Ohm's Law Calculator
Enter any two values and instantly calculate the other two — voltage (V), current (I), resistance (R), and power (P). Covers all Ohm's law and power relationships for DC and purely resistive AC circuits.
Enter any two values - the other two are calculated.
Voltage
12V
Current
2A
Resistance
6Ω
Power
24W
Key relationships
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The Ohm's law relationships
Ohm's law links four fundamental electrical quantities. Any two of the four determine the other two:
- V = I × R — voltage equals current times resistance
- I = V ÷ R — current equals voltage divided by resistance
- R = V ÷ I — resistance equals voltage divided by current
- P = V × I — power equals voltage times current
- P = I² × R — power from current and resistance
- P = V² ÷ R — power from voltage and resistance
The P = I²R formula is particularly important for cable sizing — it explains why doubling the current quadruples the heat generated in a conductor. This is why undersized cables overheat long before they trip a breaker.
Worked examples
Example 1 — load current from power:A 3 kW electric shower on a 230 V supply.
- I = P ÷ V = 3,000 ÷ 230 = 13.0 A
- Appropriate breaker: 16 A (continuous load × 1.25 = 16.3 A for NEC; BS 7671 would use a 16 A MCB)
Example 2 — resistance from voltage and current:A heating element draws 8.7 A from a 230 V supply.
- R = V ÷ I = 230 ÷ 8.7 = 26.4 Ω
- P = V × I = 230 × 8.7 = 2,001 W ≈ 2 kW
Example 3 — cable heating (P = I²R):2.5 mm² copper cable with resistance 7.41 mΩ/m, carrying 20 A over a 30 m run (60 m loop).
- Total loop resistance = 60 × 0.00741 = 0.445 Ω
- Power dissipated in cable = 20² × 0.445 = 178 W
- That is heat produced in the cable — not delivered to the load
AC circuits and power factor
For AC loads with inductance or capacitance — motors, fluorescent fittings, transformer primaries — the effective opposition to current is impedance (Z), not pure resistance. The current and voltage are no longer in phase, and the real power consumed is:
P = V × I × cosθ
Where cosθ is the power factor (0 to 1). A motor drawing 10 A at 230 V with a power factor of 0.85 consumes 230 × 10 × 0.85 = 1,955 W of real power, but demands 230 × 10 = 2,300 VA of apparent power from the supply. The conductor must be sized for the apparent power (the actual current of 10 A), not the real power.
Who is this calculator for?
Useful for electricians, electronics technicians, students, and engineers who need quick verification of voltage, current, resistance, or power in any DC or resistive circuit. Common applications include checking heating element ratings, sizing fuses, calculating voltage drop in a resistive supply, and verifying control circuit component values.
Frequently Asked Questions
- What is Ohm's law?
- Ohm's law states that the current through a conductor is directly proportional to the voltage and inversely proportional to the resistance: I = V ÷ R. Rearranged: V = I × R and R = V ÷ I. It applies to any resistive element at constant temperature.
- How do I calculate power from Ohm's law?
- Power can be found three ways from any two known values: P = V × I (from voltage and current), P = I² × R (from current and resistance), or P = V² ÷ R (from voltage and resistance). All three give the same result for a resistive load.
- Does Ohm's law work for AC circuits?
- Ohm's law applies directly to DC and purely resistive AC circuits. For AC with inductance or capacitance, resistance is replaced by impedance (Z) and a power factor is applied: P = V × I × pf. The calculator above uses resistive-only values.
- What are the units in Ohm's law?
- Voltage is measured in volts (V), current in amperes (A), resistance in ohms (Ω), and power in watts (W). For larger values: kilovolts (kV), milliamperes (mA), kilohms (kΩ), and kilowatts (kW).
- How do I use Ohm's law to check a fuse size?
- Find the load current using I = P ÷ V (for a resistive load) and choose a fuse rated above that current. For a 2 kW heater on 230 V: I = 2000 ÷ 230 = 8.7 A. A 13 A fuse is appropriate, providing protection with some margin.
- Why does resistance increase with temperature?
- In conductors (metals), higher temperature causes more atomic vibration, increasing resistance. For most metals the relationship is roughly linear: R(T) = R₀ × (1 + α × ΔT), where α is the temperature coefficient. For copper, α ≈ 0.00393 per °C — so resistance rises about 0.4% per degree of temperature increase.
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