Amps are watts divided by volts, and watts are amps multiplied by volts. A 1,500 W space heater on 120 V draws 12.5 A, and 12.5 A at 120 V is 1,500 W. Choose watts to amps or amps to watts, enter the voltage, and pick DC, AC single phase or AC three phase. For AC there is also a power factor, which is 1 to start with. The tables below the calculator answer the common cases without typing: 100 to 10,000 W at 120 V and 240 V, and 10 to 2,000 W at 12 V and 24 V DC.
Voltage sets the current for a given wattage. Double the voltage and the same watts draw half the amps: 4,800 W is 40 A at 120 V and 20 A at 240 V. Half the current lets a smaller wire carry the same load. Enter the voltage the load is rated for, from its nameplate. For load calculations the NEC uses nominal system voltages, such as 120, 240, 208Y/120 and 480Y/277 V, unless other voltages are specified (120.5(A); 220.5(A) in the 2023 edition).
On AC, watts are not always volts times amps. Power factor is the real power in watts divided by the apparent power in volt-amperes (VA), a number between 0 and 1, and the current is I = P / (V × PF). Resistive heating loads, such as heaters, toasters, ovens and incandescent lamps, have a power factor of about 1, so watts over volts gives their current. Don't assume 1 for other loads. If the nameplate gives amps, that is the current to use; a power factor entered below 1 raises the amps for the same watts.
A three-phase load shares its power across three lines, so each carries less current. Divide the watts by √3, about 1.73, times the line-to-line voltage and the power factor: I = P / (√3 × V × PF). 10,000 W at 480 V three phase draws 12.03 A per line at a power factor of 1, where the same 10,000 W on a single-phase 208 V circuit draws 48.08 A. Enter the voltage between two lines, 208 V on a 208Y/120 system or 480 V on 480Y/277, not the 120 or 277 V from a line to neutral.
DC has no power factor: watts are volts times amps, and the calculator ignores the power factor field. Low voltage makes for high current. 100 W at 12 V is 8.33 A, and 1,000 W is 83.33 A at 12 V or 41.67 A at 24 V. Wire resistance turns that current into lost volts, and on a 12 V supply each volt lost is ten times the share it is of 120 V, so check the run with the voltage drop calculator before choosing the wire.
Amps are also where sizing a circuit starts. A load that runs for three hours or more is a continuous load, and the NEC generally sizes its branch circuit for 125% of it: the conductors under 210.19(A)(1) and the breaker under 210.20(A). Turned around, a 15 A circuit carries at most 12 A of continuous load, 1,440 W at 120 V, and a 20 A circuit 16 A, or 1,920 W. The 1,500 W heater draws 12.5 A, more than 12 A, so if it runs continuously it is sized as a 15.63 A load and a 15 A circuit is too small for it. To carry the result into a wire size, with the 125% applied if you tick the continuous-load box, use the wire size calculator.