A Yagi is the classic directional antenna: a row of parallel elements on a boom, with one driven element fed by the cable, a longer reflector behind it, and shorter directors in front. Together they focus energy forward and reject signals from behind, giving gain that a simple dipole cannot match. This Yagi antenna element length calculator gives the starting lengths for the three element types, plus suggested spacing, so you can cut a first version and tune it into a working beam.
The formulas are simple fractions of the dipole family, all in feet with frequency in megahertz. The reflector is 492 divided by frequency, about 5 percent longer than the driven element. The driven element is 468 divided by frequency, the standard dipole length. Each director is 440 divided by frequency, about 6 percent shorter than the driven element. For example, at 144.2 megahertz the reflector is about 3.41 feet, the driven element about 3.25 feet, and each director about 3.05 feet, or in metres about 1.04, 0.99, and 0.93.
Each element has a job. The reflector sits behind the driven element and reflects energy forward, like a mirror behind a lamp. The driven element is the only one connected to the cable, and it does the actual radiating and receiving. The directors sit in front and guide the wave forward, each one slightly shorter so the wave speeds up along the row. Add more directors and the beam gets longer, narrower, and higher in gain, but also harder to tune and more sensitive to construction errors.
Spacing matters as much as length. A common starting point puts the reflector 0.15 wavelengths behind the driven element and the first director 0.10 wavelengths in front, with later directors spaced about 0.10 wavelengths apart. At 144.2 megahertz the wavelength is about 2.08 metres, so those spacings are about 0.31 and 0.21 metres. Closer spacing gives a shorter boom and a wider pattern, while wider spacing gives more gain but a longer boom. The tool gives the two key spacings in metres so you can mark the boom before cutting.
Element diameter shifts every length. The formulas assume thin elements, and fat tubing resonates lower for the same length, so thick elements must be cut shorter. Boom material matters too: a metal boom passing through the elements changes their effective length, while an insulating boom does not. That is why published Yagi designs always state element diameter and boom type, and why this calculator calls its answers a starting point. Copy a proven design for your band when you can, and use these lengths to check it or to scale it to a new frequency.
Tuning a Yagi is a patient job. Build the driven element first, cut all elements a little long, and assemble the beam at its final height, because height and nearby metal shift tuning. Measure the driven element standing wave ratio with an analyzer, trim the driven element to centre it, then adjust director lengths in small steps while watching forward gain or front-to-back ratio if you can measure it. Never judge a Yagi by standing wave ratio alone: a beam can show a fine match and still have poor gain if the directors are wrong.
Safety and law apply as usual. A Yagi on a mast must survive wind, so use a proper mast, guying, and a rotator rated for the wind load, and never work on a mast near power lines. The beam concentrates power forward, so keep people clear of the main beam when transmitting, and follow your licence band and power rules. This calculator is an educational estimator: it teaches the element formulas and gives a build starting point. Real transmitting antennas need proper measurement and must follow your local regulations.
In short, this tool gives the three Yagi lengths from frequency: reflector from 492 divided by frequency, driven from 468, and directors from 440, all in feet and metres, plus reflector and director spacing in metres. Cut long, respect element diameter and boom effects, tune the driven element first, and check the pattern as well as the match. A well-tuned Yagi turns modest power into real distance.