Ohm's Law Calculator
Give the Ohm's law calculator any two of volts, amps, ohms and watts and it returns the other two. The formula used is shown beside every answer, so the page doubles as the wheel of twelve.
Notice: For general informational purposes only; not electrical or engineering advice. Work on electrical circuits can cause fire, electric shock or death. Mains-voltage work must comply with the applicable electrical code and be performed by a licensed electrician.
Resistance
- Voltage Given.
- 120 V
- Current Given.
- 10 A
- Resistance R = V / I = 120 V / 10 A.
- 12 Ω
- Power P = V x I = 120 V x 10 A.
- 1,200 W
| Quantity | Formulas (DC) | Here |
|---|---|---|
| Voltage V | I x R, P / I, √(P x R) | 120 V |
| Current I | V / R, P / V, √(P / R) | 10 A |
| Resistance R | V / I, V² / P, P / I² | 12 Ω |
| Power P | V x I, I² x R, V² / R | 1,200 W |
- Ohm's law holds for resistors, heating elements and wire at a fixed temperature. It does not describe LEDs, diodes, batteries or motors, whose current is not proportional to the voltage across them.
- A conductor's resistance rises as it heats, so the cold resistance a meter reads across a lamp filament or heater element is lower than its resistance in use.
How the Ohm's Law Calculator works
Ohm's law is V = I x R: the voltage across a resistance equals the current through it times the resistance. Volts are the push, amps the flow and ohms the opposition. Add the power equation, P = V x I, and any two of the four quantities give the other two. Substituting one equation into the other gives the twelve formulas in the table, sometimes drawn as a wheel: P = I² x R and P = V² / R are P = V x I with Ohm's law put in for one of the terms.
The calculator's default. 120 V driving 10 A. R = V / I = 120 / 10 = 12 Ω, and P = V x I = 120 x 10 = 1,200 W. Turn it around and the same load checks out every way: 120² / 12 = 1,200 W, and 10² x 12 = 1,200 W. A 1,200 W heater on a 120 V outlet therefore draws 10 A, and its element has a resistance of 12 Ω while hot.
Watts and amps on AC. For a heater, a kettle or an incandescent lamp, AC behaves like DC and the formulas above apply with the RMS voltage. Motors, fluorescent ballasts and electronic power supplies draw current that is out of step with the voltage, and the real power is P = V x I x pf, where pf is the power factor, between 0 and 1. A 1,200 W load at a power factor of 0.8 on 120 V draws 1,200 / (120 x 0.8) = 12.5 A, not 10. On a balanced three-phase supply, P = 1.732 x V x I x pf with V the line-to-line voltage, because the three currents are 120 degrees apart. 10 kW at 480 V three-phase and a power factor of 0.85 is 10,000 / (1.732 x 480 x 0.85) = 14.15 A per line.
The mistakes that recur. Mixing prefixes: 4.7 kΩ is 4,700 Ω and 20 mA is 0.02 A, and a calculation done in kΩ and mA gives volts directly (4.7 kΩ x 20 mA = 94 V) only because the prefixes cancel. Using watts where volt-amps are meant: a UPS or generator rated 1,000 VA at a 0.8 power factor delivers 800 W. And on three-phase, using the line-to-neutral voltage, which understates the power by 1.732.
Where it stops applying. Ohm's law is a property of materials, not a law of nature. Metals obey it at a steady temperature, but a lamp filament's resistance rises several-fold as it heats, so the resistance measured cold with a meter does not predict the current when it is lit. Diodes, LEDs and transistors are non-ohmic: an LED's current rises steeply over a small change of voltage, which is why it needs a series resistor sized by R = (supply - LED voltage) / current. The wire carrying the current has resistance too; over a long run that becomes a voltage drop, which has its own calculator.
Ohm's Law Calculator FAQ
What is Ohm's law?
V = I x R. The voltage across a resistance equals the current through it times its resistance. It rearranges to I = V / R and R = V / I. 12 V across a 6 Ω resistor drives 2 A.
How do I convert watts to amps?
Divide watts by volts: amps = watts / volts. 1,500 W at 120 V is 12.5 A. On AC with a power factor below 1, divide by the power factor as well; on three-phase, divide by 1.732 x the line-to-line voltage x the power factor.
How do I find resistance from watts and volts?
R = V² / P. A 60 W lamp on 120 V has a hot resistance of 120² / 60 = 240 Ω. Measured cold with a meter it reads far lower, because the filament's resistance rises as it heats.
Does Ohm's law work for AC?
Yes for resistive loads such as heaters, using RMS voltage and current. For motors and electronics the ratio V / I is the impedance, which includes reactance, and power is V x I x the power factor.
What are the 12 Ohm's law formulas?
Three for each quantity. V = I x R = P / I = √(P x R). I = V / R = P / V = √(P / R). R = V / I = V² / P = P / I². P = V x I = I² x R = V² / R.