Antenna length calculator
Half-wave dipole and quarter-wave vertical from frequency. Metres and feet side by side, updated as you type.
Indicative calculation; verify before use.
Printable dipole antenna length chart for all ham bands 160 m to 70 cm, print or save as PDF
Quick picks
| Element | Metres | Feet | ft + in |
|---|
How it works
A real wire antenna resonates a little shorter than the free-space half wavelength because of the capacitance at its ends (end effect). The shortening factor k covers that. The classic ham-radio rule of thumb 468/f in feet equals k ≈ 0.952 and is printed in the table for comparison.
Wire thickness, insulation, height above ground and nearby objects all shift resonance. Cut the wire a few percent long, measure SWR, and trim. Insulated wire usually needs to be a bit shorter still.
The shortening factor here is for the antenna element itself. It is not the velocity factor of coaxial cable, which matters only for cut-to-length feed lines and stubs.
Worked example: 7.1 MHz (40 m band) with k = 0.95. λ = 299 792 458 / 7 100 000 = 42.22 m. Dipole total = 0.95 × 42.22 / 2 = 20.06 m (65.80 ft); each leg, and a quarter-wave vertical, = 10.03 m (32.90 ft). For comparison, the rule of thumb gives 468 / 7.1 = 65.92 ft (20.09 m).
Sources (checked 2026-10-30):
- c = 299 792 458 m/s, exact (SI defining constant): https://physics.nist.gov/cgi-bin/cuu/Value?c and https://www.bipm.org/en/measurement-units/si-defining-constants
- Half-wave dipole ≈ λ / 2, shortened by end effect: https://en.wikipedia.org/wiki/Dipole_antenna
- Quarter-wave vertical (monopole) ≈ λ / 4: https://en.wikipedia.org/wiki/Monopole_antenna
- 1 ft = 0.3048 m exactly (international foot): https://en.wikipedia.org/wiki/Foot_(unit)
- 468 / f and 234 / f rule of thumb, attributed to the ARRL Antenna Book (we have not checked the book itself): https://www.66pacific.com/calculators/dipole-antenna-length-calculator.aspx