Purpose-set for 12-volt systems — automotive, RV, marine, camper-van and small solar systems — with the same three solvers: drop, minimum wire size, and maximum cable length.
At 12 V, a 3% budget is just 0.36 volts — the entire allowance for the round trip through both conductors. The shortcut for any 12 V run: maximum one-way feet ≈ (0.36 × 1000) ÷ (2 × amps × Ω/kft of the wire). The calculator's Max Length tab does exactly this for every size at once.
Aim for 3% (0.36 V) for electronics, lighting and charging circuits; up to 10% is sometimes tolerated for non-critical resistive loads. Battery-to-inverter cabling should stay under 2%.