The millihertz is a unit of frequency equal to one thousandth of a hertz. Its symbol is mHz. One millihertz is one cycle every thousand seconds, a little under seventeen minutes, so the unit describes things that repeat slowly enough to watch.
The lower-case m matters. A millihertz is a million times smaller than a megahertz, and the two are distinguished only by the case of the first letter. This is one of the few places in the SI where a typing error changes a quantity by six orders of magnitude, and it is a recurring source of confusion in documents that lose their capitalisation.
Seismology is the unit's natural home. A large earthquake sets the whole Earth ringing like a bell, and these free oscillations have periods of minutes to about an hour, placing them between roughly 0.3 and 5 millihertz. The gravest mode, in which the planet alternately expands and contracts as a sphere, has a period near twenty minutes. Measuring these frequencies is how the deep structure of the Earth was mapped before seismic tomography.
The Sun oscillates in the same range. Helioseismology studies pressure waves trapped inside the Sun, whose strongest signal is a five-minute oscillation at about 3.3 millihertz. Tracking millions of these modes reveals the Sun's internal rotation and the depth of its convection zone, information no telescope can obtain by looking at the surface.
Ocean tides sit far below even this. The principal lunar semidiurnal tide repeats every twelve hours and twenty-five minutes, which is about 0.022 millihertz, and the whole family of tidal constituents that tide tables are built from occupies the range below a tenth of a millihertz.
Engineering meets the unit in the sway of tall buildings and long bridges, whose fundamental modes fall between about 100 and 500 millihertz, and in the slow control loops of heating systems and chemical plant, where a process may take many minutes to respond to a change.
One millihertz equals 0.001 hertz, one cycle per 1000 seconds, or 1000 microhertz.
| Unit | Symbol | 1 Millihertz equals | 1 of these equals | Converter |
|---|---|---|---|---|
| Nanohertz | nHz | 1000000 nHz | 0.000001 mHz | Millihertz to Nanohertz |
| Microhertz | µHz | 1000 µHz | 0.001 mHz | Millihertz to Microhertz |
| Hertz | Hz | 0.001 Hz | 1000 mHz | Millihertz to Hertz |
| Kilohertz | kHz | 0.000001 kHz | 1000000 mHz | Millihertz to Kilohertz |
| Megahertz | MHz | 0.000000001 MHz | 1000000000 mHz | Millihertz to Megahertz |
| Gigahertz | GHz | 1 × 10-12 GHz | 1000000000000 mHz | Millihertz to Gigahertz |
| Terahertz | THz | 1 × 10-15 THz | 1 × 1015 mHz | Millihertz to Terahertz |