Ohm's Law Calculator - Voltage, Current, Resistance and Power

Agarapu Ramesh — Editor and content reviewer
Editorially reviewed Reviewed by Agarapu Ramesh, science educator (chemistry). LinkedIn
Last reviewed: May 2026 | Ohm's law and standard electrical engineering formulas

Ohm law is the electrician and electronics tech daily bread: V = I x R. Once you have any two of voltage, current and resistance, the third falls out. Add power and you can also check heat and component rating.

Formula at a glance

  • V = I x R
  • I = V / R
  • R = V / I
  • P = V x I = I^2 x R = V^2 / R

Field note: The formula is simple. The field mistake is forgetting heat. A correct resistance value can still burn up if the wattage rating is too small.

Ω

Calculator Tool

Calculate V, I, R, and P - Enter any two values

V
A
Ω
W

Enter at least 2 values to calculate the others

Results
Enter 2+ values

Ohm's Law Formulas

Voltage V = I × R
Current I = V ÷ R
Resistance R = V ÷ I
Power P = V × I

Power Formulas

P = V×I Volts × Amps
P = I²R Amps² × Ohms
P = V²/R Volts² ÷ Ohms

Related Calculators

Watts to Amps Volts to Amps Voltage Divider

How to use the Ohm's Law Calculator

Use this as a bench check, then compare it with the part marking, tolerance and a meter reading when the circuit matters. Small components are cheap. Bad assumptions are not.

Worked example

Example: 12 V across 6 ohms gives 2 A. Power is 12 x 2 = 24 W, so a tiny quarter-watt resistor would not last.

Practical checks before you trust the number

  • Use resistance in ohms, current in amps and voltage in volts.
  • For AC coils and capacitors, impedance replaces plain resistance.
  • Always check component power rating, not just the ohm value.

Common mistake

The formula is simple. The field mistake is forgetting heat. A correct resistance value can still burn up if the wattage rating is too small.

Sources and references

Related calculators

Frequently Asked Questions

V = I × R. Voltage equals current times resistance. Example: 2 A through 50 Ω → V = 100 V. This is the most fundamental electrical equation, used in every circuit calculation from sensor design to high-power feeder analysis. Memorize it cold.

I = V ÷ R. Current equals voltage divided by resistance. Example: 12 V across 4 Ω → I = 3 A. This form is most common when supply voltage is fixed and you need to predict load current. For AC, replace R with impedance Z.

R = V ÷ I. Resistance equals voltage divided by current. Example: 12 V producing 0.5 A through a load → R = 24 Ω. Useful for back-calculating an unknown resistor from measurements, or for verifying a load matches its nameplate spec.

P = V × I, P = I² × R, or P = V² ÷ R. Three forms of the same power equation, derived by combining Ohm's law with P = VI. Example: 12 V × 2 A = 24 W; or 2² × 6 = 24 W (with R = 6 Ω); or 12² ÷ 6 = 24 W. All give the same answer.

Volts (V), amps (A), ohms (R), and watts (W) link through Ohm's law and power equations: V = IR, P = VI, P = I²R, P = V²/R. Knowing any two lets you compute the others. This is the foundation of every circuit-sizing calculation we do.

Yes, but use impedance Z instead of resistance R for AC. V = I × Z, where Z combines resistance, inductive reactance, and capacitive reactance. For purely resistive AC loads (heaters, incandescent lamps), Z = R and Ohm's law works as in DC. For motors and capacitors, you must use complex impedance.

Yes. Enter any two known values (V, I, R, or P) and the calculator finds the rest. Useful for quick sanity checks on circuit designs, voltage drop calculations, and component sizing. Always verify the calculator's assumptions about AC vs DC and PF before trusting the output.