Tension units
Applies to every tension in the app. Stiffness stays in lb/in, the unit Tennis Warehouse University publishes.
Back up and restore
How the numbers work
Each string's stiffness is the Tennis Warehouse University lab figure: the pounds of force it takes to stretch the string one inch during an impact, measured at 52 lb. TWU ties stiffer strings to shorter dwell time on the ball, which means more control, less power and less comfort.
Your strings are scaled to your tensions with k × (T / 52)^p, where p depends on the material, then blended with the mains carrying the larger share because they are longer and deflect more.
The 65/35 split is a stringer's rule of thumb, not a lab constant.
Lead tape
Tennis Warehouse's figures are 0.25 g per inch for 1/4 in tape and 0.5 g per inch for 1/2 in. Some brands run heavier; weigh a known length and enter it here if yours differs.
Lead verdicts judge the finished frame against the player: weight and swingweight windows for their level, balance, how big a jump the change is, MgR/I timing (most players sit between 20 and 21.5), polarization, and arm health. Highly recommended means 90 or more and inside every comfortable window.
The verdict
Every setup starts at 100 and loses points for tension outside the usual window for its material, more than 5 lb between mains and crosses, a very stiff bed (worse in a stiff frame), a bed soft enough to trampoline, and using a string against its maker's guidance. Highly recommended means 95 or more with a bed between 120 and 170 lb/in: nothing working against the player and soft enough to be kind to the arm. 80 and up is Recommended, 60 to 79 Use with caution, under 60 Not recommended.
Sources and limits
Stiffness figures come from the TWU String Stiffness Tool, 52 lb reference, fast swing. Toroline Truffle X and A5 had no TWU figure when this was built: Truffle X uses 150 because Toroline compares it with top multifilaments, and A5 uses 205 because Toroline places it just under O-Toro Tour (217). Edit either in the string database when better numbers appear. Stiffness is one number: shape, friction, gauge and tension loss also change how a bed plays, and nothing here replaces a playtest.