Estimate how far a VHF call can reach by line of sight. VHF signals travel roughly in straight lines, so range depends mostly on how high the two antennas are above the water. Enter both heights and the calculator adds the two radio horizons.
Before relying on a handheld for a call to a marina or a coast station, when deciding where to mount a masthead or radar-arch antenna, or when you want to know whether a boat you can see on AIS is within reach of a call. The answer is a planning figure for good conditions. Hills, buildings, weak batteries and bad coax all shorten it.
Height means antenna tip above the water, not above the deck. A masthead antenna on a 40 ft mast with a 4 ft freeboard is about 44 ft up; a handheld held in the cockpit is about 6 ft.
range (statute miles) ≈ 1.415 × (√h₁ + √h₂), heights in feet
range (km) ≈ 4.12 × (√h₁ + √h₂), heights in metres
Multiply statute miles by 0.869 for nautical miles. The factors include a normal allowance for the way the atmosphere bends radio waves slightly over the horizon, which is why radio range is a little longer than the visual horizon.
Powerboat calling a marina. Antenna 9 ft up, marina antenna 50 ft: 1.415 × (3.000 + 7.071) = 14.3 statute miles, about 12.4 nm.
Two small boats. Both antennas at 9 ft: 1.415 × (3 + 3) = 8.5 statute miles. That is why boat-to-boat calls fade at much shorter range than calls to shore.
Sailboat masthead to a coast station on a hill. 44 ft and 300 ft: 1.415 × (6.633 + 17.321) = 33.9 statute miles.
| Your antenna | Other boat at 9 ft | Station at 50 ft | Station at 200 ft |
|---|---|---|---|
| 4 ft | 7.1 | 12.8 | 22.8 |
| 6 ft | 7.7 | 13.5 | 23.5 |
| 9 ft | 8.5 | 14.3 | 24.3 |
| 15 ft | 9.7 | 15.5 | 25.5 |
| 25 ft | 11.3 | 17.1 | 27.1 |
| 44 ft | 13.6 | 19.4 | 29.4 |
| 60 ft | 15.2 | 21.0 | 31.0 |
Each figure comes from the formula above. A handheld is the 4 to 6 ft rows.
In an emergency, call on channel 16 even if the calculation says you are at the edge of range: other boats, aircraft and relay stations may hear you. Carry a handheld as a backup to the fixed set, and know that DSC distress alerts from a fixed set carry your position if the radio is connected to GPS.
Enter 0 for your own horizon only, or a typical height: other small boats 6 to 10 ft, marinas 30 to 60 ft, coast stations often several hundred ft.
No. It estimates geometry only.
Choose “Meters to km” and enter heights in metres.