Tennis string gauge is a number for a string's thickness: the higher the number, the thinner the string. Because gauge numbers cover overlapping ranges and are not applied identically by every brand, the diameter in millimetres is the more reliable figure to compare. Thinner strings tend to feel livelier and break sooner; thicker strings tend to last longer and feel firmer.
Key points
- Gauge runs backwards: a 17 is thinner than a 16, and an "L" (light) sits between two whole gauges.
- Gauge numbers describe a range of diameters, and brands do not all use the same ranges, so check the millimetre figure on the packet.
- Thickness affects durability most clearly. Its effect on spin is smaller and less certain than often claimed.
- To compare gauges, test the same string in two thicknesses at the same tension.
Gauge versus diameter in millimetres
Gauge is a label; diameter is a measurement. According to string manufacturer Ashaway, gauge numbers started out based on industrial wire gauge sizes, but as string makers produced more and more sizes, the industry agreed that each number would stand for a range of allowed diameters rather than one exact size. The "L" (light) versions, such as 15L and 16L, fill the gaps between whole numbers.
The table below puts two published charts side by side. It is for reference, not a recommendation, and shows how far the ranges can differ depending on who publishes them.
| Gauge | Ashaway chart | Tennis Warehouse rough guide |
|---|---|---|
| 15 | 1.41–1.49 mm | 1.35–1.40 mm |
| 15L | 1.33–1.41 mm | Not listed |
| 16 | 1.26–1.34 mm | 1.25–1.30 mm |
| 17 | 1.16–1.26 mm | 1.20–1.25 mm |
| 18 | 1.06–1.16 mm | 1.15–1.20 mm |
| 19 | Not listed | 1.10–1.15 mm |
Why gauge labels vary between brands
There are two reasons the same label can mean different things.
The ranges overlap
On Ashaway's chart, the 15L and 16 ranges overlap, so a string measuring 1.33 or 1.34 mm can correctly be called either. Ashaway puts it plainly: within that overlap, the only difference is a question of marketing. The same happens at the edge of other ranges.
There is no single universal chart
Tennis Warehouse notes that there is no standardised, universal gauge chart, and that one company's 16 might be another company's 16L. Its advice is to compare the actual millimetre sizing instead. The table above shows the problem: a 1.25 mm string sits inside Ashaway's 17 range but at the boundary of 16 and 17 on the Tennis Warehouse guide.
What to do about it
When you compare two strings, compare millimetres, not gauge numbers. Many manufacturers print both on the packet. Some, including Pro Stringer Strings, list their strings by diameter only (for example Blue Steel in 1.23 and 1.28 mm), which avoids the ambiguity. Keep in mind that a printed diameter is a nominal figure; a string that has been stretched and played with may measure a little thinner.
What string thickness changes
For the same material and construction, here is what usually changes as a string gets thinner or thicker. Material has a bigger effect than gauge, so a thin polyester and a thick multifilament will not behave like "thin" and "thick" versions of each other. Our tennis strings guide covers the materials themselves.
Durability
This is the clearest effect, and the one most sources agree on: thicker strings are generally more durable. Players who break strings often usually move to a thicker version of the same string. A thinner string tends to notch (develop grooves where the strings cross) and break sooner, especially for players who hit with a lot of topspin.
Feel and power
Tennis Warehouse's string guide summarises the general view: thinner strings give more power and spin, and thicker strings give more control and durability. In practice, many players describe a thinner string as livelier and a thicker one as firmer or more muted. These differences are real to many players, but they are usually smaller than the difference between materials.
Spin
Thinner strings are widely said to add spin. Controlled testing gives a more cautious picture. Tennis Warehouse University's string movement research lists "string gauge makes no difference to spin" among the findings from its earlier experiments, and points to string material and string movement as the bigger factors. So if spin is your main goal, changing material is likely to do more than changing gauge.
Tension and comfort
Thickness and tension interact: the same reference tension can feel different on a thin string and a thick one. Rather than repeat that here, our tennis string tension guide explains how to adjust tension, starting from the range printed on your racquet.
Choosing a gauge
There is no single best gauge. Start from your current string and ask what you want to change.
If you break strings often
Try a thicker version of the string you already like. This is the most direct way to get more life from a string without changing how it plays in other ways.
If your strings last a long time but feel dull
Try a thinner version of the same string. You may get a livelier feel, at the cost of a shorter life. If the string then breaks too quickly, go back up.
If you use a hybrid
Mains and crosses do not have to be the same thickness. Some players use a slightly thinner string in one direction to change the feel while keeping durability in the other. Treat this as one more variable to test, not a fix in itself.
If you are not sure
Choose a middle diameter from the string range you are interested in, then adjust thicker or thinner based on how long it lasts and how it feels. Most string ranges are sold in more than one diameter, so you can change thickness without changing anything else.
How to test a gauge change
Gauge differences are small, so a casual test will rarely tell you much. A structured test will.
- Keep everything else the same: the same string model, the same racquet and the same tension. Change only the diameter.
- If you can, string two identical racquets, one in each diameter, and swap between them in the same session.
- Note the date each racquet was strung and roughly how many hours you play on it.
- After each session, write down depth, comfort, how the ball feels on touch shots, and any notching or fraying.
- Record when each string bed breaks or goes dead. Durability is the one difference you can measure, not just feel.
If you are trying to separate a change in thickness from a change in the string job, consistent stringing matters. Our tennis racquet restringing guide covers what a good restring looks like, and our guide to how long tennis strings last explains the signs of a worn string bed.
Frequently asked questions
Is 16L thinner than 16?
Yes. "L" stands for light, so 16L sits between 16 and 17. Because the ranges overlap and brands use them differently, check the millimetre figure to be sure.
Why does my string's gauge not match the chart?
Because there is no single universal chart. Charts published by different companies use different ranges, and some diameters fall into two gauges at once. The diameter in millimetres is the figure to trust.
Can I measure a string's diameter myself?
Yes, with a micrometer or digital calliper on an unused piece of string. Measure new string if you want to compare with the packet; a used string may read slightly thinner.
Your next step
Find the millimetre figure for the string you use now, then decide whether you want more durability (go thicker) or a livelier feel (go thinner) and test one change at a time. If you string your own racquets, the Pro Stringer Learning Center has step-by-step stringing videos.
