VSWR Calculator (Voltage Standing Wave Ratio)

Background.

Feed a 75 Ω antenna through 50 Ω coax and the radio's meter settles at 1.5:1. Is that worth fixing? The reading means the load has sent part of the wave back down the cable, where it overlaps with the outgoing wave and leaves a fixed pattern of voltage peaks and troughs along the line. VSWR is the ratio of the highest of those voltages to the lowest. Here, 4 % of the power returns and 96 % reaches the antenna, a loss of under two tenths of a decibel.

A meter's VSWR, an analyser's return loss, a datasheet's reflection coefficient, a wattmeter's forward and reflected watts and a pair of impedances all describe one number, the magnitude of the reflection coefficient |Γ|, so any one converts into the rest.

Even 2:1 still delivers 88.89 % of the forward power. The often-quoted 2:1 limit is a convention, not a published standard, so judge the result against your own equipment's specification.

What is vswr calculator?

VSWR, the voltage standing wave ratio, is the ratio of maximum to minimum voltage along a transmission line feeding a mismatched load. It equals (1 + |Γ|) ÷ (1 − |Γ|), where the reflection coefficient Γ is the reflected voltage wave divided by the forward one. A perfect match is 1:1; a short, an open or a purely reactive load reflects everything and gives an infinite ratio.

How to use this calculator.

1. 01Choose the figure you already have; only the fields for that mode are read.
2. 02For impedances, enter the load's resistance and the cable's characteristic impedance from its jacket: 50 Ω for coax such as RG-58 or LMR-400, 75 Ω for RG-6 and video cable.
3. 03For a wattmeter, enter forward and reflected power in the same unit. They are powers, so 25 W reflected from 100 W forward means |Γ| = 0.5, not 0.25.
4. 04Enter return loss as a positive number of decibels. If your analyser shows −20 dB, enter 20.

The formula.

Every mode first recovers |Γ|. From impedances, |Γ| = |Z_L − Z_0| ÷ (Z_L + Z_0); from a VSWR reading, (VSWR − 1) ÷ (VSWR + 1); from return loss, 10^(−RL/20); from a wattmeter, √(P_reflected ÷ P_forward), since |Γ| is a voltage ratio and power goes as voltage squared. Then VSWR = (1 + |Γ|) ÷ (1 − |Γ|), return loss = −20 log₁₀|Γ|, reflected power = 100|Γ|² %, and mismatch loss = −10 log₁₀(1 − |Γ|²) dB. Marki Microwave prints all of these relations, and Keysight gives the same definitions of VSWR and return loss.

The reading beside the result sorts the mismatch at 1 %, 10 % and 50 % of power reflected (return losses of 20, 10 and 3.01 dB) as description, not as a pass mark. Impedances are treated as purely resistive, so for a load with reactance the true reflection is larger than shown. A perfect match reports return loss at a labelled 300 dB ceiling in place of infinity, and total reflection, |Γ| = 1, is refused because no finite VSWR exists.

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; this calculator reproduces five of its rows.
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. This calculator reproduces five of its rows.
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.