The Tennis String Handbook: Materials, Tension & Gauge
The Tennis String Handbook
Most players spend hours agonising over which racket to buy, then ask the stringer for "whatever you'd recommend" five minutes before handing it over. That's a mistake. The strings are the only part of the racket that actually touches the ball. Frame technology has barely shifted in two decades; string technology has been rewritten from the ground up. If you understand strings, you understand most of what makes a racket feel the way it does.
Material matters more than tension. Switching from a polyester to a multifilament will change your racket more than dropping 2 kg of tension ever could.
Polyester loses tension fast. A poly bed can drop 10–15% of its tension in the first 24 hours after stringing. By week three it plays like a different string.
Thinner = more spin and feel, but shorter life. 1.20 mm strings bite the ball harder than 1.30 mm but break sooner.
Most club players are over-strung. If you're not generating heavy racket-head speed, polyester at 25 kg is probably stealing power from you.
What strings are actually made of
Walk into any stringing shop and the wall of options can feel arbitrary. It isn't. Every tennis string sold today belongs to one of five material families, and each family has a distinct personality the moment ball meets stringbed.
01Natural gut
The original tennis string, and still the benchmark everything else is measured against. Made from a thin ribbon of cow serosa (the outer layer of the small intestine), twisted and coated. It is the most comfortable string you can play with, holds its tension better than any synthetic, and feels alive on the strings. It also costs roughly four times what a decent polyester does, hates moisture, and breaks if you look at it funny on a humid clay court.
Federer used a natural gut hybrid for his entire career. So does Djokovic. There's a reason.
02Polyester (co-poly)
The string that changed professional tennis. Co-polyester arrived properly in the late 1990s when Gustavo Kuerten won Roland Garros with Luxilon ALU Power, and within a decade the entire ATP tour had switched. Poly is stiff, durable, and lets players swing as hard as they physically can without spraying balls long. It is the reason modern tennis looks the way it does: heavy topspin, full swings, baseline rallies played from three metres behind the line.
The downside is that polyester is unforgiving. It transmits more shock to the arm than any other string type, and it loses playable tension quickly. For players who don't generate their own racket-head speed, a full poly setup feels like hitting with a board.
03Multifilament
Hundreds, sometimes thousands, of microscopic nylon or polyamide fibres bundled together and coated. Engineered to mimic the feel of natural gut at a fraction of the price. Soft on the arm, powerful, comfortable. The trade-off is durability and control — multifilaments fray, lose tension unevenly, and don't grip the ball the way poly does, so big topspin hitters tend to break them in hours.
04Synthetic gut
A solid nylon core wrapped in outer fibres. The cheap, sensible default that comes pre-strung in entry-level rackets. Synthetic gut does nothing brilliantly but also nothing badly. For a beginner playing twice a week, it's perfectly adequate and roughly the price of a coffee.
05Hybrid
Not a material, but a setup: two different strings used together, one for the mains (the long vertical strings) and another for the crosses (the shorter horizontal ones). The dominant logic is gut-mains with poly-crosses, sometimes called a "Federer hybrid" — gut provides the soft, powerful feel; poly adds control and durability where the strings cross most. A reverse hybrid (poly mains, gut or multi crosses) is cheaper and more durable, but loses much of the comfort benefit.
The tension-loss problem nobody talks about
Manufacturers print a recommended tension range on the throat of the racket — usually around 22–27 kg (50–60 lbs). What they don't print is that those numbers are only true for the first hour after stringing. Strings begin losing tension the moment they come off the machine, and different materials lose it at wildly different rates.
This is the single most under-appreciated fact in club tennis. The chart below shows roughly how each material behaves over 30 days of normal play.
Polyester drops the cliff fastest. It can lose 10–15% of its tension in the first 24 hours and another chunk in the first week of play. Natural gut, by contrast, holds its starting tension better than anything else on the market — one of the reasons it's still found in the bags of players who can afford to use it.
How thick should the string be?
String gauge is just the diameter of the string in millimetres. The numbering system is a mess of historical conventions: lower gauge numbers mean thicker strings, higher numbers mean thinner. A "16" is thicker than an "18". Then there are half-steps marked with an "L" (for "light"), so a 16L sits between 16 and 17. The good news is that millimetre measurements are universal, and that's what's worth paying attention to.
Thinner strings bite into the ball more aggressively, which produces more spin and a livelier, more textured feel at impact. Thicker strings last longer and deaden the ball slightly, which some players hear as more "control" and others as "muted". The most common pro-level choice — including most of the ATP tour — is somewhere between 1.20 and 1.25 mm.
Tighter doesn't mean better
Tension is the force the strings are pulled to before being clamped — usually quoted in kilograms in Europe and pounds in the US. Most rackets carry a recommended range printed on the throat, somewhere around 22–27 kg (50–60 lbs).
The folklore says tighter strings give more control, looser strings give more power. That's directionally true but oversimplified. What's actually happening is that tighter strings deflect less when the ball hits them, so the ball spends less time on the stringbed and the player has less margin for error — but also less help from the strings to get the ball over the net. Looser strings act more like a trampoline.
| Tension band | Feel | Best for |
|---|---|---|
| 20–22 kg (low) | Powerful, soft, springy. Larger sweet spot. Less precise. | Beginners, doubles players, anyone with arm issues |
| 23–25 kg (medium) | Balanced. The default for most stringers and most players. | Improvers and intermediates of any style |
| 26–28 kg (high) | Crisper, more direct. Less free power. Demands swing speed. | Advanced players who generate their own pace |
The arm question
Tennis elbow doesn't usually come from a single bad shot. It comes from thousands of small impacts where the arm absorbs more shock than it can comfortably dissipate. Strings are the first line of defence between the ball and the elbow, and the difference between a comfortable setup and a painful one is enormous.
Stiffness is the relevant metric. Stiffer strings transmit more vibration; softer strings absorb more of it. Polyester sits at the harsh end. Natural gut and good multifilaments sit at the comfortable end. The bands below are a rough guide.
A practical framework
Match the string to the player rather than to the racket. The table below is a starting point, not a rulebook — the right string for you depends on how often you play, how hard you swing, and whether your arm is happy.
| Player profile | Recommended string | Tension |
|---|---|---|
| Improver Plays 1–2× a week, club level, learning to swing through the ball |
Synthetic gut or soft multifilament. Skip polyester for now. | 23–25 kg |
| All-Court Plays 2–4× a week, hits with intent, no arm issues |
Multifilament, or a hybrid with soft poly mains and multi crosses | 23–25 kg |
| Baseliner Strong topspin player, fast swings, breaks strings inside 10 hours |
Full polyester, or gut/poly hybrid if budget allows | 22–24 kg |
| Arm pain Any level, but with elbow, wrist or shoulder discomfort |
Natural gut or premium multifilament. Avoid polyester entirely. | 20–23 kg |
When to re-string
The widely repeated rule of thumb is to re-string each year as many times as you play per week. Play three times a week, re-string three times a year. It's a useful starting point but it under-counts polyester users.
Polyester has the strange property of dying long before it breaks. The string can look fine, sound fine on contact, and still be playing 30% below its original tension. Many tour stringers re-string poly every 8–10 hours of court time, regardless of whether it's snapped. For a club polyester user, every 15–20 hours is a sensible target.
Multifilaments and natural gut tend to fray and snap before they fully die — the string itself signals the end. With poly, you have to track it.
The thing nobody puts in the marketing copy
String choice is the cheapest way to meaningfully change how your racket plays. A new frame costs £200–£280. A re-string costs £15–£30. The next time you're tempted to upgrade your racket because something feels off, try a different string at a different tension first. Most of the time, the racket was fine. The strings had quietly stopped doing their job a fortnight ago.
