How this Ohm’s law calculator works
Every simple electrical problem comes down to four quantities: voltage (V), current (I), resistance (R) and power (P). Know any two and the other two follow. Choose which pair you know, enter the values, and the calculator returns all four with automatic SI prefixes (mA, kΩ, MW…), plus the conductance, the heat produced in BTU/h and, if you set daily hours and a price, the energy use and running cost.
A chart shows how the same load behaves at other voltages: for a fixed resistance, current rises in proportion to voltage and power with its square. This is why a 230 V appliance run on 120 V delivers only about a quarter of its rated power, and why small over-voltages shorten the life of heating elements and incandescent lamps.
The formulas
All twelve rearrangements are listed in the results table.
Worked example: a 2 kW kettle
The default case is a 2,000 W kettle on a 230 V supply:
- Current: I = 2,000 ÷ 230 = 8.70 A
- Resistance: R = 230² ÷ 2,000 = 26.45 Ω
- One hour of boiling per day: 2 kWh, costing 0.50 per day at 0.25 per kWh (in practice a kettle runs a few minutes per boil)
- Heat output: 2,000 W × 3.412 = 6,824 BTU/h
On a 120 V supply the same element would draw 4.5 A and produce only 544 W.
Typical values
| Device | Voltage | Power | Current | Resistance |
|---|---|---|---|---|
| Phone charger (USB 5 V) | 5 V | 10 W | 2 A | 2.5 Ω (equivalent) |
| Laptop charger (USB-C PD) | 20 V | 100 W | 5 A | 4 Ω |
| LED strip | 12 V | 24 W | 2 A | 6 Ω |
| Car headlamp (halogen) | 12 V | 55 W | 4.6 A | 2.6 Ω |
| Kettle (EU) | 230 V | 2,000–3,000 W | 8.7–13 A | 17.6–26.5 Ω |
| Space heater (US) | 120 V | 1,500 W | 12.5 A | 9.6 Ω |
| Electric shower | 230 V | 9,500 W | 41 A | 5.6 Ω |
| Through-hole resistor | 5 V | 53 mW | 10.6 mA | 470 Ω |
Tips
- Check wire and fuse ratings before adding loads. Current, not power, heats cables. Two 2 kW heaters on one 230 V 16 A circuit draw 17.4 A, which is too much.
- Mind resistor power ratings. A 470 Ω resistor across 12 V dissipates 0.31 W; a ¼ W part would overheat.
- Use P = I²R for losses. Doubling current quadruples cable losses, which is why transmission lines run at high voltage.
- Low-voltage DC needs thick cables. 1 kW at 12 V is 83 A; at 48 V only 21 A. Use the power unit converter and kWh cost calculator for the energy side.
- For AC motors and electronics, include power factor: P = V × I × PF; see the kVA to kW calculator.
Limitations
Ohm’s law describes ohmic (linear) conductors at constant temperature. Real resistances change with temperature (a tungsten filament’s cold resistance is about a tenth of its hot value), and semiconductors, arcs and gas-discharge lamps are non-linear. For AC circuits with inductance or capacitance, replace R by the impedance Z and use the power factor.
Method & assumptions
Ohm's law V = I · R relates voltage, current and resistance; Joule's law P = V · I gives power. Combining them yields P = I² · R = V² ÷ R, so any two quantities determine the other two. Energy = P × hours ÷ 1,000 (kWh); cost = kWh × price.
Frequently asked questions
What is Ohm's law?
How many amps does a 2,000 W appliance draw?
How do I calculate watts from volts and amps?
What resistance does a 2 kW heater have?
Does Ohm's law apply to LEDs?
What size fuse or breaker do I need for a given wattage?
Sources & references
Embed this calculator
Free for blogs, installers, schools and NGOs. Copy the code below; it updates itself automatically.