Ohm's Law Formulas

With four quantities — voltage V, current I, resistance R and power P — there are twelve ways to calculate one from two others. They all come from V = I × R and P = V × I. The table below is the classic Ohm's law wheel laid out as rows. OhmCalc uses the same formulas in the calculator, and the 'I know' menu picks the right pair.

Units: volts (V), amps (A), ohms (Ω) and watts (W). Use base units in the formulas, or convert mA to A and kΩ to Ω first; the calculator does that for you.

To find	From V and I	From V and R	From I and R	From P and V	From P and I	From P and R
Voltage V	—	—	I × R	—	P ÷ I	√(P × R)
Current I	—	V ÷ R	—	P ÷ V	—	√(P ÷ R)
Resistance R	V ÷ I	—	—	V² ÷ P	P ÷ I²	—
Power P	V × I	V² ÷ R	I² × R	—	—	—

Unit prefixes

Prefix	Factor	Example
µ (micro)	0.000001	250 µA = 0.00025 A
m (milli)	0.001	20 mA = 0.02 A
k (kilo)	1,000	4.7 kΩ = 4,700 Ω
M (mega)	1,000,000	1 MΩ = 1,000,000 Ω

Standard resistor values (E24, 5%)

1.0, 1.1, 1.2, 1.3, 1.5, 1.6, 1.8, 2.0, 2.2, 2.4, 2.7, 3.0, 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2, 6.8, 7.5, 8.2, 9.1, and the same values × 10, × 100, × 1k and so on. The voltage divider tab suggests the nearest E24 value from the IEC 60063 series.

OhmCalc assumes ideal components and DC (or resistive AC) circuits. For AC loads with a power factor, real power is V × I × PF. Mains voltage can kill: these tools are for learning and low-voltage design.

Frequently asked questions

What does each letter stand for?

V (or E) is voltage in volts, I is current in amps, R is resistance in ohms and P is power in watts.

How do I find resistance from watts and volts?

R = V² ÷ P. A 60 W bulb rated for 120 V has a hot resistance of 240 Ω.

How do I find current from watts and ohms?

I = √(P ÷ R). 1 W in 100 Ω is 0.1 A.

Is OhmCalc free?

Yes, with no sign-up or ads.

Are my values saved?

Yes, in this browser only.

Ohm's law formulas