What gauge extension cord do I need for a given length and amperage?

For 120V tools, keep voltage drop under 3%. At 50 feet, 14-gauge works for 15 amps. At 100 feet, use 12-gauge for 15 amps. At 150 feet, step up to 10-gauge. For 20 amps, double those gauges (12 at 50ft, 10 at 100ft, 8 at 150ft).