Dipole Antenna Length Calculator
Use this tool to estimate the total wire length and each leg length for a half-wave dipole antenna.
What this dipole antenna calculator does
A dipole antenna is one of the most popular and effective antenna designs in amateur radio. This calculator helps you quickly estimate wire lengths for a half-wave dipole based on your operating frequency. It returns both the total tip-to-tip length and the length of each side (leg), in meters and feet.
Core formula used
The theoretical half-wave dipole length is derived from wavelength:
Half-wave dipole total length = λ / 2
Adjusted cut length = (λ / 2) × Velocity Factor × (1 − Trim%)
Where c is the speed of light and f is frequency in hertz. A common radio shortcut is:
This shortcut is convenient, while the calculator above is a bit more flexible because it lets you set your own velocity factor and trim amount.
How to use the calculator
- Enter your target operating frequency (for example, 7.2 for the 40-meter band in MHz).
- Select the correct frequency unit (Hz, kHz, or MHz).
- Set a velocity factor. A default of 0.95 is a good practical starting point for many wire installations.
- Click Calculate.
- Cut the antenna slightly long in real life, then trim during tuning for best SWR.
Example calculation
20-meter band example
Suppose you want to operate around 14.2 MHz. Enter 14.2, keep velocity factor at 0.95, and press calculate. You will get a total length and each-leg length you can use as a starting cut. After installation height, surroundings, and feedline effects are considered, final tuning is usually done by trimming both legs equally.
Practical installation and tuning tips
- Install the antenna as high and in the clear as possible for better performance.
- Keep both legs equal length to preserve balance.
- Tune in the final installed position; antennas often shift resonance after raising them.
- Trim in small increments (1 to 2 cm / under 1 inch per side) to avoid overcutting.
- Use a choke/balun at the feed point to reduce common-mode current on the coax.
Common mistakes to avoid
- Using the wrong frequency unit (MHz vs kHz).
- Cutting to exact calculated length without leaving room for tuning.
- Ignoring nearby metal objects, gutters, towers, or power lines that detune the antenna.
- Assuming one formula works perfectly for every wire type and mounting style.
Final note
This dipole antenna calculator gives a strong starting point for building a resonant HF wire antenna. Real-world tuning is always part of the process, but with accurate calculations and careful trimming, you can quickly dial in a highly effective antenna for your target band.