2
out pitch_speed | Theoretical pitch speed | mph | (pitch / 12) * rpm * 60 / 5280 | 1