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SWR Calculator

Calculate Standing Wave Ratio from forward and reflected power readings. Convert SWR to return loss and reflected power percentage.

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Inputs that matter
Forward Power, Reflected Power
Output to expect
SWR Calculator
  • Check the units and required inputs before comparing results.
  • Keep the assumptions with a copied result so you can reproduce the calculation later.
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Reference & details

How it works

SWR from Power

Use square roots of forward and reflected power readings. SWR of 1:1 is perfect match; 1.5:1 or below is acceptable for most transceivers.

SWR = (√P_fwd + √P_ref) ÷ (√P_fwd − √P_ref)

Return Loss

Return loss expresses mismatch in dB. Higher return loss (more negative) is better. 20 dB RL ≈ 1.22:1 SWR.

RL (dB) = −10 × log10(P_ref ÷ P_fwd)

Reflected Power Percentage

At 1.5:1 SWR with 100W forward, roughly 4% (4W) reflects. Modern transceivers fold back power above ~2:1 SWR.

Reflected % = (P_ref ÷ P_fwd) × 100

Updated: July 2026

Example Scenarios

SWR calculates to approximately 1.33:1 — acceptable match for HF operation without tuner.

50W forward, 12.5W reflected yields SWR ~3:1 — investigate connector, antenna length, or mount ground before transmitting.

Multi-band antenna showing 1.2:1 on 20m but 2.5:1 on 15m — tuner recommended for 15m operation or adjust antenna for broader bandwidth.

FAQ

Below 1.5:1 is good for most HF work. Below 2:1 is acceptable. Above 3:1 risks transmitter protection foldback or damage on some rigs. Always check manufacturer limits.

Modern transceivers have SWR protection that reduces power. Extended operation at high SWR can damage finals in unprotected radios. Fix antenna mismatch rather than forcing transmission.

Hams traditionally use SWR ratio. Engineers prefer return loss in dB. This calculator provides both from the same power measurements.

Antennas are resonant at specific frequencies. Off-resonance, impedance mismatch increases SWR. Multi-band antennas or tuners compensate across bands.

Built-in meters are adequate for tuning. External directional wattmeters at the antenna feedpoint are more accurate — coax losses hide reflected power seen at the radio.

About SWR Calculator

Standing Wave Ratio (SWR) indicates antenna system impedance match. Enter forward and reflected power from your wattmeter to calculate SWR, return loss in dB, and percentage of power reflected back to the transmitter.