VSWR Calculator (Voltage Standing Wave Ratio)

Background.

A transmission line only delivers all its power when the load at the far end has the same impedance as the line itself. When it does not, part of the wave turns round at the load and travels back towards the source. The forward and reflected waves add along the cable, and because they drift in and out of phase with distance they produce a stationary pattern of voltage maxima and minima. The ratio of the largest of those voltages to the smallest is the voltage standing wave ratio — VSWR — and it is the number most commonly quoted to describe how well an antenna, a filter or an amplifier is matched.

VSWR is only one of five ways the same fact gets written down. A vector network analyser is more likely to give you return loss in decibels. A component datasheet may quote a reflection coefficient. A transmitter's directional wattmeter gives you a forward and a reverse power reading. And if you know the load and the line impedance you can compute the lot from first principles. All five say exactly the same thing, and this page converts freely between them so that a figure from one instrument can be compared with a specification written in another.

The number worth watching is usually not the VSWR itself but the fraction of power that actually reaches the load. The relationship is less alarming than the ratio looks. A 1.5:1 VSWR reflects 4 % of the power and delivers 96 %. Even a 2:1 VSWR — the figure most often treated as a red line — still delivers 88.9 %, which is a loss of half a decibel. What makes a high VSWR genuinely worth fixing is rarely the delivered power: it is the extra heating in a lossy cable, the voltage peaks on the line, and the fact that most solid-state transmitters reduce their own output to protect themselves when they see too much reflected power.

Two limits are stated here rather than in a collapsed answer below. This page works with the magnitude of the reflection coefficient and treats both impedances as purely resistive, so a load with significant reactance is at least as badly matched as the figure shown and usually worse. And the bands in the result are descriptive landmarks at 20, 10 and 3 decibels of return loss, not a pass mark. What counts as an acceptable VSWR is set by your own system's specification; the familiar 2:1 is a convention rather than a published standard.

What is vswr calculator?

The reflection coefficient Γ is the ratio of the reflected voltage wave to the forward one at the load. For purely resistive impedances it is (ZL − Z0) divided by (ZL + Z0), which is zero when the two match, positive when the load is the larger, and negative when it is the smaller. Its magnitude is what every quantity on this page is built from.

VSWR is (1 + |Γ|) divided by (1 − |Γ|). It runs from 1 for a perfect match upwards without limit, and it is infinite for a short circuit, an open circuit or a purely reactive load, because all three reflect the entire wave. Return loss is the same information as a decibel figure, −20·log|Γ|, quoted as a positive number in which a bigger value is a better match. Mismatch loss, −10·log(1−|Γ|²), is the power lost purely because it was reflected.

Because power goes as the square of voltage, the reflection coefficient squared is the fraction of power that comes back. That single fact explains why the numbers feel gentler than they look: a |Γ| of 0.2 reflects 4 % of the power, not 20 %, and even |Γ| = 0.5 — a 3:1 VSWR — still delivers three quarters of it.

How to use this calculator.

1. 01Choose whichever figure you already have. The impedances mode is for design work; the other four are for reading an instrument.
2. 02In the impedances mode, enter the load's resistance and the cable's characteristic impedance — 50 Ω for most RF coax, 75 Ω for RG-6 and video. The order does not matter: 25 Ω on a 50 Ω line and 100 Ω on a 50 Ω line both give 2:1.
3. 03In the powers mode, enter the forward and reverse readings from a directional wattmeter in the same unit. Remember they are powers: 25 W back out of 100 W forward is a reflection coefficient of 0.5, not 0.25.
4. 04Enter return loss as a positive number of decibels. If your analyser shows it as a negative reflection figure, drop the sign.
5. 05Read the power reaching the load as well as the ratio. It is the figure that tells you whether a mismatch actually matters, and it is usually far less dramatic than the VSWR suggests.
6. 06Compare the result against your own system's specification rather than against a remembered rule of thumb. The bands here describe the size of the mismatch; they do not pass or fail it.

The formula.

Everything on this page is a rearrangement of the reflection coefficient, so each mode's job is to recover |Γ| and then run the same four expressions.

From impedances, Γ is (ZL − Z0)/(ZL + Z0), and the magnitude is taken because the sign only records which of the two is larger. That is why a 25 Ω load and a 100 Ω load on the same 50 Ω line both come out at 2:1. From a VSWR reading, |Γ| is (VSWR − 1)/(VSWR + 1). From a return loss, |Γ| is ten raised to minus the return loss over twenty. From a pair of wattmeter readings, |Γ| is the square root of the reflected power divided by the forward power — the square root matters, because the coefficient is defined on voltage while the meter reads power.

With |Γ| in hand, VSWR is (1+|Γ|)/(1−|Γ|), return loss is −20·log|Γ|, the reflected power fraction is |Γ|² and the mismatch loss is −10·log(1−|Γ|²).

The worked example is a 75 Ω load on a 50 Ω line. Γ is 25/125 = 0.2 exactly, so the VSWR is 1.2/0.8 = 1.5 exactly, the return loss is −20·log(0.2) = 13.9794 dB, the reflected power fraction is 0.04 and the mismatch loss is −10·log(0.96) = 0.1773 dB. Two independently published conversion tables agree: Mini-Circuits lists a 1.50 VSWR against 14.0 dB of return loss, 0.177 dB of transmission loss, |Γ| of 0.20 and a 96.0 / 4.0 per cent split, and Marki Microwave lists 14 dB of return loss against a VSWR of 1.50, |Γ| of 0.200, 0.176 dB of mismatch loss and 3.98 / 96.02 per cent.

Two singular cases are handled differently, on purpose. A perfect match has infinite return loss but a perfectly ordinary VSWR of 1, zero mismatch loss and 100 % delivery — five of six figures are meaningful, so the page accepts it and reports the return loss at a labelled ceiling of 300 dB. Total reflection is different: both the VSWR and the mismatch loss are infinite and nothing at all reaches the load, so the page refuses that input and explains why instead of inventing a number.

All arithmetic runs as exact decimals at forty significant digits and is rounded once, at the return boundary, to ten decimal places. The band in the verdict is decided on the unrounded reflected-power fraction, so a figure that displays as exactly 1.0000000000 % but is really a shade above it is described as being in the band above.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Marki Microwave, "Return Loss to VSWR Conversion Table". Read directly from the PDF on 2026-07-29. Prints every relation implemented on this page — Γ = 10^(−RL/20), Return Loss (dB) = −20 log|Γ|, VSWR = (1+|Γ|)/(1−|Γ|), Γ = (VSWR−1)/(VSWR+1), Mismatch Loss (dB) = −10 log(1−Γ²), Through Power (%) = 100(1−Γ²), Reflected Power (%) = 100·Γ² — and tabulates return loss from 1 to 40 dB against all of them. The 14 dB row reads VSWR 1.50, Γ 0.200, mismatch loss 0.176 dB, 3.98 % reflected, 96.02 % through; five of its rows are asserted directly in this calculator's tests.
2. [2]Mini-Circuits, application note AN-40-013 rev A (14 April 2015), "The Effect of VSWR on Transmitted Power". Read directly from the PDF on 2026-07-29. An independent table from a different manufacturer, indexed by VSWR rather than by return loss, with columns for VSWR in dB, return loss, transmission loss, voltage reflection coefficient, and power transmitted and reflected as percentages. The VSWR 1.50 row reads 14.0 dB return loss, 0.177 dB transmission loss, |Γ| 0.20, 96.0 % transmitted and 4.0 % reflected — the same point as Marki's 14 dB row, reached from the opposite direction. Five of its rows are asserted directly in this calculator's tests.
3. [3]Keysight Technologies, "VSWR / Return Loss Calculator" knowledge page. Retrieved 2026-07-29. A test-and-measurement manufacturer's statement of the same two core relations: VSWR = (1+|Γ|)/(1−|Γ|) and Return Loss (dB) = −20 log10|Γ|. Cited as a third, instrument-maker's confirmation of the definitions the two conversion tables above are built on.
4. [4]IEC 60050-726, International Electrotechnical Vocabulary, Part 726: Transmission lines and waveguides — the definitional authority of record for the terms standing wave ratio, reflection coefficient, return loss and characteristic impedance. Access: gated. The IEC sells this part and only its front matter was retrievable; no clause of it was opened, and nothing on this page is attributed to a specific IEV entry. It is named here so that a reader who needs the formal definitions knows where they live rather than being sent to a secondary source that paraphrases them. IEEE Std 145, "IEEE Standard for Definitions of Terms for Antennas", is the equivalent authority on the antenna side and is likewise gated and unopened.