Pitch Calculator Sheet: How to Use It

A pitch calculator sheet is a structured worksheet, often a spreadsheet, that lets you enter gear or propeller dimensions and instantly get the corresponding pitch, speed, or ratio. You input values like diameter, pitch, RPM, and gear ratio, and the sheet outputs the calculated result, saving you from manual math. This article explains what these sheets contain, how to use them, and the formulas behind them.

Formula

The core formula for theoretical speed from pitch and RPM is:

Speed (mph) = (RPM / Gear Ratio) × Pitch (inches) / 1056

Where:

• RPM – propeller or wheel rotations per minute (rev/min)
• Gear Ratio – reduction ratio (e.g., 1.5 means the output shaft turns 1.5 times slower than the motor)
• Pitch – the theoretical forward distance per revolution, in inches
• 1056 – conversion factor: (12 inches/ft × 5280 ft/mile) / 60 min/hr = 1056

The result is the theoretical speed assuming zero slip – the maximum possible if the propeller or wheel “screwed� through a solid medium. Real-world values are lower due to slip (air or water yielding).

Worked Example

Using the default values from the calculator: diameter = 10 in, pitch = 12 in, RPM = 3000, gear ratio = 1.5.

1. First, adjust RPM by gear ratio: 3000 / 1.5 = 2000 RPM (effective output RPM).
2. Multiply by pitch: 2000 × 12 = 24,000 inches per minute.
3. Convert to miles per hour: 24,000 / 1056 = 22.7 mph.

The calculator outputs 22.7 mph, matching the manual calculation.

What Is a Pitch Calculator Sheet?

A pitch calculator sheet is typically an Excel or Google Sheets template that automates the pitch-speed calculation. It usually includes input cells for diameter, pitch, RPM, and gear ratio, and output cells for speed, sometimes also showing propeller slip percentage or boat speed. Many model airplane and drone enthusiasts use these sheets to compare propellers before testing. The sheet may also include a table of common propeller sizes and their expected performance.

These sheets are popular because they let you quickly test “what-if� scenarios: change the pitch or RPM and see the speed change instantly. They also reduce unit errors by embedding conversion factors. Some advanced sheets include a slip factor input to estimate real speed more accurately.

How to Choose the Right Pitch

Selecting the correct pitch depends on your application:

• Boats: Lower pitch gives faster acceleration and better towing, higher pitch gives more top speed. A good rule: pitch should be about 1.5 to 2 times the diameter for average recreational boats.
• Drones/RC aircraft: Higher pitch increases speed but draws more current and may overheat the motor. Start with a pitch that matches the motor’s KV rating and battery voltage.
• Bicycles: Gear pitch is analogous to gear inches – larger pitch (or gear ratio) means more speed per pedal, but harder pedaling.

Always check the manufacturer’s recommended range for your motor or engine. Using too high a pitch can overload the system.

Common Mistakes and Misconceptions

One common error is mixing units: using pitch in centimeters but RPM in minutes, or forgetting to convert inches to feet. Another is ignoring gear ratio – if your motor spins 3000 RPM but the propeller shaft only spins 2000 RPM (via a reduction gear), you must divide by the ratio. A frequent misconception is that pitch speed equals actual speed. In reality, propellers operate at 70-90% efficiency in air, and boat propellers have even more slip (10-30%). The pitch speed is a theoretical ceiling, not a prediction.

Also, diameter does not appear in the speed formula – it only affects thrust and efficiency. Many people mistakenly think a larger diameter automatically gives more speed; it gives more thrust, but may reduce top speed if the pitch is too low.

Limits of the Formula

The pitch-speed formula assumes no slip and no losses. In real conditions, air or water yields, so actual speed is lower. For boats, hull drag, water density, and propeller cavitation affect speed. For aircraft, air density and drag matter. The formula also ignores the blade shape, number of blades, and pitch distribution (constant vs. variable). It is a first-order approximation useful for comparisons, not for exact predictions.

For more accuracy, you need a slip factor. For example, if you know your boat achieves 85% of theoretical speed, multiply the result by 0.85. Some advanced sheets include a slip input. But without empirical data, the theoretical value is the best you can do.

Alternatives to a Spreadsheet

If you don’t want to build your own sheet, you can use online calculators (like the one on this page) that do the same math. Many RC and boating forums have downloadable pitch-speed charts. There are also mobile apps for prop selection. However, a spreadsheet gives you full control and lets you store multiple configurations. If you prefer manual calculation, the formula above is simple enough for a pocket calculator.

For boaters, a more accurate method is to use a GPS speedometer and tachometer to measure actual speed and RPM, then back-calculate slip. That empirical data is more valuable than any theoretical sheet.

Frequently Asked Questions

What is the difference between pitch and diameter?

Pitch is the theoretical distance a propeller or wheel advances in one revolution (like the thread pitch of a screw). Diameter is the width of the propeller’s circle. Pitch affects speed, diameter affects thrust and load. Both are usually stamped on the propeller (e.g., 10×12 means 10-inch diameter, 12-inch pitch).

How do I measure pitch if it’s not marked?

You can measure pitch by placing the propeller flat on a table, measuring the rise of the blade over a known chord length, then using the formula: pitch = (rise / chord length) × (2 × π × radius). This is approximate because pitch can vary along the blade. For a precise measurement, use a pitch gauge.

Does gear ratio affect pitch speed?

Yes, because gear ratio changes the effective RPM at the propeller. If your motor turns 3000 RPM but the gearbox reduces it by 1.5:1, the propeller only turns 2000 RPM, so pitch speed is lower. Always divide motor RPM by the gear ratio before calculating speed.

Can I use this calculator for car tires?

Yes, the same formula applies to any wheel that “screws� through a medium. For car tires, pitch is the circumference (π × diameter), and gear ratio is the transmission/differential ratio. But tire slip and deformation mean actual speed is lower. For cars, it’s more common to use tire circumference directly.

What is a good slip percentage for a boat?

For a planing boat, slip is typically 10-20%. For a displacement hull, it can be 30-40%. If your calculated slip is over 50%, you likely have the wrong propeller or a measurement error. Slip = (1 – (actual speed / theoretical speed)) × 100%.

Why does the calculator not ask for diameter?

Diameter does not appear in the pitch-speed formula because it doesn’t affect speed directly – only thrust and efficiency. That’s why the calculator only needs pitch, RPM, and gear ratio. Diameter is still important for matching the propeller to the engine’s power, but it’s not part of the speed equation.

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in d | Diameter | in | 10 | 0
in p | Pitch | in | 12 | 0
in rpm | Rotations per minute | rpm | 3000 | 0
in gr | Gear ratio | - | 1.5 | 0.1
out speed | Theoretical speed | mph | (rpm / gr) * p / 1056 | 1
out pitch_speed | Pitch speed | mph | (rpm / gr) * p / 1056 | 1