Dipole antenna calculator
This dipole antenna calculator gives the length of a half-wave dipole and of each leg for a frequency in MHz. A half wavelength in free space is 491.8 ÷ f feet; a wire dipole is cut about 5 % shorter. For 14.2 MHz (20 m band) that is 32.90 ft in total, or two legs of 16.45 ft.
Source: NIST CODATA – speed of light in vacuum c = 299 792 458 m/s, exact (read 09.10.2026). Updated: .
How it is calculated
Dipole length = k × c ÷ (2 × f). With the speed of light c = 299,792,458 m/s (NIST) a half wavelength is 149.9 ÷ f meters or 491.8 ÷ f feet (f in MHz). A real wire dipole resonates a bit lower than that, so it is cut shorter by the factor k. With k = 0.95 the formula becomes 142.4 ÷ f meters or 467.2 ÷ f feet; the well-known rule of thumb 468 ÷ f feet equals k ≈ 0.952.
Dipole length by band (k = 0.95)
| Frequency | Total length | Each leg |
|---|---|---|
| 3.65 MHz (80 m) | 128.00 ft (39.01 m) | 64.00 ft |
| 7.1 MHz (40 m) | 65.80 ft (20.06 m) | 32.90 ft |
| 14.2 MHz (20 m) | 32.90 ft (10.03 m) | 16.45 ft |
| 21.2 MHz (15 m) | 22.04 ft (6.72 m) | 11.02 ft |
| 28.5 MHz (10 m) | 16.39 ft (5.00 m) | 8.20 ft |
| 27.205 MHz (CB) | 17.17 ft (5.23 m) | 8.59 ft |
| 145 MHz (2 m) | 3.22 ft (0.982 m) | 1.61 ft |
Examples
- 40 m band, 7.1 MHz: 65.80 ft – the 468/f rule gives 65.92 ft.
- 2 m band, 145 MHz: 0.982 m end to end, legs of 0.491 m.
- Free space (k = 1) at 14.2 MHz: half wavelength 10.56 m, wavelength 21.11 m.
The shortening factor of 0.95 is a guide value, not a law of physics: thicker conductors, insulation, the height above ground and an inverted-V shape all lower the resonant frequency. Start a little long and trim both legs equally. The wavelength calculator converts between frequency and wavelength, and the meters to feet converter helps with wire on metric spools.
Frequently asked questions
How do you calculate the length of a dipole antenna?
Total length = k × 299.79 ÷ (2 × f) in meters with f in MHz. With k = 0.95: 142.4 ÷ f m or 467.2 ÷ f ft. Each leg is half of that.
Where does 468 ÷ f come from?
A half wavelength in free space is 491.8 ÷ f feet. Multiplying by about 0.952 for the end effect of a wire gives 468 ÷ f.
How long is a 40 m dipole?
At 7.1 MHz about 65.80 ft (20.06 m) end to end, with two legs of 32.90 ft.
How long is a 20 m dipole?
At 14.2 MHz about 32.90 ft (10.03 m), two legs of 16.45 ft.
Which shortening factor should I use?
0.95 is a common starting point for thin wire. The real value depends on the wire, insulation and height, so trim the antenna while checking the SWR.
Can I use it for a quarter-wave vertical?
Yes – a quarter-wave radiator is the same length as one dipole leg, e.g. 16.45 ft at 14.2 MHz.
Sources and legal basis
- NIST CODATA – speed of light in vacuum c = 299 792 458 m/s, exact (read 09.10.2026)
- NASA Radio JOVE – TM2.0 Basic Radio Telescope Operation: tip-to-tip dipole length L = 468/f (feet, MHz), i.e. k ≈ 0.9516 (read 09.10.2026)
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