Antenna Length Calculator (Half-Wave Dipole & Quarter-Wave)
By Santaji GadepatilinUpdated October 30, 2026
A resonant antenna is cut to a fraction of the wavelength — half a wavelength for a dipole, a quarter for a ground-plane whip — shortened slightly because of end effects and conductor thickness. This antenna length calculator gives the free-space wavelength and the practical dipole, dipole leg and quarter-wave lengths for any frequency.
How to use the antenna length calculator
Enter the operating frequency and a velocity (end-effect) factor — 0.95 suits a thin bare-wire dipole. The calculator gives the free-space wavelength, the total half-wave dipole length, the length of each dipole leg and the quarter-wave whip or monopole length.
Antenna length formulas
λ = c ÷ f · Ldipole = k × λ ÷ 2 · Lmonopole = k × λ ÷ 4
c = 299,792,458 m/s. With k = 0.95 the dipole formula becomes L (m) = 142.5 ÷ f (MHz), or L (ft) ≈ 468 ÷ f (MHz).
Example
A dipole for 433.92 MHz with k = 0.95:
- λ = 299,792,458 ÷ 433.92 × 10⁶ = 69.09 cm
- Half-wave dipole = 0.5 × 69.09 × 0.95 = 32.82 cm (16.41 cm per leg)
- Quarter-wave whip = 16.41 cm
Antenna lengths for common bands (k = 0.95)
| Band | Frequency | Wavelength | Half-wave dipole | Quarter-wave |
|---|---|---|---|---|
| CB radio | 27.185 MHz | 11.028 m | 5.238 m | 2.619 m |
| 2 m amateur | 146 MHz | 2.053 m | 97.54 cm | 48.77 cm |
| ISM 433 MHz | 433.92 MHz | 69.09 cm | 32.82 cm | 16.41 cm |
| LoRa / ISM 868 MHz (EU) | 868 MHz | 34.54 cm | 16.41 cm | 8.20 cm |
| ISM 915 MHz (US) | 915 MHz | 32.76 cm | 15.56 cm | 7.78 cm |
| GPS L1 | 1575.42 MHz | 19.03 cm | 9.04 cm | 4.52 cm |
| Wi-Fi / Bluetooth 2.4 GHz | 2440 MHz | 12.29 cm | 5.84 cm | 2.92 cm |
| Wi-Fi 5 GHz | 5500 MHz | 5.45 cm | 2.59 cm | 1.29 cm |
Notes
- Cut slightly long and trim — it is easier to shorten an antenna than to lengthen it.
- PCB trace antennas and helical or ceramic chip antennas follow their own design notes, not these formulas.
- Estimate the range of the link with the free-space path loss calculator.
Frequently asked questions
How long should a half-wave dipole be?
About 0.95 × λ ÷ 2. In feet that is the familiar 468 ÷ f (MHz); in metres, 142.5 ÷ f (MHz). At 433.92 MHz that is about 32.8 cm tip to tip.
How long is a 433 MHz quarter-wave antenna?
About 16.4 cm with a 0.95 factor (17.3 cm for a full free-space quarter wavelength). Trim while watching SWR or range for the best result.
What does the velocity factor do?
Currents near the ends of a real conductor make it behave electrically longer, so the physical length is a few percent shorter than the free-space fraction of a wavelength. Thicker elements need a lower factor.
Does a quarter-wave antenna need a ground plane?
Yes. A monopole uses a ground plane (a metal surface, PCB copper or radials about λ/4 long) as the other half of the dipole; without it, performance and matching suffer.