The units of frequency
Frequency counts how many times something repeats per second, and the hertz is exactly that: one cycle per second. Processor clocks, radio bands and audio sample rates all use it with metric prefixes. Revolutions per minute is the same idea with a different time base, which is why the conversion is a division by 60 rather than a change of prefix.
| Unit | Symbol | Equals |
|---|---|---|
| Hertz | Hz | 1 Hz |
| Kilohertz | kHz | 1,000 Hz |
| Megahertz | MHz | 1,000,000 Hz |
| Gigahertz | GHz | 1,000,000,000 Hz |
| Revolutions per minute | rpm | 0.01666667 Hz |
How to convert by hand
Every frequency conversion follows the same procedure: take the amount to the base unit, then from there to the target unit. In practice it is a single multiplication, because the two steps combine into one factor.
The metric prefixes are simple decimal shifts: a kilohertz is 1,000 Hz, a megahertz a million, a gigahertz a billion. For rpm, divide by 60 to get hertz.
The equivalents people look up most
| Conversion | Result |
|---|---|
| Mains electricity | 50 Hz or 60 Hz |
| An FM radio station | 100 MHz · 100,000,000 Hz |
| A processor clock | 3.5 GHz · 3,500 MHz |
| A car engine at idle | 800 rpm · 13.3 Hz |
| CD audio sampling | 44.1 kHz · 44,100 Hz |
Common mistakes
- Confusing MHz with Mbps. One is a frequency and the other a data rate; a higher clock does not automatically mean more throughput.
- Treating rpm as a metric prefix problem. It is a different time base, so the factor is 60, not a power of ten.
- Assuming clock speed alone measures processor performance. Work per cycle differs enormously between designs.
- Reading Hz as Hz per second. The hertz already contains the per-second.