How to size a drone propeller: diameter and pitch

By Keayvan Keramati · ~6 min read

Picking a propeller is one of the first real decisions in a drone or UAV design, and it drives everything downstream — thrust, top speed, efficiency and battery size. The good news is that a solid first estimate needs only three inputs: your target airspeed, the thrust you need per propeller, and the motor RPM. Here is a clean, physics-based way to turn those into a diameter and a pitch.

Diameter comes from thrust

A propeller's static thrust follows the thrust-coefficient relation, where n is revolutions per second (RPM ÷ 60), ρ is air density and C_T is a non-dimensional thrust coefficient (typically 0.08–0.15 for common props):

T = C_T · ρ · n² · D⁴

Propeller type	Typical C_T
Aircraft propellers	0.02–0.20
Multirotor/drone propellers (hover)	0.08–0.15
Marine propellers	0.10–0.50
High-loaded marine propellers	Up to 0.8 (rare)

Rearranging for the diameter D you need to hit a required thrust T:

D = ( T / (C_T · ρ · n²) )^(1/4)

Because thrust scales with D⁴, small changes in diameter make a big difference — and a bigger disc is dramatically more efficient (a point we come back to in the battery article).

Pitch comes from speed and slip

Pitch is the theoretical distance the prop would advance in one revolution, like a screw in wood. Multiply by rev/s and you get the pitch speed — the ideal no-slip airspeed the prop is "geared" for:

V_pitch = pitch · n

Real props always "slip", so the drone cruises below the pitch speed. If slip is the fraction lost (0.2–0.3 is typical), then your wanted airspeed V and the required pitch are:

V_pitch = V / (1 − slip) → pitch = V_pitch / n

The key rule: the wanted speed must sit safely below the pitch speed. As the drone accelerates toward pitch speed, thrust falls to zero — see static vs dynamic thrust.

Two ratios worth checking

Pitch/Diameter (P/D): ≈ 0.3–0.8 for efficient "cruise" props; above 1.0 is an aggressive high-speed prop.
Advance ratio J = V / (n·D): a compact way to describe the operating point; P/D = J / (1 − slip).

Don't break the tip-speed limit

The blade tips move fast. Keep the tip Mach number below ~0.7 to avoid compressibility losses and noise:

V_tip = π · D · n (keep V_tip / 343 < 0.7)

This is why very high RPM pushes you to smaller diameters, and why big slow props are quiet and efficient.

Worked example

Suppose you want 200 km/h (55.6 m/s), need 50 N per prop, spin at 5000 RPM (n = 83.3 rev/s), with C_T = 0.10, ρ = 1.225 and slip = 0.25:

D = (50 / (0.10·1.225·83.3²))^¼ ≈ 0.49 m (≈19 in)
V_pitch = 55.6 / 0.75 ≈ 74 m/s → pitch = 74 / 83.3 ≈ 0.89 m (≈35 in)

That is a huge, high-pitch prop — a signal the RPM is too low for the target speed. Raise the RPM and both diameter and pitch shrink into normal ranges (D ∝ RPM^−½, pitch ∝ RPM⁻¹). Iterating this RPM trade is exactly what the free tool visualises.

Try it on your own numbers

The Propeller dashboard in the free tool computes diameter, pitch, P/D, advance ratio and tip Mach live, and plots how they change with RPM. Need detailed prop/duct CFD for a real design? Get in touch.

Note: this is a preliminary, static-C_T model. For final design, replace C_T with a measured prop map or blade-element/CFD data.