Four ways to find a frequency.
Pick the tab that matches what you know — everything recalculates live as you type, and every result explains itself.
How long one full cycle takes — a 2 ms period is a 500 Hz wave.
The rate you already know — 440 Hz is the A4 note.
How fast the wave travels — 343 m/s is sound in air at 20 °C, 299,792,458 m/s is light.
The length of one full cycle — 0.78 m in air is the 440 Hz A4 note.
Radians per second — one full revolution is 2π ≈ 6.283 rad.
Energy of a single photon — 2 eV is deep-red light, 1 keV is a soft X-ray.
From question to answer in four moves.
The fastest path through the panel — though it is simple enough to just start typing.
Choose a mode
Four tabs cover the four classic starting points: a measured period, a wave's speed and wavelength, an angular speed, or a photon's energy.
Type your values
Enter numbers and pick units from the dropdowns — seconds or nanoseconds, meters or nanometers. Everything recalculates live as you type.
Read the results
The big value is your answer in the cleanest unit, and the rows unpack the period, angular frequency, RPM and more alongside it.
Copy & verify
Check the formula line to see the exact arithmetic, then hit COPY to take the whole result as plain text for notes or homework.
Six lines that power the panel.
Every mode on this page runs on one of these — worth keeping them in arm's reach.
From period
The definition. Frequency is just the reciprocal of the period — a cycle lasting 2 ms repeats 500 times each second.
From a wave
Divide the wave's speed by its wavelength — sound at 343 m/s over a 0.78 m wave is the 440 Hz A4 note.
From rotation
One full turn is 2π radians, so dividing angular speed by 2π gives cycles per second: 377 rad/s → 60 Hz.
Photon energy
Planck's constant ties a photon's energy to its frequency — a 2 eV red photon oscillates about 484 trillion times per second.
Light's wavelength
In a vacuum, wavelength times frequency is always c — 2.4 GHz Wi-Fi waves are 12.5 cm long.
Back to period
The reverse move. A 50 Hz mains cycle lasts 20 ms; a 100 MHz FM wave repeats every 10 ns.
Frequency, answered.
The things students ask most about frequency, periods, waves and photons — in plain language.
What is frequency, and what is a hertz?+
Frequency measures how often a repeating event happens per unit of time. Its SI unit is the hertz (Hz): 1 Hz means one cycle per second, so a 50 Hz current completes 50 full cycles every second. Frequency is used for sound, light, electricity, rotation, radio waves and any other oscillation.
How do I calculate frequency from period?+
Take the reciprocal: f = 1 ÷ T, with the period T in seconds. For example, a cycle lasting 20 ms = 0.02 s has a frequency of 1 ÷ 0.02 = 50 Hz. The Period tab converts any time unit — nanoseconds to days — and shows the full working.
How do I calculate frequency from wavelength and wave speed?+
Use f = v ÷ λ: divide the wave's speed by its wavelength, both in SI units. Sound at 343 m/s with a 0.78 m wavelength gives 343 ÷ 0.78 ≈ 440 Hz, while light at 299,792,458 m/s with a 650 nm wavelength gives about 461 THz. Keep speed in m/s and wavelength in meters to get hertz directly.
How are frequency and angular frequency related?+
Angular frequency ω counts radians per second instead of cycles per second, and one full cycle is 2π radians, so ω = 2πf and f = ω ÷ 2π. A 60 Hz power grid therefore has ω ≈ 377 rad/s.
How do I convert RPM to hertz?+
RPM is revolutions per minute and hertz is cycles per second, so divide by 60: 3,000 rpm = 50 Hz. To go the other way, multiply hertz by 60 — every result panel here shows the rotation rate in rpm automatically.
How do I find the frequency of light from its energy?+
A photon's energy and frequency are linked by Planck's constant: E = hf, so f = E ÷ h with h = 6.62607015×10−34 J·s. Convert the energy to joules first (1 eV = 1.602176634×10−19 J): a 2.33 eV green photon has a frequency of about 563 THz. The Photon energy tab does all conversions and also outputs the wavelength.
Does frequency change when a wave enters a different medium?+
No. The frequency of a wave is fixed by its source. When light passes from air into glass it slows down, and its wavelength shrinks to λ = v/f — but the frequency, and therefore the color, stays the same.
Is this frequency calculator free, and is my data private?+
It is completely free and runs entirely in your browser — nothing you type is uploaded, logged, or sent to a server. No account, no sign-up, no limits.