Ohm's Law Calculator
This Ohm's Law calculator solves V = IR for any unknown — voltage, current, or resistance — and computes power simultaneously. Select which variable you want to find, enter the two known values, and get instant results.
Enter known values
Select which quantity to solve for, then fill in the two known values.
Ohm's Law and the V = IR triangle
Ohm's Law is the cornerstone of circuit analysis. It relates three quantities — voltage (V), current (I), and resistance (R) — in a single equation: V = I × R. Rearranging gives you the other two forms:
V = I × R (solve for voltage)
I = V ÷ R (solve for current)
R = V ÷ I (solve for resistance)
A classic memory aid is the VIR triangle: cover the unknown letter with your thumb and the remaining two show the calculation — cover V and you see I × R; cover I and you see V ÷ R; cover R and you see V ÷ I.
Power formulas — the fourth variable
Power (P), measured in watts, describes how fast electrical energy is consumed. Starting from P = V × I and substituting Ohm's Law yields two more useful variants:
P = V × I (voltage × current)
P = I² × R (current and resistance only)
P = V² ÷ R (voltage and resistance only)
Power tells you how much heat a resistor dissipates and whether it needs a heatsink. A 100 Ω resistor carrying 0.1 A dissipates P = (0.1)² × 100 = 1 W — right at the limit of a standard ¼ W component, so choose a higher-rated resistor in practice.
Worked example — a simple LED circuit
You want to run a red LED (forward voltage 2 V, target current 20 mA) from a 5 V supply. The resistor must drop 5 − 2 = 3 V at 0.02 A. Using Ohm's Law: R = V ÷ I = 3 ÷ 0.02 = 150 Ω. Power in the resistor: P = 3 × 0.02 = 60 mW, well within a ¼ W rating. Enter these values into the calculator above — select Resistance, enter 3 V and 0.02 A — to confirm.
Frequently asked questions
- What is Ohm's Law?
- Ohm's Law states that the voltage (V) across a conductor is directly proportional to the current (I) flowing through it, with the constant of proportionality being the resistance (R). The formula is V = I × R. It is one of the most fundamental relationships in electrical engineering and physics, applicable to resistors and many other linear circuit elements at constant temperature.
- How do I use this Ohm's Law calculator?
- Select which quantity you want to solve for — Voltage, Current, or Resistance — then enter the two known values into the fields that appear. The calculator instantly applies V = IR to find the unknown value, and also computes power (P = V × I) as a bonus output.
- What units does Ohm's Law use?
- Voltage is measured in volts (V), current in amperes (A), resistance in ohms (Ω), and power in watts (W). If you work with milliamperes, kilohms, or milliwatts, convert to base units first — for example, 10 mA = 0.01 A and 4.7 kΩ = 4700 Ω.
- How is power calculated alongside Ohm's Law?
- Power is the rate at which energy is consumed or delivered. When you know voltage and current, P = V × I. Substituting Ohm's Law gives two equivalent forms: P = I² × R (useful when current and resistance are known) and P = V² ÷ R (useful when voltage and resistance are known). All four formulas give the same result for consistent inputs.
- Does Ohm's Law apply to all components?
- Ohm's Law strictly applies to ohmic (linear) resistors — components where resistance stays constant regardless of voltage or current. It does not directly apply to non-ohmic components like diodes, transistors, light-emitting diodes, or batteries, where the V-I relationship is non-linear. For those components, more specialised models are required.
- What is a real-world example of Ohm's Law?
- A car headlight bulb rated at 12 V draws about 4.17 A of current. By Ohm's Law the bulb's resistance is R = V ÷ I = 12 ÷ 4.17 ≈ 2.88 Ω, and the power consumed is P = V × I = 12 × 4.17 ≈ 50 W. This is exactly the kind of calculation used when sizing fuses, wiring, and power supplies.
- What happens if resistance is zero?
- If resistance is zero (a short circuit), any non-zero voltage would theoretically produce infinite current — which is why short circuits are dangerous and cause fuses or breakers to trip. In practice, source impedance and wire resistance limit the actual fault current. This calculator will show an error if you attempt to divide by a zero resistance value.
- How do I find total resistance in series and parallel circuits?
- For resistors in series, total resistance is the sum: R_total = R1 + R2 + … For resistors in parallel, use the reciprocal formula: 1/R_total = 1/R1 + 1/R2 + … Once you have the total resistance, enter it here along with your source voltage or current to find the overall circuit behaviour.