Wavelength Calculator
Convert a frequency to its wavelength or back again, with a velocity factor for cable and the common half-wave and quarter-wave antenna lengths.
How it works
Wavelength is how far a wave travels during one cycle: the speed of light divided by the frequency. Higher frequencies give shorter wavelengths, in strict inverse proportion.
Inside a cable, waves travel slower than in free space. The velocity factor — typically 0.66 for solid polyethylene coax, 0.82 for foam, around 0.95 for open ladder line — scales the wavelength down accordingly. Cutting a cable stub to a free-space length is a standard and expensive mistake.
Antennas are cut in fractions of a wavelength because that is what makes them resonant. A half-wave dipole is close to half a wavelength, a quarter-wave whip close to a quarter — both trimmed slightly shorter in practice, since end effects make a real conductor behave as though it were a little longer than it is.
The formula
Wavelength
λ = c ÷ f
c = 299,792,458 m/s in a vacuum
Frequency
f = c ÷ λ
In a medium
λ = (c × velocity factor) ÷ f
Half-wave dipole
length ≈ 0.5 × λ (trimmed ~5% shorter in practice)
Worked examples
| Scenario | Working | Result |
|---|---|---|
| 100 MHz FM broadcast | 299792458 ÷ 100000000 | 2.998 m |
| 2.4 GHz Wi-Fi | 299792458 ÷ 2.4e9 | 12.49 cm, quarter wave 3.12 cm |
| A 1 m stub in 0.66 coax | λ scaled by 0.66 | Electrically longer than 1 m of free space |
When you'd use it
- Cutting an antenna to length for a band
- Working out cable stub lengths with the right velocity factor
- Checking whether a PCB trace is long enough to behave as a transmission line
- Converting between the frequency and wavelength conventions in a datasheet
Common questions
Should I use 3 × 10⁸ or the exact speed of light?
This tool uses the exact figure, 299,792,458 m/s. The rounded value is about 0.07% high, which is irrelevant for a rough antenna cut and starts to matter for precise cable lengths at high frequencies.
Why is a real antenna cut shorter than the calculated length?
End effects. Capacitance at the ends of the conductor makes it behave electrically longer than it physically is, so the resonant point comes in low. Cutting around 5% short and trimming to the measured resonance is the usual approach.
What is velocity factor and when do I need it?
It is the fraction of the speed of light at which a wave travels in a given medium. It applies inside cables and along conductors, not through the air — so use it for coax stubs and delay lines, and leave it at 1 for free-space wavelengths.
Does this work for sound as well?
The relationship does, but not with these numbers — sound travels at roughly 343 m/s in air rather than the speed of light. This calculator is built for electromagnetic waves.
Is anything I enter sent to a server?
No. Every calculation runs in your browser, so component values, circuit figures and measurements are never uploaded, logged or stored. There is no account and no record of what you typed.