How a guitar feels depends on string tension: how hard each string pulls. Tension rises with the note, the thickness of the string and the scale length. That is why a set that feels right in standard tuning goes floppy when you tune down to drop C, and why the same strings feel tighter on a 25.5 inch Fender scale than on a 24.75 inch Gibson scale.
Choose your scale length, tuning and string type, then pick a common set or type your own gauges, thinnest first. Every string shows its tension in kilograms or pounds with a bar against the range most players like, and the change compared with standard tuning. Tap any row to hear that string at its pitch.
When you change tuning, the calculator suggests the common set that keeps the feel you are used to, and a balanced custom set with even tension across the strings. One tap loads either one.
The maths is the standard formula string makers publish: tension equals unit weight times (2 × scale length × frequency) squared, divided by 386.4. Plain steel strings are worked out from the density of steel, and wound strings use D’Addario’s published unit weights for nickel-wound electric and phosphor bronze acoustic strings. Gauges between published sizes are estimated and marked.