Convert Petahertz to Millihertz

Petahertz (PHz) to Millihertz (mHz) conversion via f = c / λ — enter any value below to get an instant result, or use the table for common values.

Results from this calculator are estimates provided for general informational purposes only, based on formulas, rates, and standards commonly accepted as of 2026. Figures may differ slightly from other calculators or professional sources due to rounding methods, differing assumptions, or regional regulations, and rules may change over time. Always consult a qualified professional — such as a financial advisor, healthcare provider, or other relevant specialist — before making decisions based on these results.

Value

From

Result

1 PHz = 1E+18 mHz

1 Petahertz = 1E+18 Millihertz

1 Millihertz = 1E-18 Petahertz

1 PHz in every supported unit

Conversion chart: Petahertz to Millihertz

Conversion table

Petahertz (PHz)	Millihertz (mHz)
0.01 PHz	1E+16 mHz
0.1 PHz	1E+17 mHz
1 PHz	1E+18 mHz
2 PHz	2E+18 mHz
3 PHz	3E+18 mHz
5 PHz	5E+18 mHz
10 PHz	1E+19 mHz
20 PHz	2E+19 mHz
50 PHz	5E+19 mHz
100 PHz	1E+20 mHz
1000 PHz	1E+21 mHz

Petahertz (PHz)

Definition: One quadrillion (10¹⁵) hertz — an SI-prefixed multiple of the base unit.

History: Formed by attaching the SI prefix "peta-" (adopted in 1975) to hertz.

Current use: Sits in the ultraviolet and soft X-ray region of the spectrum — visible light itself tops out around 0.75-0.8 PHz, so this unit is used mostly in spectroscopy and photonics research just above the visible band.

Millihertz (mHz)

Definition: One thousandth (10⁻³) of a hertz — an SI-prefixed fraction of the base unit.

History: Formed by attaching the SI prefix "milli-" to hertz.

Current use: Used to describe very slow periodic processes such as ocean tides, some circadian-adjacent biological rhythms, and low-frequency vibration monitoring in structural and mechanical engineering.

Supported Units

Unit	Symbol	Equivalent Frequency (Hz)
Hertz	Hz	1 Hz
Exahertz	EHz	1E+18 Hz
Petahertz	PHz	1E+15 Hz
Terahertz	THz	1E+12 Hz
Gigahertz	GHz	1E+09 Hz
Megahertz	MHz	1000000 Hz
Kilohertz	kHz	1000 Hz
Hectohertz	hHz	100 Hz
Dekahertz	daHz	10 Hz
Decihertz	dHz	0.1 Hz
Centihertz	cHz	0.01 Hz
Millihertz	mHz	0.001 Hz
Microhertz	µHz	1E-06 Hz
Nanohertz	nHz	1E-09 Hz
Picohertz	pHz	1E-12 Hz
Femtohertz	fHz	1E-15 Hz
Attohertz	aHz	1E-18 Hz
Cycle/Second	cycle/s	1 Hz
Wavelength in Exametres	Em	1 unit = 2.99792E-10 Hz
Wavelength in Petametres	Pm	1 unit = 2.99792E-07 Hz
Wavelength in Terametres	Tm	1 unit = 0.0002997925 Hz
Wavelength in Gigametres	Gm	1 unit = 0.29979246 Hz
Wavelength in Megametres	Mm	1 unit = 299.79246 Hz
Wavelength in Kilometres	km	1 unit = 299792.46 Hz
Wavelength in Hectometres	hm	1 unit = 2997924.6 Hz
Wavelength in Dekametres	dam	1 unit = 29979246 Hz
Wavelength in Metres	m	1 unit = 2.9979246E+08 Hz
Wavelength in Decimetres	dm	1 unit = 2.99792E+09 Hz
Wavelength in Centimetres	cm	1 unit = 2.99792E+10 Hz
Wavelength in Millimetres	mm	1 unit = 2.99792E+11 Hz
Wavelength in Micrometres	µm	1 unit = 2.99792E+14 Hz
Wavelength in Nanometres	nm	1 unit = 2.99792E+17 Hz
Electron Compton Wavelength	λC (e⁻)	1 unit = 1.23559E+20 Hz
Proton Compton Wavelength	λC (p)	1 unit = 2.26873E+23 Hz
Neutron Compton Wavelength	λC (n)	1 unit = 2.27186E+23 Hz

About These Parameters

Value: The frequency or wavelength value you want to convert, expressed in the "From" unit. Accepts decimals, and can be a very large or very small number since this page spans everything from millihertz to exahertz.
From Unit: The unit your input value is currently measured in — pick a frequency unit (hertz, megahertz, gigahertz...) if you have a rate, or a wavelength unit (metres, nanometres...) if you have a physical wavelength.
To Unit: The unit you want the result converted into — can be a frequency unit, a wavelength unit, or the same category as From. Use the swap button to flip From and To instantly.

How Frequency Wavelength Conversion Works

The Formula

Two frequency units convert the normal way — multiply by the ratio of their factors. Converting between a frequency and a wavelength (or between two wavelength units) instead uses the physics relationship between them:

frequency (Hz) = speed of light (c) ÷ wavelength (m)
wavelength (m) = speed of light (c) ÷ frequency (Hz)

For Petahertz → Millihertz: 1 PHz = 1E+18 mHz, computed via each unit's fixed relationship to hertz and, where wavelength is involved, the exact speed of light (299,792,458 m/s).

Why This Converter Is Different: Inverse, Not Multiplicative

Every other converter on this site works by multiplying a value by a fixed ratio — 1 foot is always 0.3048 metres, no matter what the input value is. Frequency and wavelength don't work that way: doubling a wavelength HALVES its frequency, and doubling a frequency HALVES its wavelength, because f and λ are reciprocally related through f = c / λ rather than being simple multiples of each other. This converter still fits the site's shared table/chart machinery — it just uses the reciprocal formula internally whenever a wavelength unit is involved, the same technique the Fuel Consumption Converter uses for its "volume per distance" units (L/100km, gal/mile), which are also reciprocals of the more common "distance per volume" units.

The Electromagnetic Spectrum, by Wavelength

Different wavelength ranges correspond to familiar names: radio waves span from kilometres down to about a millimetre, microwaves run from roughly a millimetre to 30 centimetres, infrared (heat) radiation runs from about a micrometre to a millimetre, visible light sits in a narrow band from about 380 to 750 nanometres, and ultraviolet and X-rays lie below that, down into the picometre range covered by the terahertz-and-above frequency units on this page.

Example

1 PHz equals 1E+18 mHz. For scale, a typical FM radio station broadcasts around 100 MHz (a wavelength of about 3 metres), household WiFi runs at 2.4 or 5 GHz (wavelengths of about 12.5 cm or 6 cm), and visible green light has a wavelength around 530 nanometres, equivalent to roughly 566 THz.

Frequently Asked Questions

How many Millihertz are in 1 Petahertz?

1 Petahertz (PHz) equals exactly 1E+18 Millihertz (mHz).

Why isn't this a simple multiply-by-a-factor conversion like other unit converters?

Because frequency and wavelength are reciprocally related (f = c / λ), not proportionally related. A longer wavelength always means a lower frequency, so converting between them requires dividing the speed of light by the value rather than multiplying by a fixed ratio.

What speed of light does this converter use?

The exact SI-defined value, 299,792,458 metres per second — the speed of light in a vacuum, which has been an exactly fixed constant (not a measured approximation) since the metre itself was redefined against it in 1983.

What are the Compton wavelengths doing on a frequency-wavelength converter?

The Compton wavelength of a particle (electron, proton, or neutron) is a fixed physical constant with units of length, so it converts to an equivalent frequency through the exact same f = c / λ relationship as any other wavelength — it's included as a fixed reference point useful in particle and nuclear physics calculations.

Does this work for sound waves too?

No — the f = c / λ relationship here uses the speed of light, which only applies to electromagnetic waves (radio, light, X-rays, etc). Sound waves travel at the much slower speed of sound (about 343 m/s in air), so sound frequency-to-wavelength conversion needs a different constant and isn't covered by this tool.

Convert Petahertz to Other Frequency/Wavelength Units

Possible Frequency/Wavelength Conversions