Ohm’s law calculator
Any two of V, I, R and P — the other two, worked out.
Output
Enter any two of voltage, current, resistance and power — the other two are worked out.
Your files stay on your device
This tool runs entirely in your browser. What you paste is processed on your device and is never sent to a server, so it is safe to use with tokens, keys and unpublished text.
Example
Example: how many amps a 1500-watt space heater draws from a 120 V outlet
A space heater’s label says 1500 W, and you want to know how much current it takes from a 120 V outlet. Type 120 in Voltage and 1500 (or 1.5k) in Power, and leave Current and Resistance empty.
- Current: I = P / V = 1.5 kW / 120 V = 12.5 A.
- Resistance: R = V² / P = (120 V)² / 1.5 kW = 9.6 Ω, the element’s resistance while it runs.
- Type 12 in Current as well and the page says the power does not agree: 120 V and 12 A give 1.44 kW, 4.2 % off the 1.5 kW entered.
| What you enter | What it works out |
|---|---|
| 120 V and 1500 W (space heater) | 12.5 A, 9.6 Ω |
| 120 V and 1875 W (hair dryer) | 15.63 A, 7.68 Ω |
| 12 V and 60 W (car accessory) | 5 A, 2.4 Ω |
| 5 V and 3 A (USB charger) | 1.667 Ω, 15 W |
| 1.3 V and 10 mA (resistor for a 2 V LED on a 3.3 V pin) | 130 Ω, 13 mW |
12.5 A for 1500 W at 120 V. Every answer comes with its unit and prefix — 13 mW rather than 0.013 W, 1.5 kW rather than 1500 W — and the formulas are shown with your numbers in them.
How it works
- Fill in any two of the four fields — for example voltage and current.
- Use prefixes as you would write them: 4k7 or 4.7k for 4,700 Ω, 10mA, 500 mW, 2.2M.
- Read the other two values and the formulas used, with your numbers in them.
Frequently asked questions
- What is Ohm’s law?
- Voltage equals current times resistance: V = I × R. With power P = V × I you can find all four values from any two.
- An example?
- 12 V and 0.5 A give 24 Ω (R = V / I) and 6 W (P = V × I). 5 V across 4k7 gives 1.064 mA and 5.319 mW.
- What if I fill in three or four fields?
- The first two in the order voltage, current, resistance, power are used, and the page says whether the others agree within 1 % — handy for checking a design.
- Which resistor wattage do I need?
- Pick one rated at least twice the power shown: for 0.2 W, a ½ W resistor.
- How do I convert watts to amps?
- Fill in Power and Voltage: amps = watts ÷ volts. 1500 W at 120 V is 12.5 A; 60 W at 12 V is 5 A. The resistance comes with it, 9.6 Ω and 2.4 Ω.
- Why are some results in mA or kW?
- Each value takes the prefix that keeps the number readable: 0.02 A shows as 20 mA, 1500 W as 1.5 kW, 4700 Ω as 4.7 kΩ. You can type them the same way.
- Is anything saved?
- No. It is worked out in your browser.
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