The math first
Revolutions per minute and hertz (cycles per second) describe the same thing on different clocks. Divide RPM by 60 to get Hz; multiply Hz by 60 to get RPM. Two anchors cover most of what you'll meet: 1,800 RPM = 30 Hz and 3,600 RPM = 60 Hz — the two most common motor speed classes in North America.
Method 1 — mark it and count (free)
Stick a piece of tape on the shaft or one blade, run the machine, and count passes against a stopwatch — 30 seconds of counting, doubled, is your RPM. Human counting falls apart above roughly 120 RPM (2 passes per second), so this is for ceiling fans, barbecue rotisseries, and slow rollers, not motors.
Method 2 — strobe apps (free, fiddly)
A strobe app flashes the phone's flashlight at an adjustable rate. Put a contrast mark on the rotating part, dim the room, and tune the flash rate until the mark appears frozen — the flash rate then equals the rotation rate. Two real problems: it's trial and error, and the mark also freezes at half, a third, and double the true speed, so you must sanity-check (double the rate: one frozen mark means you were right; two ghost marks means you were at half). Phone flashlight strobes also top out around 100 Hz — about 6,000 RPM — and get dim near the top.
Method 3 — slow-motion video, counted by hand (free, tedious)
Record the marked part at 240 fps, then step through frames and count how many frames one revolution takes: RPM = 60 × 240 ÷ frames-per-revolution. This is accurate and free, and it's the manual version of what camera measurement automates. Mark one blade first — a symmetric 3-blade fan looks identical after a third of a turn, and you'll count triple the true speed.
Method 4 — audio spectrum (free, ambiguous)
A spectrum-analyzer app on the microphone will show peaks from the machine's hum and blade-pass. It works when you can't even see the shaft — but you have to guess which peak is the shaft rate, which is a blade-pass multiple, and which is electrical hum. Mains interference sits at 50/60 Hz and its harmonics (100/120/180 Hz…) — exactly the range most motors live in — so audio readings need corroboration.
Method 5 — read the frequency straight off the camera
VibraLens ($4.99 one-time, iPhone) automates method 3: point the phone at the running machine and it analyzes 240 fps video on-device, paints a color overlay on whatever moves periodically, and shows the dominant frequency in Hz, live. Multiply by 60 — or use the converter on the home page — and you have RPM. Because the reading is per-location, a belt-driven setup shows the motor and the driven pulley as two different frequencies in the same frame: motor at 28.8 Hz (1,725 RPM) and, say, the blower wheel at whatever the pulley ratio makes it.
Technique: brace the phone, aim at a textured spot on the housing or pulley guard rather than the spinning blades, add light or use the built-in torch — under LED or fluorescent lighting the torch's steady light also keeps 50/60 Hz flicker (which VibraLens filters and flags automatically) from masquerading as a machine reading. Self-test on an electric toothbrush — about 75–80 Hz — before trusting a reading that matters.
Honest limits: the reliable band is ~8–115 Hz, roughly 480–6,900 RPM. Below that (ceiling fans) use methods 1–3; above it you need real instrumentation. It needs light on the target, reads frequency and location only — no amplitude, no severity verdicts — and the phone never touches the machine.
Reference: common motor speeds ↔ Hz
- 2-pole induction motor: 3,600 RPM synchronous / ~3,450 RPM loaded → 60 / 57.5 Hz
- 4-pole: 1,800 RPM / ~1,725 RPM → 30 / 28.8 Hz
- 6-pole: 1,200 RPM / ~1,150 RPM → 20 / 19.2 Hz
- Ceiling fans: roughly 80–300 RPM → 1.3–5 Hz (below the camera band — use methods 1–3)
- Blade-pass: shaft Hz × number of blades — a 3-blade condenser fan at 1,075 RPM hums near 54 Hz, three times its 18 Hz shaft rate
Quick answers
How do I convert Hz to RPM? Multiply by 60. A reading of 28.8 Hz is 1,725 RPM — a loaded 4-pole motor. Going the other way, divide RPM by 60: a nameplate that says 3,450 RPM means you should read about 57.5 Hz.
Why does my fan look like it spins backward on video? Aliasing — the wagon-wheel effect. When the rotation rate is near a multiple of the frame rate, each frame catches the blades slightly behind or ahead of the previous position, so they appear slow, frozen, or reversed. The same trap makes a strobe "freeze" at half or double the true speed, which is why methods 2 and 3 need a single asymmetric mark and a sanity check.
What RPM range can a phone camera measure? At 240 fps the practical frequency ceiling is about 115 Hz (the Nyquist limit is 120 Hz), so a camera-based readout covers roughly 480–6,900 RPM. Below ~480 RPM, count or step frames by hand; above ~6,900 RPM, use dedicated instrumentation. More in the FAQ.