Ohm's Law Calculator

Enter any two of voltage, current, resistance and power and get the other two, with every formula shown.

Voltage (V)

Volts

Current (I)

Amperes

Resistance (R)

Ohms

Power (P)

Watts

How to use the Ohm's law calculator

Fill in any two of the four boxes and leave the other two empty. The calculator gives you the missing two and shows the formula it used. Pick the unit next to each box, for example mA for a small current or kΩ for a large resistance. If you type a third value, the calculator asks you to clear one. The button "Clear all fields" empties everything so you can start with a different pair.

The 12 Ohm's law formulas

Ohm's law links voltage, current and resistance with V = I × R. Electrical power adds P = V × I. Putting one into the other gives P = I² × R and P = V² ÷ R. Each of the four quantities can be found from three different pairs of the others, which makes twelve formulas. This is the same information as the familiar Ohm's law wheel, written as a table.

To find	Formula	You need
Voltage (V)	V = I × R	Current and resistance
Voltage (V)	V = P ÷ I	Power and current
Voltage (V)	V = √(P × R)	Power and resistance
Current (I)	I = V ÷ R	Voltage and resistance
Current (I)	I = P ÷ V	Power and voltage
Current (I)	I = √(P ÷ R)	Power and resistance
Resistance (R)	R = V ÷ I	Voltage and current
Resistance (R)	R = V² ÷ P	Voltage and power
Resistance (R)	R = P ÷ I²	Power and current
Power (P)	P = V × I	Voltage and current
Power (P)	P = I² × R	Current and resistance
Power (P)	P = V² ÷ R	Voltage and resistance

All six combinations for the same circuit

A circuit with 12 V across 6 Ω carries 2 A and uses 24 W. Whichever pair you start from, the calculator returns the same circuit.

You enter	You get
12 V and 2 A	6 Ω and 24 W
12 V and 6 Ω	2 A and 24 W
12 V and 24 W	2 A and 6 Ω
2 A and 6 Ω	12 V and 24 W
2 A and 24 W	12 V and 6 Ω
6 Ω and 24 W	12 V and 2 A

Example: a 1,500 W heater on 120 V

You know the voltage is 120 V and the label says 1,500 W. Type 120 in the voltage box and 1,500 in the power box. The calculator finds the current and the resistance.

Step	Operation	Result
Current	I = P ÷ V = 1,500 ÷ 120	12.5 A
Resistance	R = V² ÷ P = 120² ÷ 1,500 = 14,400 ÷ 1,500	9.6 Ω
Check	R = V ÷ I = 120 ÷ 12.5	9.6 Ω

The check row shows the two formulas agree. The result describes a purely resistive load. A real appliance can behave differently, and the safe circuit for it is a job for the NEC and an electrician, not for this page.

Using milliamps, kilohms and other prefixes

Small and large values are usually written with a prefix. The calculator converts them for you when you pick the unit, and it writes the results with the prefix that reads best. These are the SI prefixes it uses, as listed by NIST (checked October 2026).

Prefix	Symbol	Factor	Example
micro	µ	0.000001	1 µA = 0.000001 A
milli	m	0.001	20 mA = 0.02 A
(none)		1	1 A = 1 A
kilo	k	1,000	4.7 kΩ = 4,700 Ω
mega	M	1,000,000	2 MΩ = 2,000,000 Ω

For example, a current of 20 mA at 5 V is 0.02 A. The resistance is 5 ÷ 0.02 = 250 Ω, and the power is 5 × 0.02 = 0.1 W, which the calculator shows as 100 mW.

When Ohm's law does not apply

Ohm's law describes parts whose resistance stays constant, such as a resistor or a heating element. Diodes, LEDs and other parts do not follow a straight line between voltage and current, and the resistance of a filament or a heater changes as it warms up. In those cases the calculator gives you the ratio at the values you type, not a property of the part.

For alternating current, the results are only valid when the load is resistive. Motors and many other loads add a power factor, and three-phase circuits add a factor of √3 when you use the line-to-line voltage. For those, use the watts to amps calculator.

Frequently asked questions

What is Ohm's law?

Ohm's law says that the current through a conductor is proportional to the voltage across it. The three quantities are linked by V = I × R, where V is the voltage in volts, I is the current in amperes and R is the resistance in ohms. If you know any two of them, you can find the third.

What are the 3 Ohm's law formulas?

V = I × R gives the voltage, I = V ÷ R gives the current and R = V ÷ I gives the resistance. They are the same equation rearranged. Add the power formula P = V × I, and its two variants P = I² × R and P = V² ÷ R, and you have the full set of 12 formulas in the table above.

How do I calculate ohms from watts and volts?

Divide the voltage squared by the power: R = V² ÷ P. For 120 V and 1,500 W, R = 120² ÷ 1,500 = 14,400 ÷ 1,500 = 9.6 ohms. Type the voltage and the power in the calculator and it gives the resistance and the current (12.5 A in this case).

How do I convert ohms to voltage?

You cannot convert ohms to volts on their own, because voltage also depends on the current. Multiply the resistance by the current: V = I × R. A current of 2 A through 6 ohms gives 12 V. If you know the power instead of the current, use V = √(P × R).

Can I use it for series or parallel circuits?

Yes, as long as you enter the resistance of the whole circuit. Resistors in series add up to the total. For resistors in parallel, the reciprocals add up (1 ÷ R total = 1 ÷ R1 + 1 ÷ R2 + ...). This calculator does not combine resistors for you, so work out the total first and then type it in.

Does Ohm's law work for AC circuits and three-phase power?

This calculator is for direct current and for purely resistive loads, where voltage and current are in step. Alternating current with motors, transformers or other loads also involves a power factor, and three-phase circuits add a factor of √3 when you use the line-to-line voltage. This page does not handle those cases.

Can I use this to choose wire or a breaker?

No. The result tells you how voltage, current, resistance and power relate in a simple circuit. Choosing conductors and overcurrent protection for a real installation depends on the National Electrical Code (NEC), the load, the length of the run and the conditions, and it needs a qualified electrician.