liminfoPropeller Calculator
Free web tool: Propeller Calculator
Speed = Pitch × RPM / 1056 (mph). Adjust for slip factor.
Theoretical Speed
153.4 mph
Actual Speed (-15% slip)
130.4 mph
Tip Speed
848 ft/s (M0.760)
Speed at Various RPM
| RPM | Theoretical (mph) | Actual (mph) |
|---|---|---|
| 1500 | 85.2 | 72.4 |
| 1800 | 102.3 | 86.9 |
| 2000 | 113.6 | 96.6 |
| 2200 | 125.0 | 106.3 |
| 2400 | 136.4 | 115.9 |
| 2700 | 153.4 | 130.4 |
| 3000 | 170.5 | 144.9 |
About Propeller Calculator
The Propeller Speed Calculator computes the theoretical and actual forward speed of a propeller-driven vehicle from three parameters: propeller diameter (inches), pitch (inches), and RPM. The core formula is theoretical speed = pitch × RPM / 1056, which converts pitch-per-revolution × revolutions-per-minute into miles per hour. Actual speed accounts for propeller slip — the efficiency loss caused by the fluid being accelerated rather than rigidly pushed — and is calculated as theoretical speed × (1 − slip/100). A typical slip value is 10–20% for aircraft and 15–25% for boats.
Propeller pitch is the theoretical distance a propeller would travel forward in one complete revolution if it were moving through a solid medium with no slip, similar to the pitch of a screw thread. A 60-inch pitch propeller at 2700 RPM gives a theoretical speed of 60 × 2700 / 1056 = 153.4 mph. This calculation is widely used by homebuilt aircraft builders, ultralight pilots, UAV designers, and marine engineers when selecting or evaluating a propeller for a specific engine and airframe combination.
Beyond forward speed, the tool also calculates propeller tip speed in feet per second and expresses it as a Mach number (tip speed / 1116 ft/s at sea level standard conditions). Tip speed is critical because when the blade tips approach transonic speeds (Mach 0.85+), efficiency drops sharply and noise increases dramatically. The tool additionally generates a comparison table of theoretical and actual speeds at seven standard RPM values (1500, 1800, 2000, 2200, 2400, 2700, 3000), which is useful when evaluating how speed changes across the operating RPM range.
Key Features
- Calculates theoretical speed using the formula: pitch × RPM / 1056 (mph)
- Calculates actual speed by applying user-specified slip percentage to theoretical speed
- Displays both MPH and KPH for theoretical and actual speed
- Computes blade tip speed in ft/s and converts to Mach number at sea-level standard conditions
- Generates a 7-row RPM comparison table (1500, 1800, 2000, 2200, 2400, 2700, 3000 RPM)
- Accepts diameter in inches for tip speed calculation (tip speed = π × D(ft) × RPM / 60)
- Pre-filled defaults (72 in diameter, 60 in pitch, 2700 RPM, 15% slip) for immediate results
- 100% client-side processing — no server calls, works on all devices
Frequently Asked Questions
What is propeller pitch and how is it measured?
Propeller pitch is the theoretical distance a propeller advances in one full rotation, assuming no slip — analogous to the pitch of a screw thread. It is measured in inches for aviation and boating applications. A 60-inch pitch propeller advances 60 inches (5 feet) per revolution in theory. Actual advance is less due to slip.
What is the formula for propeller speed?
The standard formula is: Theoretical Speed (mph) = Pitch (inches) × RPM / 1056. The constant 1056 comes from 12 inches/foot × 5280 feet/mile ÷ 60 minutes/hour = 1056 in/(mph·RPM). Actual speed = theoretical speed × (1 − slip fraction).
What slip percentage should I use?
Slip depends heavily on the vehicle and conditions. For light aircraft (cruise flight), 10–15% is typical. For ultralights and trikes, 15–20%. For boats in calm water, 10–20%. For boats at high throttle or in rough water, 20–30%. Higher slip means the propeller is less efficient at converting rotation into forward motion.
Why is tip speed important?
When propeller blade tips approach the speed of sound (Mach 1 = 1116 ft/s at sea level), compressibility effects create a dramatic increase in drag, noise, and vibration. Most aircraft designers aim to keep tip speed below Mach 0.85 (about 949 ft/s). The tool flags tip speed as a Mach number so you can quickly check whether your combination is within acceptable limits.
What is the difference between diameter and pitch for a propeller spec like "72×60"?
Propeller specs are always written as diameter × pitch, both in inches. A 72×60 propeller has a 72-inch (6-foot) diameter and a 60-inch pitch. The diameter determines the disk area swept and contributes to tip speed, while the pitch determines the theoretical advance per revolution.
Can this calculator be used for boat propellers?
Yes. The formula pitch × RPM / 1056 is universal for any propeller in any medium. For marine applications, typical slip values are slightly higher (15–25%) than aviation due to water density. The MPH and KPH outputs correspond to boat speed through the water. The Mach number calculation assumes air and is not meaningful for underwater tip speed.
How do I use this calculator to select a propeller for my engine?
First, determine your engine's cruise RPM. Enter different pitch values (keeping diameter within your airframe/hull clearance limits) and observe which combination gives your target cruise speed at that RPM with a realistic slip percentage. Then check that the tip speed stays below Mach 0.85–0.9 to ensure acceptable noise and efficiency.
Why is the constant 1056 used in the formula?
1056 is a unit conversion constant that converts (inches × RPM) to miles per hour: 1056 = 12 in/ft × 5280 ft/mi ÷ 60 min/hr. Breaking it down: pitch in inches × RPM gives inches per minute. Dividing by 12 converts to feet per minute, then by 5280 to miles per minute, then multiplying by 60 gives miles per hour. 12 × 5280 / 60 = 1056.