Standing wave ratio, SWR, is the number every radio operator watches, but it is not itself a power figure. It describes how much of your signal bounces back from the antenna instead of radiating. This SWR reflected power calculator turns that ratio into watts: enter the SWR and the forward power from your transmitter, and it gives the reflection coefficient, the reflected percentage, the reflected watts, and the watts actually delivered to the antenna. That translation is what makes SWR readings useful.
The maths starts with the reflection coefficient, called gamma. Gamma equals SWR minus one, divided by SWR plus one. At a perfect SWR of 1.0, gamma is zero and nothing reflects. At an SWR of 2.0, gamma is one third, about 0.333. At an SWR of 3.0, gamma is one half. The reflected power fraction is gamma squared, so at SWR 2.0 about 11.1 percent of power reflects, and at SWR 3.0 about 25 percent reflects. Multiply that fraction by forward power to get reflected watts, and subtract from forward power to get delivered watts.
Here is a worked example. Your transmitter sends 100 watts forward and the meter shows SWR 2.0. Gamma is 0.333, the reflected fraction is 0.111, so about 11.1 watts reflect and about 88.9 watts reach the antenna. Raise the SWR to 3.0 at the same 100 watts and about 25 watts reflect, leaving 75 watts delivered. At SWR 1.5 only 4 percent reflects, so 96 watts of 100 reach the antenna. These numbers show why chasing SWR from 1.5 down to 1.1 gains almost nothing: you recover only about 3 watts in 100.
Delivered power is not the same as radiated power. The watts that reach the antenna still lose some energy as heat in the antenna, the matching network, and the ground system before the rest radiates. An antenna with a perfect SWR can still be a poor radiator if it is low, lossy, or badly grounded, while an antenna with a moderate SWR can radiate well. SWR tells you about the match between cable and antenna, not about efficiency. Judge an antenna by match plus field results, never by match alone.
Reflected power causes real problems beyond lost watts. It heats the transmitter output stage, it raises voltage on the cable, and in extreme cases it can damage a transmitter that lacks protection. Many modern radios reduce power automatically when SWR climbs, which protects the radio but also cuts your signal. Cable loss complicates the picture further: a long lossy cable absorbs reflected energy on the way back, so the meter at the radio can show a better SWR than the antenna really has, while the cable quietly turns your power into heat.
A few habits keep SWR readings honest. Measure at the antenna feed point when you can, because that is where the match actually lives. If you measure at the radio, remember that cable loss flatters the reading. Measure across the whole band, not at one frequency, since SWR rises toward the band edges on most antennas. And check connectors and cable first when SWR suddenly worsens: a corroded connector or a crushed cable is a more common cause than a broken antenna element.
What SWR should you accept? Below 1.5 is good for most amateur work, and below 2.0 is usable with most modern radios, losing only about 11 percent of power. Above 2.0, fix the antenna or add a tuner, and above 3.0, reduce power until you understand the fault, because reflected power and voltage climb fast. A tuner at the radio improves the match the radio sees, but it does not fix the antenna or recover cable losses: the tuner only hides the mismatch from the transmitter, while the antenna system still wastes energy.
In short, this tool converts SWR and forward power into the numbers that matter: reflection coefficient from SWR minus one over SWR plus one, reflected percentage from gamma squared, reflected watts, and delivered watts. Use it to judge whether a match problem is worth fixing, to protect your transmitter, and to keep perspective: below 2.0 the loss is small, and a good antenna with a modest match beats a poor antenna with a perfect one.