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Ohm's Law Calculator

Calculate voltage, current, resistance or power using Ohm's law

Reference Info & Formulas
Ohm's Law

V = I × R

I = V / R

R = V / I

P = V × I

Power Formulas

P = V × I

P = I² × R

P = V² / R

Units

V — Volts (voltage)

I — Amps (current)

R — Ohms (resistance)

P — Watts (power)

Ohm's Law
Select what to solve for, then enter known values
V
Ω

Data: Standard electrical formulas (V = I × R, P = V × I)

For guidance only. The responsibility for any electrical installation lies with the qualified person carrying out the work. Always verify calculations independently and apply professional judgement.

How This Calculator Works

Ohm's Law and the power formula together give 12 equations — enter any two known values to calculate the other two.

Ohm's Law states that voltage equals current multiplied by resistance. Combined with the power formula (P = V × I), any two of the four quantities — voltage, current, resistance, and power — are sufficient to derive the remaining two. The calculator applies the appropriate formula pair based on which values you provide.

V = I × R
V
= Voltage in volts (V)
I
= Current in amps (A)
R
= Resistance in ohms (Ω)
P = V × I
P
= Power in watts (W)
V
= Voltage in volts (V)
I
= Current in amps (A)

Quick Reference — All 12 Formula Variations

Ohm's Law and power wheel formulae

FindFormula 1Formula 2Formula 3
Voltage (V)V = I × RV = P / IV = √(P × R)
Current (I)I = V / RI = P / VI = √(P / R)
Resistance (R)R = V / IR = V² / PR = P / I²
Power (P)P = V × IP = I² × RP = V² / R

All formulae assume DC or purely resistive AC circuits. For reactive AC loads, use impedance (Z) in place of resistance (R).

Practical Notes

DC and Resistive AC Only

Ohm's Law in its basic form applies to DC circuits and purely resistive AC circuits. For AC circuits containing inductance or capacitance, use impedance (Z) instead of resistance (R). In practice, most domestic cable sizing and fault loop calculations use resistance values because reactance is negligible at typical circuit lengths.

The Power Wheel

The power wheel is a quick visual reference — place the quantity you want to find at the centre and read the two-variable formulae from the surrounding segments. It is particularly useful during exams and on-site calculations where speed matters.

Frequently Asked Questions

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