Antenna Gain to Beamwidth Calculator

How it is calculated

The gain in dBi is first turned into a plain ratio: G = 10(gain/10). For a beam that is the same width in both planes, the width in degrees is estimated as θ = √(K / G). The tool shows two versions: one with K = 41253 (the number of square degrees on a full sphere, for an ideal pattern) and one with K = 52525 (the Kraus constant, often used for real parabolic dishes). The linear gain is shown too. These are rules of thumb for a symmetric pencil beam; the true half-power width depends on the antenna's shape and efficiency. At gains around 0 dBi the formula gives angles above 180°, which are not meaningful for a single beam.

Common mistakes

• Entering gain as a plain ratio. The field is dBi, so a gain of 100 times must be entered as 20.
• Treating the two results as separate antennas. They are two estimates for the same gain, using different constants, so expect a difference of roughly 10%.
• Applying it to a sector or omnidirectional antenna. The formula assumes one narrow beam of equal width in both planes, so it misleads for fan-shaped patterns.
• Mixing up dBi and dBd. The tool expects dBi, which is 2.15 dB higher than dBd; entering dBd makes the beam look wider than it is.
• Reading the result as the exact width quoted on a data sheet. It is a rough estimate from gain alone.