Tennis String Gauge Guide
Short answer: Gauge is the string’s diameter. On the US scale a higher number means a thinner string: 16 gauge is about 1.26–1.34 mm and 17 gauge about 1.16–1.24 mm. Thinner strings are more flexible but less durable. The International Tennis Federation (ITF) finds little empirical evidence that thinner strings add spin, and none of our sources gives a tension adjustment for gauge, so check the millimetres, change one thing at a time and measure the result.
Tennis string gauge chart
The ITF says the popular gauges lie between 1 and 1.5 mm, or 15 to 18 on the American scale. The rows below are taken from the ITF’s comparison of European and US gauges.
| US gauge | Millimetres | Inches |
|---|---|---|
| 15 | 1.41–1.49 mm | 0.056–0.059 in |
| 15L | 1.33–1.41 mm | 0.052–0.056 in |
| 16 | 1.26–1.34 mm | 0.050–0.053 in |
| 16L | 1.22–1.30 mm | 0.048–0.051 in |
| 17 | 1.16–1.24 mm | 0.046–0.049 in |
| 18 | 1.06–1.16 mm | 0.042–0.046 in |
| 19 | 0.90–1.06 mm | 0.035–0.042 in |
Read the millimetres, not just the number. Gauge labels are ranges, and brands do not all map them the same way. Wilson’s string guide, for example, lists 15 gauge as 1.40 mm and 17 gauge as 1.25 mm, each just outside the ITF ranges above. Two strings both sold as “17” can therefore differ in diameter, so compare the millimetre figure when you switch brands and record it with each string job.
Thin vs. thick tennis strings
The ITF states the core trade-off: a thinner gauge makes the string more flexible but less durable, because small movements between the strings wear them until they snap where they cross. Wilson’s guidance points the same way in playing terms.
| Property | Thinner (higher number) | Thicker (lower number) | Evidence |
|---|---|---|---|
| Flexibility | More flexible | Less flexible | ITF technical guidance |
| Durability | Wears through sooner | Lasts longer | ITF technical guidance; Wilson |
| Feel | More power, comfort, touch and feel | More control | Wilson (manufacturer guidance) |
| Spin | Widely believed to add spin | — | ITF: little empirical evidence |
Wilson’s own gauge list follows that pattern: it suggests 15 gauge for maximum durability and control, 16 for competitive players who break strings often, 17 for players wanting power and comfort, and 17L or 18 for players wanting more touch and feel. Those are one manufacturer’s recommendations, not measured outcomes.
Do thinner strings give more spin?
Not reliably, on the evidence available. The ITF writes that although thinner strings are believed to impart more spin, there is little empirical evidence to support the idea. In the same passage it says there is no appreciable difference in spin between rough or smooth strings, or between low- and high-tension strings.
Choose a gauge for durability and feel, then judge spin on court. If spin is your goal, string movement and your swing matter more; the spin guide explains what the laboratory studies found.
Should you change tension when you change gauge?
None of the sources on this page gives a pound or kilogram adjustment for gauge. Wilson’s starting ranges are set by material, not thickness, which is why our tension calculator records gauge as context but does not change the range for it.
What the sources do support is that thickness and tension both change how the stringbed feels, so changing both at once makes the result hard to read. A practical routine:
- Keep the reference tension the same the first time you try a new gauge of the same string.
- Take a fresh baseline reading after stringing, using the routine in our guide to measuring string tension. Treat the new gauge as a new setup with its own baseline, rather than comparing it with the old string’s readings.
- Adjust tension in a later string job if the feel is not right, by one small step inside the frame’s printed range.
In a hybrid, gauge is set separately for the mains and the crosses. Babolat notes that playing with the gauges, tensions and mains-cross choices is what gives a hybrid its range; the hybrid tension guide covers how to record and compare those choices.
Choosing a gauge
These rules are String Tension AI’s reading of the sources above. They are starting points to test.
- You break strings within a few sessions: try one step thicker in the same string, for example 17 to 16.
- Your strings last well but feel harsh or dull: try one step thinner, accepting that they may wear through sooner.
- You are changing material: keep the gauge the same for the first string job, so you can tell what the material changed. The guide to tennis string types explains the materials.
- You are comparing two strings: compare them in the same gauge, or at least note both millimetre figures.
For the vocabulary used here, see the tennis stringing glossary. To understand how string spacing interacts with thickness and tension, read string pattern and tension.
Tennis string gauge: FAQ
What gauge is 1.25 mm?
On the ITF’s gauge table, 1.25 mm falls just above the 17 gauge range (1.16–1.24 mm) and inside 16L (1.22–1.30 mm), yet Wilson labels 1.25 mm as 17. Because labels vary, compare strings by their millimetre figure rather than the gauge number alone.
Is a thinner string better?
It depends on what you want. The ITF says a thinner gauge is more flexible but less durable, and Wilson links thinner strings with power, comfort and feel and thicker strings with durability and control. If you break strings often, a thicker gauge is the usual first change; if strings last well but feel harsh, try one step thinner.
Do thinner strings give more spin?
The evidence is weak. The ITF writes that although thinner strings are believed to impart more spin, there is little empirical evidence to support it. String movement and your swing matter more, so choose gauge for durability and feel and judge spin on court.
Do I need a different tension for a different gauge?
None of our sources publishes a tension adjustment for gauge, and Wilson’s starting ranges are set by material. Keep the reference tension the same the first time you try a new gauge, take a fresh baseline reading, and change tension in a later string job only if the feel is not right.
What the sources support
The ITF document supports the gauge-to-millimetre table, the flexibility and durability trade-off, and the weak evidence for thinner strings adding spin. Wilson’s guidance supports its gauge-by-player recommendations and its millimetre labels. Babolat’s guidance supports adjusting gauges separately in a hybrid. The decision rules and the testing routine are String Tension AI’s method.
Accessed October 3, 2026
- International Tennis Federation — Rackets and Strings: Strings (November 2019) for the gauge table, flexibility and durability, and spin evidence (governing-body technical guidance).
- Wilson — How to Choose Tennis String (updated January 18, 2024) for its gauge list in millimetres and its thicker-versus-thinner recommendations (manufacturer guidance).
- Wilson — What String Tension Should I Use in My Tennis Racket? for starting ranges set by material rather than gauge (manufacturer guidance).
- Babolat — Tennis Hybrid for adjusting gauges and tensions separately in mains and crosses (manufacturer guidance).
Give each new gauge its own baseline
Measure the racquet fresh after stringing, then compare later readings with that baseline to see how the new setup settles.
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