Dipole Antenna Calculator – Length | Formula
Cut a half-wave dipole to resonance for any frequency. Enter the frequency and the velocity factor of your wire or tubing, and this dipole antenna calculator works out the wavelength, the total dipole length, and the length of each leg, in meters, feet, inches, or centimeters. A quarter-wave vertical mode is included, because it is the same physics at half the length.
The velocity factor field is the point. The famous 468/f rule hides that factor inside a constant chosen for average wire, so builders working with insulated wire or aluminum tubing cut the wrong length and then wonder why the SWR dip sits in the wrong place. Here you enter your conductor’s actual factor and see the classic rule checked against your result on every calculation.
Typical velocity factors: 0.95 bare copper wire, 0.92 aluminum tubing, 0.85–0.92 insulated wire.
Starting lengths: cut 3–5% long and trim to lowest SWR with an analyzer. Height, nearby objects, and insulation shift resonance.
Quick Answer
- Quick Answer
- Using the calculator, from frequency to a cut length
- Where 468/f comes from: the formula in full
- Worked example: a 20 meter dipole for 14.150 MHz
- Velocity factor: the input most dipole tools hide
- Common amateur band lengths at VF 0.95
- Dipole or quarter-wave vertical: the honest trade-off
- Feedpoint, balun, and trimming to resonance
- What competing calculators miss
- Frequently Asked Questions
Using the calculator, from frequency to a cut length
Four fields, in the order you would think about a real build.
Enter the frequency in MHz. Design for the middle of the segment you actually use, not the band edge: a dipole cut for 14.150 MHz covers the 20 meter band comfortably, while one cut for an edge leaves the other edge with a rising SWR.
Set the velocity factor for your conductor. Bare copper wire 0.95, aluminum tubing 0.92, insulated wire somewhere in 0.85 to 0.92. This single choice moves the answer by half a meter on a 20 meter dipole, as the table below shows, which is why a tool without it cannot be right for every build.
Pick the antenna type and the unit your tape measure uses. Half-wave dipole returns a total and a per-leg length; quarter-wave vertical returns the single element length. Results come out in meters, feet, inches, or centimeters, and the formula readout underneath always shows meters for cross-checking.
Cut long, then trim. Add 3 to 5 percent to each leg, hoist the antenna to its working height, and trim both legs equally toward the lowest SWR. While you are choosing the element wire itself, the conductor size calculator covers how gauge, material, and run length trade off for the feedline and elements.
Where 468/f comes from: the formula in full
The dipole length formula is L = VF × c / (2f): wavelength first, halved, then shortened by the velocity factor. Everything else is unit conversion.
The physics underneath is resonance. A conductor cut near half a wavelength carries a standing wave with current maximum at the center feedpoint and voltage maxima at the tips, and at resonance its feedpoint impedance is nearly pure resistance, around 73 ohms in free space, which is why the dipole matches coax so gracefully. Two effects make the resonant physical length a few percent shorter than the free-space λ/2. The wave travels slower on a real conductor than in vacuum, and capacitance at the wire tips (the end effect) makes the antenna behave as if it were slightly longer than it is. The velocity factor field in this tool is where both effects are charged: pick the factor for your conductor and the tool shortens the free-space length by exactly that factor.
The 468 constant has a traceable history rather than a mystical one. Ward Silver’s ARRL research, “Where Does 468 Come From?”, found the value in the 1929 ARRL Handbook’s antenna chapter and in the first ARRL Antenna Book of 1939, where the roughly 5 percent reduction from 492/f was credited to the end effect of the insulators. In other words, 468 was always an average for typical wire at typical heights, measured, not derived. A calculator that lets you replace the average with your conductor’s factor is strictly more honest than the rule alone, and the tool proves the connection on every run by printing the classic rule check beside your result: at 14.15 MHz, 468 / 14.15 = 33.07 ft against the full calculation’s 33.0174 ft.
The c in the formula is not an approximation to argue about. Since 1983 the meter itself has been defined from the speed of light, fixed at exactly 299,792,458 m/s, a history the National Institute of Standards and Technology account of the meter definition lays out. Every length this tool returns inherits that exactness; the uncertainty in a real build lives entirely in the velocity factor and the installation, never in c.
Worked example: a 20 meter dipole for 14.150 MHz
The calculator is pre-filled with the most-built HF dipole there is: the 20 meter band at 14.150 MHz, bare copper wire, VF 0.95. These are its exact outputs, in both unit systems, plus the vertical for the same frequency.
| Result | Meters | Feet |
|---|---|---|
| Wavelength (λ) | 20.1274 m | 66.0348 ft |
| Half-wave dipole, total | 10.0637 m | 33.0174 ft |
| Each leg (λ/4) | 5.0319 m | 16.5087 ft |
| Quarter-wave vertical | 5.0319 m | 16.5087 ft |
| Classic rule check (468 / 14.15) | 10.0810 m equivalent | 33.07 ft |
Two numbers in that table deserve a second look. The vertical’s length equals the dipole’s leg length to the digit, which is no coincidence: a quarter-wave vertical is one leg of the dipole stood upright and worked against a ground image, so any frequency gives the same figure in both rows. And the 40 meter case shows the rule and the tool diverging a little more: at 7.15 MHz the tool returns 19.9163 m (65.3421 ft) against the rule’s 65.45 ft. The gap stays under 0.2 percent across HF, small enough that either gets you a working antenna, and large enough that insulated wire at VF 0.88, which the rule cannot express at all, is off by about three quarters of a meter if you trust 468 blindly.
Velocity factor: the input most dipole tools hide
Same frequency, same everything, five different conductors. At 14.15 MHz the choice of velocity factor alone swings the total length by more than 1.5 m from the slowest insulated wire to the free-space ideal.
| Velocity factor | Typical conductor | Total dipole | Each leg |
|---|---|---|---|
| 0.85 | Thickly insulated wire | 9.0044 m | 4.5022 m |
| 0.88 | Insulated wire, common estimate | 9.3222 m | 4.6611 m |
| 0.92 | Aluminum tubing (Yagi elements) | 9.7459 m | 4.8730 m |
| 0.95 | Bare copper wire | 10.0637 m | 5.0319 m |
| 1.00 | Free-space ideal, no shortening | 10.5934 m | 5.2967 m |
Treat these factors as starting points, not promises. Insulation thickness and material vary between spools of wire that look identical, thicker conductors resonate slightly shorter than thin ones at the same factor, and the only instrument that settles the question for your antenna is an SWR meter or analyzer at the finished height. One boundary worth knowing: the tool accepts any factor above 0 up to the physical limit of 1 and rejects anything over 1, since no passive conductor carries the wave faster than light in vacuum. Real antenna conductors live between about 0.85 and 1.00; a value far below 0.85 usually means a data entry slip, not an exotic wire.
Common amateur band lengths at VF 0.95
Design frequencies near each band’s center, bare copper wire, from the tool itself. Per-leg lengths are half the totals, and a quarter-wave vertical for any row is the per-leg figure.
| Band | Frequency | Total dipole | Each leg |
|---|---|---|---|
| 160 m | 1.9 MHz | 74.9481 m | 37.4741 m |
| 80 m | 3.7 MHz | 38.4869 m | 19.2434 m |
| 40 m | 7.15 MHz | 19.9163 m | 9.9581 m |
| 20 m | 14.15 MHz | 10.0637 m | 5.0319 m |
| 15 m | 21.15 MHz | 6.7329 m | 3.3665 m |
| 10 m | 28.5 MHz | 4.9965 m | 2.4983 m |
| 6 m | 51 MHz | 2.7922 m | 1.3961 m |
| 2 m | 146 MHz | 0.9754 m | 0.4877 m |
The band names are wavelength nicknames, not promises: the 20 meter band’s wavelength in this table is 20.1274 m at the design frequency, close to its name because the names were rounded from the same physics. United States allocations behind these bands are administered by the Federal Communications Commission’s Amateur Radio Service, and other countries slice the spectrum slightly differently, so design for the frequencies your license actually lets you use.
Dipole or quarter-wave vertical: the honest trade-off
The tool’s second mode exists because builders genuinely choose between these two at the same frequency. Neither wins outright.
Half-wave dipole
- Pro: balanced and self-contained; no ground system needed, works at any height.
- Pro: feedpoint near 73 ohms in the clear, a comfortable match for 50 ohm coax with a 1:1 balun.
- Con: needs span: 19.92 m of clear space for 40 m, plus room for the feedline drop.
- Con: directional broadside; signals off the ends are weaker, so orientation is a real choice.
Quarter-wave vertical
- Pro: omnidirectional and a small footprint: one 5.0319 m element covers 20 m in every direction.
- Pro: low radiation angle suits long-distance work from a ground-mounted installation.
- Con: only half an antenna without radials or a ground plane; the earth system does the other half of the work.
- Con: verticals pick up more local electrical noise, which is mostly vertically polarized.
Height changes the dipole’s character more than its length. Near half a wavelength up, the pattern favors low angles and long-distance contacts; low heights push radiation upward for regional NVIS work. Either way, signal strength falls off with distance once the wave leaves the antenna, by the same inverse-square geometry that governs how loudness drops in the decibel distance calculator, which is why height and a clear takeoff usually beat a few extra watts.
Feedpoint, balun, and trimming to resonance
A dipole is a balanced antenna and coax is an unbalanced line. Feed one directly with the other and RF current flows on the outside of the coax shield, making the feedline part of the antenna: it radiates, it picks up noise, and the SWR you measure belongs partly to the cable. A 1:1 choke balun at the center insulator is the standard fix, and it costs almost nothing next to the wire. While you are at the transmitter end of that coax, the amp and watt calculator is handy for translating between the amplifier’s current draw and the RF power you are actually asking the antenna to handle.
Cut long is not superstition, it is arithmetic: this formula describes a wire in clear space, and your yard adds ground, gutters, fences, and a feedline. The 3 to 5 percent extra the tool’s note recommends is the budget that absorbs all of it. Trimming in small, equal steps from both ends keeps the feedpoint at the electrical center, and stopping as soon as the dip centers on your frequency leaves the antenna exactly as long as your site wants it, which is the only length that was ever going to be right.
What competing calculators miss
The dipole tools ranking for this term lean heavily on a single hard-coded constant. Three gaps kept showing up in that set.
Miss 1: 468/f with no velocity factor. The most common competitor design takes a frequency and returns 468/f, which is correct for exactly one imaginary average wire. Insulated wire at VF 0.88 needs 9.3222 m at 14.15 MHz, not the rule’s 10.08 m equivalent; a tool that cannot express the difference sends that builder three quarters of a meter long with no warning.
Miss 2: the rule presented as magic. Several pages state 468/f or 143/f with no derivation, so a reader cannot tell when the rule applies or why it fails. This page shows the full chain (wavelength, halving, the 492 constant, the 0.95 factor that turns it into 468, and the constant’s documented ARRL history back to 1929) and prints the rule check inside every result, so the approximation is always visibly an approximation.
Miss 3: the vertical treated as a separate hunt. Multi-antenna tools bury the quarter-wave vertical among Yagis and loops, or skip it. Here it is one select field away, with its own 234/f rule check, because it is the same standing-wave physics at a quarter wavelength, and builders comparing the two shapes deserve both answers from one set of inputs.
One honest limit: this is a starting-length tool, not an antenna modeler. It does not simulate patterns, impedance across a band, or the effect of a specific height and soil. For a fixed wire dipole or a simple vertical, a calculated length plus an analyzer trim gets you to resonance; for arrays and exact impedance work, model in NEC-based software first and use this page to sanity-check the element lengths.
Frequently Asked Questions
L = VF x c / (2f), where c is the speed of light and f is the frequency. In feet with f in MHz and typical wire at VF 0.95, it simplifies to the classic L = 468 / f. In meters the same rule is quoted as about 143 / f.
Half the total length, a quarter wavelength each. For 14.15 MHz at VF 0.95 the tool gives 5.0319 m (16.5087 ft) per leg, out of a 10.0637 m total.
Use 0.95 for bare copper wire, about 0.92 for aluminum tubing, and 0.85 to 0.92 for insulated wire, as listed under the calculator. When in doubt, choose the lower value and trim, because cutting long and shortening is easier than adding wire.
End-effect capacitance at the wire tips makes the antenna electrically longer than its physical length, and the wave travels slower on the conductor than in free space. Resonance therefore happens a few percent short of the free-space half wavelength.
It is strongly recommended. A dipole is balanced and coax is not, so a 1:1 choke balun at the feedpoint stops RF current flowing on the outside of the coax shield, which would otherwise radiate and distort the pattern.
L = VF x c / (4f): exactly half the dipole length, or 234 / f in feet with typical wire. Select Quarter-wave vertical in the tool. A vertical also needs radials or a ground plane to work against.
As high as practical. Around half a wavelength up favors long-distance DX contacts, while lower heights concentrate radiation upward for regional NVIS contacts. For HF wire antennas, 30 to 40 ft is a good all-round compromise.
Because height, nearby metal, insulation, and the feedline all shift resonance. If the SWR dip sits below your target, the antenna is electrically long: trim both legs equally. If the dip sits above target, it is too short: lengthen both legs or fold wire back less.
Yes for length: a folded dipole uses the same half-wave formula and the same velocity factor choice. Its feedpoint impedance is different, around 300 ohms, so match it to your feedline accordingly.
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References
- Silver, H. Ward (N0AX), “Where Does 468 Come From?”, QST (ARRL) – archival research tracing the 468/f constant to the 1929 ARRL Handbook and the 1939 ARRL Antenna Book, including the end-effect explanation for the reduction from 492/f. ARRL QST: Where Does 468 Come From? (N0AX). Accessed October 2026.
- National Institute of Standards and Technology, “The 1983 Definition of the Meter and the Speed of Light” – the metrology history behind c = 299,792,458 m/s exactly, the constant the wavelength calculation starts from. NIST: The Meter Definition and the Speed of Light. Accessed October 2026.
- Antenna-Theory.com, “Half-Wave Dipole Antenna” – the reference treatment of the half-wave dipole’s current distribution, resonance, and roughly 73 ohm free-space feedpoint impedance. Antenna Theory: Half-Wave Dipole Antenna. Accessed October 2026.
- Electronics Notes, “Dipole Antenna: What is a Dipole Antenna?” – construction and feed guidance for center-fed dipoles, including balun use at the feedpoint. Electronics Notes: Dipole Antenna Basics. Accessed October 2026.
- Radio Society of Great Britain, amateur licence training lesson on antennas and feeders (G3ZPD mirror) – the end factor correction of about 5 percent and the 468/f design equation as taught for the UK licence. RSGB Training Lesson: Antenna Length and End Factor. Accessed October 2026.
- Federal Communications Commission, “Amateur Radio Service” – the United States band allocations behind the design frequencies in the band table. FCC: Amateur Radio Service. Accessed October 2026.