Read the guide: Vibration frequency explained
How to convert RPM to Hz
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Step 1: Find the speed in RPM. It's on the motor's nameplate, in the manual, or on a tachometer.
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Step 2: Type it in as RPM. The Hz value updates as you type.
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Step 3: Compare the result with 30 Hz. Below that, a phone can measure the vibration directly. Above it, a phone can't.
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Step 4: Work out the other frequencies to look for: 2× the result for misalignment or looseness, and cylinders ÷ 2 × the result for a 4-stroke engine.
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Step 5: Measure with the vibration frequency analyzer and look for a peak near the frequencies you worked out.
RPM to Hz means dividing by 60. RPM counts revolutions per minute, and hertz counts cycles per second. Converting first tells you which frequency to look for in a vibration reading, and whether a phone can see it at all. Most phones can measure up to about 30 Hz, which is 1,800 RPM.
RPM to Hz formula
Hz = RPM ÷ 60
A shaft turning at 1,200 RPM turns 1,200 ÷ 60 = 20 times each second. If it has a heavy spot, it shakes the machine 20 times a second, so you’d look for a peak at 20 Hz. This once-per-turn frequency is called the running speed, or 1×.
Need the reverse? Use the Hz to RPM converter.
Worked examples
- Front-load washer spinning at 1,400 RPM. 1,400 ÷ 60 = 23.3 Hz. A phone can measure that, so an unbalanced load should show a clear peak near 23 Hz.
- Car engine idling at 750 RPM. 750 ÷ 60 = 12.5 Hz for the crankshaft. A 4-cylinder engine fires twice per turn, so 25 Hz. A 6-cylinder fires three times per turn, so 37.5 Hz, which is above the phone limit.
- Ceiling fan at 90 RPM. 90 ÷ 60 = 1.5 Hz. An unbalanced blade makes the fan wobble once per turn, so the wobble shows up near 1.5 Hz.
RPM to Hz chart for common machines
| Machine | Typical RPM | Hz | Can a phone measure it? |
|---|---|---|---|
| Dryer drum | 45–60 | 0.75–1 | Too slow to read well |
| Ceiling fan on low | 60–100 | 1–1.7 | Yes |
| Top-load washer spin | 600–900 | 10–15 | Yes |
| Engine crankshaft at idle | 750 | 12.5 | Yes |
| Front-load washer spin | 1,000–1,400 | 17–23 | Yes |
| 4-pole electric motor, 60 Hz power | about 1,750 | about 29 | At the edge |
| 2-pole electric motor, 60 Hz power | about 3,500 | about 58 | No |
| Fixed-speed fridge compressor | 3,000–3,600 | 50–60 | No |
What the result tells you
Below about 30 Hz: measure it
Put the phone flat on the machine and run it at the speed you converted. Look for a peak at 1× or 2× the frequency you worked out. The machine vibration test does this match for you when you enter the RPM. For engines, the car idle vibration test also works out the firing frequency.
Above about 30 Hz: a phone can’t see it directly
A phone takes about 60 readings a second, so it can’t follow anything faster than about 30 Hz. The shake may still show up, but at a false, lower frequency called an alias. At 60 readings a second, a 45 Hz shake can appear near 15 Hz, which is 60 minus 45. The sensor tools on this site warn you when this may be happening.
You can still use the phone’s strength reading as a rough check. For fast motors, pumps and compressors, a proper vibration meter or a technician is the right tool.
Very slow parts
Below about 1 Hz, a phone has trouble picking out the frequency. Dryer drums fall here. For slow parts, the strength of the shake tells you more than the frequency.
Where the numbers come from
The formula is simple unit math: there are 60 seconds in a minute. The machine speeds are typical ranges for common appliances and motors, not specs for your model. Check the nameplate, the manual or the maker’s support page for your exact RPM. Electric motor speeds follow from the power frequency and the number of poles. Real motors run a little slower than that because of slip.
Frequently asked questions
How many Hz is 1,000 RPM?
1,000 RPM is about 16.7 Hz. Divide by 60: 1,000 ÷ 60 = 16.67.
What is 3,600 RPM in Hz?
3,600 RPM is exactly 60 Hz. That's about the speed of a 2-pole electric motor on US 60 Hz power, though real motors run a little slower. It's twice as fast as a phone can measure.
Why does my engine shake at a different Hz than RPM ÷ 60?
RPM ÷ 60 is how often the crankshaft turns. A 4-stroke engine fires each cylinder once every two turns, so the firing shake is at RPM ÷ 60 × cylinders ÷ 2. At a 750 RPM idle, that's 25 Hz for a 4-cylinder and 50 Hz for a V8.
Can my phone measure a motor running at 1,750 RPM?
Just barely. 1,750 RPM is about 29 Hz, right at the edge of the roughly 30 Hz limit, and your phone's exact limit may be a little lower. A 3,450 RPM motor (57.5 Hz) is out of range and may show up as a false, lower frequency.
What's the difference between the Hz and RPM on a motor's nameplate?
The Hz is the power supply frequency, usually 60 Hz in the US. The RPM is how fast the shaft turns. Convert the RPM, not the supply Hz, to find the shaft's vibration frequency.
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