How do I calculate voltage drop for an extension cord without doing the math myself?

Skip the math — use a voltage drop calculator. Just Google “voltage drop calculator” and plug in the gauge, length, amps, and voltage. For the manual way: voltage drop = (2 × length × current × resistance per foot) / 1000. On a 120V circuit, a drop up to 3% (3.6V) is fine; over 5% and you’ll notice dim lights or slow tools.