Ohm's Law Calculator
- Last reviewed:
- Reviewed calculations using Ohm's Law to ensure accurate voltage, current, resistance, and power conversions across DC, single-phase, and three-phase electrical systems.
Table of Contents
Use this Ohm’s Law calculator to solve for voltage, current, resistance or power. Enter any two known values and the calculator works out the remaining two — with the formula used and full step-by-step working shown.
Ohm's Law Calculator
Enter any two values — the calculator works out the other two with full working shown
💡 How to use: Enter any two known values and leave the others blank. The calculator will solve for the remaining two automatically.
V
Volts (V)
A
Amperes (A)
Ω
Ohms (Ω)
W
Watts (W)
Results
Voltage
—V
Current
—A
Resistance
—Ω
Power
—W
Step-by-Step Working
What is Ohm's Law?
Ohm’s Law states that the current through a conductor is directly proportional to the voltage across it, provided the temperature remains constant. It is one of the most fundamental principles in electrical engineering and is used daily by electricians, engineers and students worldwide.
The three core Ohm’s Law formulas are:
- V = I × R (Voltage = Current × Resistance)
- I = V ÷ R (Current = Voltage ÷ Resistance)
- R = V ÷ I (Resistance = Voltage ÷ Current)
When combined with the power formula P = V × I, all four variables can be solved from any two known values:
- P = V × I
- P = I² × R
- P = V² ÷ R
Where:
- V = Voltage in volts (V)
- I = Current in amps (A)
- R = Resistance in ohms (Ω)
- P = Power in watts (W)
Worked Example
Example 1 — Known: Voltage and Resistance A circuit has a supply voltage of 230 V and a resistance of 46 Ω.
Find current: I = V ÷ R = 230 ÷ 46 = 5 A Find power: P = V² ÷ R = 230² ÷ 46 = 1,150 W
Example 2 — Known: Current and Resistance A circuit carries 13 A through a resistance of 17.69 Ω.
Find voltage: V = I × R = 13 × 17.69 = 229.97 V Find power: P = I² × R = 13² × 17.69 = 2,990 W
Example 3 — Known: Power and Voltage A 60 W lamp operates at 230 V.
Find current: I = P ÷ V = 60 ÷ 230 = 0.26 A Find resistance: R = V² ÷ P = 230² ÷ 60 = 881.67 Ω
Common Mistakes
- Entering more than two values. The calculator needs exactly two known values to solve for the others. If you enter three or four values, it will ask you to remove the extras.
- Confusing watts and kilowatts. Power must be entered in watts. If your value is in kilowatts, multiply by 1,000 first. For example, 2.5 kW = 2,500 W.
- Applying Ohm’s Law to AC circuits with reactive loads. Ohm’s Law in its basic form applies to resistive circuits. For circuits with motors, capacitors or inductors, impedance and power factor must also be considered.
- Confusing resistance and impedance. In AC circuits, opposition to current flow is called impedance (Z), not resistance (R). Ohm’s Law still applies but uses Z in place of R.
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Disclaimer: This calculator is provided for learning and revision purposes only. Ohm’s Law applies to resistive circuits under constant temperature conditions. For AC circuits with reactive or non-linear loads, additional principles apply. Results must not be used as the basis for electrical design or any on-site decision. Always seek advice from a competent qualified electrician or engineer.
Frequently Asked Questions
What is Ohm's Law?
Ohm’s Law states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. It is expressed as V = I × R.
How do you use this Ohm's Law calculator?
Enter any two known values — voltage, current, resistance or power — and leave the other two blank. Click Calculate and the tool works out the remaining values with full working shown.
What are the three formulas in Ohm's Law?
V = I × R (voltage), I = V ÷ R (current), R = V ÷ I (resistance). Combined with P = V × I, all four variables can be solved from any two known values.
Does Ohm's Law apply to AC circuits?
Ohm’s Law applies to resistive AC circuits. For circuits with inductive or capacitive loads, impedance (Z) replaces resistance (R). The basic relationship V = I × Z still holds but the calculation is more complex.
What is the difference between resistance and impedance?
Resistance (R) is the opposition to current flow in a DC or purely resistive AC circuit, measured in ohms. Impedance (Z) is the total opposition to current flow in an AC circuit, including resistance and reactance from inductors and capacitors.
What units does this calculator use?
Voltage in volts (V), current in amps (A), resistance in ohms (Ω) and power in watts (W). Make sure all values are in these base units before entering them.
Is this calculator suitable for electrical design?
No. This calculator is for learning and revision only. For electrical design, always refer to BS 7671 and consult a qualified electrician or engineer.
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