The formula
String tension follows from the string's mass per unit length, its vibrating length and its frequency:
T = UW × (2 × L × F)² ÷ 386.4
where UW is the unit weight in pounds per inch, L is the scale length in inches, F is the pitch in Hz, and 386.4 in/s² is gravitational acceleration. Because tension grows with the square of scale length, the same string at the same pitch carries more tension on a longer scale.
Examples
A .010 plain steel string tuned to E4 (329.63 Hz) on a 25.5" scale comes out at about 16.2 lb (7.3 kg). A .046 nickel-wound string tuned to E2 (82.41 Hz) on the same scale is about 18.0 lb (8.2 kg).
Using the presets
The instrument presets fill in a whole string set: scale length, gauges, materials and tuning for 6 to 9-string guitars, 12-string, classical, basses, ukuleles and mandolin. Tap any string to adjust its gauge, material, note or octave, and compare the per-string tensions to see whether a set is balanced.
About the numbers
Unit weights come from a physics-based model of each string type, not a copy of any manufacturer's chart, so results are estimates and can differ slightly from a maker's published tension. They are accurate enough for comparing sets and planning a build.
Frequently asked questions
Does scale length change string tension?
Yes. For the same string and pitch, tension is proportional to the square of the scale length, so a 25.5" scale carries more tension than a 24.75" scale.
Why do acoustic guitars need a different set than electrics?
Acoustic tops are designed to carry more total tension than an electric guitar neck and bridge, and acoustic strings are heavier for tone and volume. Always check the recommended range for your bracing and truss rod.
How do I compare two string sets?
Choose the preset, enter the gauges of each set, and compare the tension per string and the total. A balanced set has similar feel across strings.