How do I work out the length of a dipole antenna?

A half-wave dipole uses the ham rule of thumb length in metres equals 142.5 divided by the frequency in MHz, which many people round to 143 divided by f. At 14.2 MHz that is about 10.0 metres tip to tip, so each of the two legs is about 5.0 metres. The 142.5 number already folds in a 0.95 velocity factor for the end effect of real wire, so you do not subtract anything extra.

What is the velocity factor and why is it 0.95?

Radio waves travel slightly slower along a real wire than through free space, and the wire ends radiate, so a resonant antenna comes out a few percent shorter than the ideal half wavelength. The velocity factor captures that. For thin bare wire 0.95 is the standard value. Insulated wire, thick tubing or wire near other conductors runs lower, often 0.90 to 0.93, which is why this tool lets you change it.

How long is a quarter-wave vertical antenna?

A quarter-wave element is exactly half of the half-wave dipole, so its length in metres is about 71.25 divided by the frequency in MHz. For the 27 MHz CB band that is roughly 2.64 metres, which is why a full quarter-wave CB whip is so tall and base-loaded mobile whips are physically shortened. A quarter-wave vertical needs a ground plane or radials to work as the missing half of the antenna.

Why does a higher frequency need a shorter antenna?

Antenna length tracks wavelength, and wavelength is the speed of light divided by frequency, so the two are inversely related. Double the frequency and you halve every length. A 2 metre band dipole at 146 MHz is under a metre tip to tip, while an 80 metre band dipole at 3.6 MHz spans about 39 metres. That is the whole reason VHF handheld antennas are tiny and HF wire antennas fill a backyard.

What is the difference between a half-wave dipole and a full-wave loop?

A half-wave dipole is a straight wire one half wavelength long, fed in the centre, and it is the most common HF antenna. A full-wave element uses a whole wavelength of wire, usually bent into a loop. The full-wave loop in this tool is simply twice the dipole length, about 285 divided by f in metres. Loops are quieter on receive and have a different radiation pattern, which is why contesters like them on the lower bands.

Should I trim the antenna to the exact calculated length?

Treat the calculated length as a starting point, then trim for the lowest SWR with an analyser. Real installations shift resonance through height above ground, nearby trees and roofs, wire sag and insulation. Cut a little long, measure SWR, and shorten in small steps, because adding wire back is harder than removing it. The calculator gets you within a few percent so you only fine tune from there.

Cut an HF dipole for a ham band

You want a 40 metre band dipole centred on 7.1 MHz. Type 7.1 and the half-wave length comes out near 20 metres tip to tip, about 10 metres per leg. Cut each leg a touch long, hang the antenna at its real height, then trim a few centimetres at a time for lowest SWR. The calculator lands you within a few percent so the analyser only has to fine tune.

Size a quarter-wave vertical with radials

Building a ground-mounted vertical for 14.2 MHz? The quarter-wave element reads about 5 metres, and you read the same number this tool gives for one dipole leg, since a quarter wave is exactly half the dipole. Cut your radials around the same length and you have the missing half of the antenna in place.

Check a 2 metre or 70 cm whip before you buy

A VHF handheld at 146 MHz wants a quarter-wave around 49 centimetres, and a 70 cm UHF whip at 435 MHz drops to about 16 centimetres. Punch in the frequency before ordering a replacement antenna so you know the stock rubber duck length is in the right ballpark rather than a random stub.

Plan a full-wave loop for the low bands

Contesters chase quiet receive on 80 metres with a full-wave loop. Set the frequency to 3.6 MHz, switch the result to the full-wave row, and you get the total wire length, roughly 79 metres, before you decide whether the loop even fits the property and how to support the corners.

Antenna Length Calculator for Dipole and Quarter-Wave

Frequency in MHz to dipole, quarter-wave and full-wave length, with velocity factor, in metres or feet, browser-only

Runs locally
Category Calculator
Best for Getting a realistic range before a purchase, plan, workout, or schedule decision.

Enter a frequency in MHz and read the resonant length for a half-wave dipole, a quarter-wave vertical and a full-wave loop. A velocity factor of 0.95 accounts for the end effect that makes real wire resonate a little short. Everything runs in your browser.

Frequency (MHz)Velocity factorUnit

The velocity factor shortens the ideal length. 0.95 suits thin bare wire; insulated wire runs nearer 0.90 to 0.93.

Antenna lengths

Full wavelength λ: 21.112 m
Half-wave dipole (tip to tip): 10.028 m
Each dipole leg: 5.014 m
Quarter-wave (vertical / monopole): 5.014 m
Full-wave (loop): 20.057 m

All math runs in your browser. The frequency you type is never uploaded.

What this tool does

Free antenna length calculator for ham radio, CB, scanner and antenna tinkering. Type a frequency in MHz and read four resonant lengths at once: the full wavelength lambda = c divided by frequency, the half-wave dipole (the classic 143 divided by f rule of thumb), the quarter-wave vertical or monopole, and a full-wave loop. A velocity factor of 0.95 bakes in the end effect that makes real wire resonate a few percent short of the textbook ideal, and you can drop it toward 0.90 to 0.93 for insulated wire. Read every length in metres or feet, see how much each dipole leg should be, and copy the whole set with one click. Higher frequencies give shorter antennas, so 2 metre VHF needs centimetres of wire while an 80 metre band dipole spans tens of metres. Everything runs in your browser, and a shareable URL reopens the exact frequency and velocity factor you used. 100 percent client-side, nothing is uploaded.

Tool details

Input: Numbers; The page exposes text boxes, numeric controls, file pickers, or structured inputs depending on the tool.
Output: Live result + Copy; The result area focuses on usable output, with copy, download, or preview actions when supported.
Privacy: Browser-side processing; The main tool logic does not call an external API, so inputs normally stay in the current tab.
Save / share: Shareable URL state; Key settings are encoded in the URL so another person can reopen the same setup.
Performance budget: Initial JS <= 9 KB; No WASM budget is declared, keeping the tool quick to open on mobile.
Best fit: Calculator · Developer; Category and role tags drive related tools, internal links, and quick fit checks.

How to use

1. Input

Paste or drop your content into the tool panel.
2. Process

Click the button. All processing is local in your browser.
3. Copy / Download

Copy the result or download to disk in one click.

How Antenna Length Calculator fits into your work

Use it for fast estimates, comparisons, and planning numbers before you make the final call.

Calculation jobs

Getting a realistic range before a purchase, plan, workout, or schedule decision.
Comparing scenarios by changing one input at a time.
Turning rough assumptions into a number you can discuss.

Calculation checks

Double-check units, dates, rates, and rounding assumptions.
Treat health, finance, tax, and legal outputs as planning aids, not professional advice.
Save the inputs that produced an important result so you can reproduce it later.

Good next steps

These links move the current task into a more complete workflow.

Real-world use cases

Cut an HF dipole for a ham band
You want a 40 metre band dipole centred on 7.1 MHz. Type 7.1 and the half-wave length comes out near 20 metres tip to tip, about 10 metres per leg. Cut each leg a touch long, hang the antenna at its real height, then trim a few centimetres at a time for lowest SWR. The calculator lands you within a few percent so the analyser only has to fine tune.
Size a quarter-wave vertical with radials
Building a ground-mounted vertical for 14.2 MHz? The quarter-wave element reads about 5 metres, and you read the same number this tool gives for one dipole leg, since a quarter wave is exactly half the dipole. Cut your radials around the same length and you have the missing half of the antenna in place.
Check a 2 metre or 70 cm whip before you buy
A VHF handheld at 146 MHz wants a quarter-wave around 49 centimetres, and a 70 cm UHF whip at 435 MHz drops to about 16 centimetres. Punch in the frequency before ordering a replacement antenna so you know the stock rubber duck length is in the right ballpark rather than a random stub.
Plan a full-wave loop for the low bands
Contesters chase quiet receive on 80 metres with a full-wave loop. Set the frequency to 3.6 MHz, switch the result to the full-wave row, and you get the total wire length, roughly 79 metres, before you decide whether the loop even fits the property and how to support the corners.

Common pitfalls

Forgetting the velocity factor and cutting to the ideal half wavelength. Free space says half a wavelength of a 14.2 MHz signal is about 10.55 metres, but a real wire dipole resonates near 10.0 metres because of the end effect. Cut to the textbook number and the antenna resonates low, forcing you to chop a lot of wire back off.
Mixing up the half-wave dipole length with one leg. The 143 divided by f number is the total tip-to-tip span. Each of the two legs is half of that. People cut two legs each at the full length and end up with a dipole twice as long as intended, resonant an octave too low.
Entering kilohertz or hertz instead of megahertz. The formulas expect MHz, so 14.2 means 14.2 MHz. Type 14200 thinking in kHz and you get a millimetre-scale antenna, an obvious tell that the unit is wrong. Always convert to MHz first.

Privacy

Every length here, the wavelength, the half-wave dipole, the quarter wave and the full-wave loop, is plain JavaScript that runs in your browser tab. The frequency and velocity factor you type never leave the page and nothing is logged. The one caveat: the shareable URL encodes the frequency and velocity factor in the query string, so a share link pasted into chat records those values in the recipient server access log. Those are not sensitive for an antenna, but it is worth knowing where the numbers go.

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Antenna Length Calculator for Dipole and Quarter-Wave

What this tool does

Tool details

How to use

1. Input

2. Process

3. Copy / Download

How Antenna Length Calculator fits into your work

Calculation jobs

Calculation checks

Good next steps

Real-world use cases

Cut an HF dipole for a ham band

Size a quarter-wave vertical with radials

Check a 2 metre or 70 cm whip before you buy

Plan a full-wave loop for the low bands

Common pitfalls

Privacy

FAQ

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