Ohm's Law Calculator
Enter any two of volts, amps, ohms and watts to get the other two with the formula used, plus amps to watts and kVA to amps for single and three phase. Pick mV, kV, mA, kΩ, MΩ or kW next to each box and the answer comes back in the unit that reads best.
Enter any two values
Fill in two of the four boxes and leave the other two empty. Pick a unit next to each box.
Result
Voltage (V)
12 V
Entered
Current (I)
2 A
Formula: I = V ÷ R
Resistance (R)
6 Ω
Entered
Power (P)
24 W
Formula: P = V² ÷ R
All 12 Ohm's law formulas
The twelve formulas for voltage, current, resistance and power. The ones used for your result are highlighted.
Voltage (V)
- V = I × R
- V = P ÷ I
- V = √(P × R)
Current (I)
- I = V ÷ R
- I = P ÷ V
- I = √(P ÷ R)
Resistance (R)
- R = V ÷ I
- R = V² ÷ P
- R = P ÷ I²
Power (P)
- P = V × I
- P = I² × R
- P = V² ÷ R
Amps to watts or watts to amps
Line to neutral or line to line, for example 120 or 240 V.
1 for heaters and incandescent lamps. Motors run about 0.8 to 0.9 at full load; check the nameplate.
Result
Real power
1.2 kW
1,200 W
Apparent power
1.2 kVA
1,200 VA
Current
10 A
Formula: P = V × I × PF
kVA to amps or amps to kVA
Line-to-line voltage, for example 208, 480 or 600 V.
Result
Current
90.2 A
kW at power factor 0.8
60 kW
kW at power factor 0.9
67.5 kW
Formula: I = kVA × 1,000 ÷ (√3 × V)
Everything is worked out in your browser. Nothing you type is sent to ParrotNotes.
How the Ohm's law calculator works
Ohm's law says voltage equals current times resistance: V = I × R. Power is voltage times current: P = V × I. Put the two together and each of the four quantities can be found three ways, which gives the twelve formulas in the table under the calculator.
Fill in any two boxes and leave the other two empty. The calculator picks the formula that uses exactly what you gave it, shows the answer and highlights the formula, so you can check the step by hand.
Worked example
A 12 V supply across a 6 Ω resistor: current I = V ÷ R = 12 ÷ 6 = 2 A, and power P = V² ÷ R = 144 ÷ 6 = 24 W. A resistor rated 1/4 W would burn out; you need one rated well above 24 W.
A 100 W lamp on 120 V: I = P ÷ V = 0.83 A and R = V² ÷ P = 144 Ω. That is the hot resistance; a cold filament reads much lower on a meter.
Amps to watts: DC, single phase and three phase
DC: watts = volts × amps. Single-phase AC: watts = volts × amps × power factor. Three-phase AC: watts = √3 × line-to-line volts × amps × power factor, with √3 ≈ 1.732.
So 10 A at 120 V with a power factor of 1 is 1,200 W, and a 1,500 W heater on 120 V draws 12.5 A. A three-phase motor drawing 20 A at 480 V with a power factor of 0.85 uses about 14,134 W of real power (14.1 kW), and its apparent power is 16.6 kVA.
kVA to amps
For a transformer or generator rated in kVA: three-phase amps = kVA × 1,000 ÷ (√3 × volts), and single-phase amps = kVA × 1,000 ÷ volts. Power factor does not enter, because kVA already counts the full current.
A 75 kVA transformer gives 90.2 A at 480 V three phase and 208.2 A at 208 V. A 25 kVA single-phase transformer at 240 V gives 104.2 A. To get kW, multiply kVA by the load's power factor: 75 kVA at 0.8 is 60 kW.
What the numbers mean
Volts (V) push the current; amps (A) are how much current flows; ohms (Ω) resist it; watts (W) are the power turned into heat, light or motion. Power factor is the share of the current that does real work in an AC circuit: 1 for heaters, about 0.8 to 0.9 for motors at full load.
kW is real power and kVA is apparent power. Breakers, wire and transformers are sized on amps and kVA, while the electricity bill counts kWh.
Results are for steady DC or for AC at the power factor you enter. Motor starting current, harmonics and unbalanced loads are not included. Size wire, breakers and transformers from the NEC tables and the equipment nameplate, and have a licensed electrician check the design.
Selling electrical or HVAC equipment? See electrical distributor sales, HVAC sales, solar sales, what a sales engineer does.
Keep the nameplate numbers from the site walk
On a site walk or a call, the customer reads out volts, amps and kVA faster than you can type them. Record the conversation with ParrotNotes and you get a transcript, a summary and the action items, so the numbers for the quote are already written down.
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