Dipole calculator with trim from measurement

The half-wave dipole length for your frequency, and the cut to make once your analyzer shows where the wire really resonates.

The formula

A half-wave dipole is a little shorter than half a wavelength in free space. The difference is the end effect, written as a factor K:

L = K × c / 2f

c is the speed of light, 299.792458 m per microsecond, and f is the frequency in MHz, so L comes out in metres. The old rule 468 / f in feet is the same formula with K = 0.9516. For thin bare wire high in the clear, K is usually between 0.95 and 0.97. Insulated wire, thick wire and low wires come out shorter.

Flat half-wave dipole, thin wire: total length tip to tip
Band	Frequency	K 0.95	468 / f	468 / f, other unit
160 m	1.9 MHz	245′ 10 ¹¹⁄₁₆″	246′ 3 ¹³⁄₁₆″	75.08 m
80 m	3.65 MHz	128′ 0″	128′ 2 ⅝″	39.08 m
60 m	5.36 MHz	87′ 1 ¹⁵⁄₁₆″	87′ 3 ¾″	26.61 m
40 m	7.1 MHz	65′ 9 ⅝″	65′ 11″	20.09 m
30 m	10.125 MHz	46′ 1 ¹¹⁄₁₆″	46′ 2 ¹¹⁄₁₆″	14.09 m
20 m	14.175 MHz	32′ 11 ½″	33′ 0 ³⁄₁₆″	10.06 m
17 m	18.118 MHz	25′ 9 ⁷⁄₁₆″	25′ 9 ¹⁵⁄₁₆″	7.87 m
15 m	21.225 MHz	22′ 0 ⅛″	22′ 0 ⁹⁄₁₆″	6.72 m
12 m	24.94 MHz	18′ 8 ¹³⁄₁₆″	18′ 9 ³⁄₁₆″	5.72 m
10 m	28.5 MHz	16′ 4 ¹¹⁄₁₆″	16′ 5 ¹⁄₁₆″	5.01 m
6 m	50.15 MHz	9′ 3 ¹³⁄₁₆″	9′ 4″	2.84 m

Trimming from what your analyzer reads

Cut the wire a little long, hang it where it will live, and find the frequency with the lowest SWR. Resonance moves in inverse proportion to length, so the length that hits your target is:

L new = L × f measured / f target

On a centre-fed dipole half of the change comes off each leg. Small changes are accurate in one step. For a change of more than about 10 %, cut half, measure again and finish from the new reading.

A 40 m dipole, first cut
Step	Value
Cut for 7.100 MHz with K 0.95	65′ 9 ⅝″ · 20.06 m
Analyzer shows the lowest SWR at	6.95 MHz
Target	7.1 MHz
Whole wire after the cut	64′ 4 ¹⁵⁄₁₆″ · 19.63 m
Cut from each leg	8 ⁵⁄₁₆″ · 21.2 cm
This wire's own K	0.9299

Your wire has its own K

The first measurement also tells you the K of this wire, in this place: K = L / (λ / 2) at the frequency it resonates. Height, insulation, the ground and nearby metal all go into it. Use that K for the next band, a fan dipole leg or a link dipole section, and the first cut lands much closer.

The app keeps every measurement. From two or more cuts it also learns a fixed offset for the feed region and the balun leads, which removes the slow "cut, measure, still long" approach.

Inverted-V and other shapes

An inverted-V starts from the same length. Drooping legs lower the resonance by a few percent, and how much depends on the apex angle, the height and the ground, so no fixed factor is honest. Start from the flat length and trim. The app covers inverted-V, end-fed half-wave, off-centre-fed, verticals with radials, J-poles, loops and more, each with trim from measurement.

Sources

Calculations are aids; go by the rules in force and your own measurement.

Checked against the sources on November 2, 2026.