Antenna Length Calculator (Half-Wave Dipole, Quarter-Wave)
Calculate the length of a half-wave dipole, quarter-wave vertical or 5/8-wave vertical from the operating frequency and the wire's velocity factor (0.95 gives the classic 468/f feet). Results in metres, cm, feet or inches, with each dipole leg and the free-space wavelength.
Last updated: October 2026
Antenna length
97.54 cm
Formula below · 4 sources (physics.nist.gov, kb6nu.com, hamradio.my, radio-workshop.co.uk) · Updated Oct 2026
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About this calculator
A resonant wire antenna is cut to a fraction of the wavelength of the operating frequency. In free space one wavelength is λ = c ÷ f, with c = 299,792,458 m/s, so a half wavelength in feet is 491.8 ÷ f (MHz). A real wire is electrically longer than its physical length (end effect from the insulators, the finite thickness of the conductor, nearby objects), so it resonates about 5% shorter than the free-space figure. That correction is the velocity factor:
length = (c ÷ f) × fraction × velocity factor
With 0.95 the half-wave dipole becomes 467 ÷ f feet, which is where the ham-radio rule of thumb 468 ÷ f (MHz) = length in feet comes from (143 ÷ f in metres). A quarter-wave vertical is half of that, 234 ÷ f feet, and a 5/8-wave vertical radiator is 585 ÷ f feet.
With the defaults, 146 MHz (the 2 m band): λ = 2.053 m; half-wave dipole = 2.053 × 0.5 × 0.95 = 0.975 m = 97.5 cm = 3.20 ft, so each leg is 48.8 cm (19.2 in). A quarter-wave ground-plane radiator for the same frequency is 48.8 cm.
Velocity factor
0.95 suits bare or thin-insulated wire at HF and VHF. Insulated wire (PVC or silicone jacket) is electrically longer, so use about 0.92-0.94 and expect to trim; thick tubing elements at VHF/UHF also need a few percent less; a dipole low to the ground (under 0.2 λ up) resonates lower and is cut shorter still. The formula is a starting point: cut a few percent long, measure the SWR or resonance, and trim to the final length.
Limits
This is the physical length of a straight, centre-fed resonant element. It does not cover loaded or coil-shortened antennas, Yagi element spacing, full-wave loops (which use about 1,005 ÷ f feet, a factor slightly over 1) or the feed-line length (for coax, use its own velocity factor, typically 0.66 for solid polyethylene and 0.80-0.85 for foam).
Half-wave dipole lengths by band at velocity factor 0.95 (this calculator; 468/f in brackets)
Scroll sideways to see all columns →
| Band | Frequency (MHz) | Dipole length (ft) | Dipole length (m) | Each leg (m) |
|---|---|---|---|---|
| 80 m | 3.75 | 124.59 (124.80) | 37.974 | 18.987 |
| 40 m | 7.1 | 65.80 (65.92) | 20.057 | 10.028 |
| 20 m | 14.2 | 32.90 (32.96) | 10.028 | 5.014 |
| 15 m | 21.2 | 22.04 (22.08) | 6.717 | 3.359 |
| 10 m | 28.4 | 16.45 (16.48) | 5.014 | 2.507 |
| CB (11 m) | 27.185 | 17.19 (17.22) | 5.238 | 2.619 |
| 6 m | 50.1 | 9.33 (9.34) | 2.842 | 1.421 |
| 2 m | 146 | 3.20 (3.21) | 0.975 | 0.488 |
| 70 cm | 446 | 1.05 (1.05) | 0.319 | 0.160 |
How to use
Enter the frequency, choose the antenna type, keep the velocity factor at 0.95 for ordinary wire, and pick the unit for the result. Example 1 (2 m band): 146 MHz, half-wave dipole. λ = 299,792,458 ÷ 146,000,000 = 2.0534 m. Length = 2.0534 × 0.5 × 0.95 = 0.9754 m = 97.54 cm = 3.20 ft (468 ÷ 146 = 3.21 ft). Each leg: 48.77 cm (19.2 in). Example 2 (40 m band): 7.1 MHz dipole. λ = 42.224 m; length = 42.224 × 0.475 = 20.06 m = 65.80 ft (468 ÷ 7.1 = 65.9 ft); each leg 10.03 m. Example 3 (CB): 27.185 MHz quarter-wave vertical. λ = 11.028 m; radiator = 11.028 × 0.25 × 0.95 = 2.619 m = 103.1 in (8.59 ft).
Frequently asked questions
Where does the 468/f formula come from?
Half a free-space wavelength in feet is 491.8 ÷ f (MHz). Multiply by the 0.95 end-effect correction and you get 467 ÷ f, rounded to the traditional 468 ÷ f. This calculator uses the exact speed of light and your own velocity factor, so it gives 467.2 ÷ f at 0.95.
How long is each leg of the dipole?
Half of the total: the tile under the result shows it. For 146 MHz the dipole is 97.5 cm tip to tip, so each leg is 48.8 cm measured from the centre insulator.
What velocity factor should I use?
0.95 for bare wire or thin enamel at HF and VHF (the 468/f convention). Use about 0.92-0.94 for PVC- or silicone-insulated wire, and a little less for thick aluminium tubing at VHF/UHF. Always cut slightly long and trim to the lowest SWR.
Does the 5/8-wave option include the loading coil?
No. It gives the length of the radiator (585 ÷ f feet). A 5/8-wave vertical is not self-resonant and needs a matching coil at the base, which the calculator does not size.
Why does my dipole resonate below the calculated frequency?
Insulated wire, thick elements, low height above ground and nearby metal all make the antenna electrically longer, so it resonates lower. Shorten it a little at a time (a 1% cut raises the resonant frequency about 1%) until the SWR minimum lands on your frequency.
Can I use this for the coax feed line?
Not with 0.95. Coaxial cable has its own velocity factor, about 0.66 for solid polyethylene and 0.80-0.85 for foam dielectric; enter that value and 0.5 or 0.25 wavelength to cut a phasing or matching section.
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