If using a stiffer string material like polyester, we’d recommend stringing looser to avoid arm injuries. Here are the tension ranges we’d aim for the first time you get your racket strung: Nylon/Gut: 50-60lbs (22.5-27kg) Hybrid: 46-56lbs (21-25.5kg) *Because polyester is a stiffer material, string 2lbs (1kg) looser than nylon
First, remember that tension is really a matter of personal preference. Even the pros' tensions are all over the map -- some string as low as the mid-to-high 30-lbs range, while others are as high as 70+ lbs, with most others sprinkled somewhere in the middle.
20-27 kg / 45-60 lb. You may also find some of the tennis strings you purchase offer a recommended tension range. For example, Babolat’s RPM Blast recommends 48 – 66 lbs or 22 – 30 kg, a rough guide for understanding where they have determined the string performs its best.
As soon as a racquet is strung, it will lose tension before it goes anywhere near a tennis ball. It's said that strings can lose up to 10% within 24 hours. So if you string at 55 lbs but don't plan to use your racquet for a week, then it will have lost quite a bit of tension by the time you finally play.
High String Tension . A high string tension would be anything from around 55lbs to 65lbs. Similarly to what we talked about during the low tension section, there are exceptions to this too – some players opting for even higher tensions! Unless you want an injury coming your way though, we wouldn’t advise you to go over 60lbs.
Junior rackets have entirely different requirements, so if you have kids (with junior rackets), they’ll have different tension ranges. There are plenty of professional tennis players who use higher tension as well, but 55-65 lbs is the standard range. Federer, for example, has his rackets strung at 55-60 lbs.