Find optimal propeller size for your motor KV and battery voltage. Estimates thrust, power, and efficiency.
Recommended Prop Size
3" prop (3×2 typical pitch)
Unloaded RPM
At 14.8V (4S)
Thrust @ 100%
Thrust @ 75%
Thrust @ 50%
Approximate hover thrust for most builds
Power @ 100%
Current @ 100%
Within motor rating of 30A
Efficiency @ 100%
Moderate — typical for high-performance builds
Motor KV is the velocity constant — RPM per volt of applied voltage under no load. It is the single most important factor in prop selection. The relationship is simple: RPM = KV × Voltage. A 2400KV motor on 4S (14.8V) spins at approximately 35,520 RPM unloaded. Under load with a propeller attached, actual RPM drops 10–20% depending on the aerodynamic drag of the propeller.
Every propeller has a maximum safe tip speed, which translates directly to a maximum RPM for a given diameter. Exceed that limit and the prop risks catastrophic failure — carbon fibre props can shatter into high-velocity shards at unsafe RPM. The prop-matching calculation is therefore fundamentally a safety constraint: find the largest prop where the motor's RPM stays within the prop's structural limits.
Larger props are always more efficient at generating thrust — they move a larger mass of air at lower velocity, which requires less kinetic energy per gram of thrust. The trade-off is that large props require low KV motors to keep RPM safe, and low KV motors have less torque bandwidth for rapid throttle changes. Racing pilots use small high-RPM props precisely because fast throttle response matters more than hover efficiency.
Diameter
The total tip-to-tip span in inches. Larger diameter moves more air per revolution and generates more thrust at the same RPM, but requires more torque and limits maximum RPM due to tip-speed constraints. Frame size directly limits maximum prop diameter — a 5-inch frame physically cannot fit a 7-inch prop.
Pitch
The theoretical advance per revolution in inches, as if the prop were a screw cutting through a solid medium. A 5×3 prop has a 5-inch diameter and 3-inch pitch. Higher pitch increases top-end air velocity and speed potential but requires more torque and current. Lower pitch favours hover efficiency and acceleration over maximum speed.
Blade Count
2-blade props have the highest efficiency — each blade operates in less disturbed air. 3-blade props generate more thrust from the same diameter by moving more air per revolution, at the cost of 5–15% more current draw. 4-blade and tri-blade props are used in applications where minimum prop diameter is constrained but maximum thrust is needed. Most FPV freestyle builds use 3-blade 5-inch props as a practical balance.
Prop notation combines diameter and pitch: a 5140 prop is 5.1 inches diameter with 4.0 inches pitch. Some manufacturers add blade count: 5140-3 is the tri-blade version. The tool models typical pitch ratios for each diameter class — actual performance varies by specific prop model.
Frame size dictates maximum prop diameter, which determines the KV range you need. The table below shows common pairings used in production builds:
| Frame / Use | Prop Size | Voltage | KV Range | Notes |
|---|---|---|---|---|
| Micro Whoop (65–75mm) | 2–3" | 1S | 15000–25000KV | Indoor acro |
| Toothpick / 3" Cinewhoop | 3" | 2S–3S | 4000–6000KV | Ultra-light builds |
| 4" Freestyle | 4" | 3S–4S | 2400–3000KV | Sub-250g builds |
| 5" FPV Freestyle | 5" / 5.1" | 4S | 2000–2700KV | Most popular class |
| 5" FPV Freestyle (6S) | 5" / 5.1" | 6S | 1600–2000KV | Higher efficiency |
| 7" Long Range | 7" | 4S–6S | 1200–1800KV | Efficiency priority |
| 10" Mapping / Survey | 10" | 4S–6S | 700–1100KV | Heavy payload |
| 12–15" Heavy Lift | 12–15" | 6S–8S | 300–600KV | Commercial payload |
Propeller efficiency is measured in grams of thrust per watt of electrical power consumed (g/W). Larger, slower-spinning props are dramatically more efficient than small, fast-spinning ones. A 15-inch prop at hover may achieve 12–15 g/W, while a 3-inch racing prop at the same thrust might only manage 3–5 g/W. This is why long-range builds use large, low-KV setups: every watt of battery energy stretches further.
The Actuator Disk Model
Momentum theory explains the efficiency gap: thrust equals the rate of change of momentum of the air. Moving a large mass of air slowly (big prop) requires less kinetic energy than accelerating a small mass rapidly (small prop) to produce the same thrust. The induced power scales as T^(3/2) / (2ρA)^(1/2), where A is the disk area. Doubling prop diameter quadruples disk area, roughly halving the power needed at the same thrust.
Throttle and Efficiency
Efficiency is highest near hover throttle (typically 30–50%) and decreases at both extremes. At very low throttle the motor becomes inefficient due to switching losses and poor magnetic flux utilisation. At 100% throttle, the high current and voltage drop across motor windings reduces efficiency. Most motors achieve peak g/W efficiency at 40–60% throttle — which is why hover efficiency dominates flight time more than maximum power.
Over-propping
Using a prop too large for the motor increases current draw dramatically. The motor overheats, ESC thermal-limits, and efficiency actually decreases. Always verify that estimated current is within the motor's continuous rating.
Under-propping
A prop too small wastes motor capability — you get high RPM but little thrust. The motor runs at high speed with minimal load, which also reduces efficiency. Match prop diameter to the motor's rated optimal range.
Ignoring voltage
Switching from 4S to 6S without changing the motor raises RPM by 50%. The same prop that was fine on 4S may now exceed its safe RPM on 6S. Always recalculate when changing battery voltage.
Trusting unloaded RPM
The RPM shown here is unloaded (no prop). Under load with a propeller, RPM drops 10–20%. Thrust calculations already account for this through empirical coefficients, but be aware that your flight controller's telemetry will show actual loaded RPM which will be lower.