How to use the Ohm’s law calculator
- Under Known values, choose the pair you have: Voltage and current, Voltage and resistance, Current and resistance, Power and voltage, Power and current, or Power and resistance.
- Enter the two values that appear: Voltage (V), Current (A), Resistance (Ω) or Power (W). Both must be above zero.
- Read all four values. Prefixes such as m (milli), µ (micro), k (kilo) and M (mega) keep the numbers short.
What it does and when to use it
Ohm’s law is the first formula most people meet in electronics, and it keeps coming up: choosing a resistor for an LED, checking whether a fuse is big enough, working out what current a heater draws, or sizing a power supply for a hobby project. Instead of rearranging the formula by hand for each case, pick the two values you know and read off the rest.
For energy and running cost rather than circuit values, the electricity cost calculator turns watts and hours into kWh and money. To convert between power units such as watts, kilowatts and horsepower, use the power converter. For other everyday sizing sums, the aspect ratio calculator is in the same group of tools.
How it works
The calculator uses Ohm’s law, V = I × R, together with the power formula, P = V × I. Combining them gives:
| You know | It calculates |
|---|---|
| V and I | R = V ÷ I, P = V × I |
| V and R | I = V ÷ R, P = V² ÷ R |
| I and R | V = I × R, P = I² × R |
| P and V | I = P ÷ V, R = V² ÷ P |
| P and I | V = P ÷ I, R = P ÷ I² |
| P and R | I = √(P ÷ R), V = √(P × R) |
Results are shown to four decimal places after choosing a prefix, so 0.2608696 A becomes 260.8696 mA.
Worked examples
These figures come from the calculator’s own function.
12 V across 6 Ω. Voltage and resistance: current 2 A and power 24 W.
A 60 W lamp on 230 V. Power and voltage: current 260.8696 mA and resistance 881.6667 Ω (its resistance when hot).
An LED resistor drop. Voltage and current, 5 V and 0.02 A: resistance 250 Ω and power 100 mW. In a real LED circuit, subtract the LED’s forward voltage from the supply first, then use the remaining voltage.
A speaker load. Power and resistance, 100 W into 4 Ω: 20 V and 5 A.
Limits and tips
- Positive values only. Zero or negative entries return a prompt to enter two values above zero.
- Resistive loads. Reactive AC circuits need impedance and power factor, which this tool does not handle.
- Resistance changes with temperature; a lamp’s cold resistance is much lower than its hot value.
- Safety first. Mains voltage is dangerous; leave mains wiring to a qualified electrician.
- Calculations happen in your browser.
Frequently asked questions
What is Ohm's law?
How do I find the current a device draws?
Does it work for household AC appliances?
Why do I see mA, kΩ or µA?
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