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About Ohm's law
Ohm's law links three values in an electric circuit: voltage (V), current (I) and resistance (R):
V = I × R
Add power (P) and you have four values. Know any two, and you can work out the other two. This calculator does exactly that. Type two values in the units you like — mV, V, µA, mA, A, Ω, kΩ, MΩ, mW or W — and the other two appear, with each formula and your own numbers in it.
Features
- Any two values — voltage, current, resistance or power; the other two are worked out
- Units with prefixes — pick mA, kΩ or mW in the list, or type 4k7, 20m or 2.2M in the box
- Easy to read — each answer in the unit that reads best: 22.73 mA, not 0.02273 A
- The formulas with your numbers — like I = V ÷ R = 5 V ÷ 220 Ω = 22.73 mA
- The Ohm's law wheel — all 12 formulas, with the ones used lit up
- Resistor wattage — the smallest common power rating that is safe for your resistor
- Six examples — from an LED resistor to a 1500 W heater
The 12 formulas
| To find | Formulas |
|---|---|
| Voltage V | I × R · P ÷ I · √(P × R) |
| Current I | V ÷ R · P ÷ V · √(P ÷ R) |
| Resistance R | V ÷ I · V² ÷ P · P ÷ I² |
| Power P | V × I · V² ÷ R · I² × R |
The units are volts (V), amperes (A), ohms (Ω) and watts (W). Use the base units in the formulas: 20 mA is 0.02 A, and 4.7 kΩ is 4,700 Ω. The calculator does this for you.
Worked examples
| You know | You get |
|---|---|
| 5 V across 220 Ω | I = 22.73 mA, P = 113.6 mW |
| 3 V across a resistor, 20 mA through it | R = 150 Ω, P = 60 mW |
| 12 V bulb, 60 W | I = 5 A, R = 2.4 Ω |
| 9 V battery, 1 kΩ | I = 9 mA, P = 81 mW |
| 120 V heater, 1500 W | I = 12.5 A, R = 9.6 Ω |
| 1/4 W resistor of 100 Ω, at its limit | V = 5 V, I = 50 mA |
The first one is a classic Arduino check: a 5 V pin with a 220 Ω resistor to ground gives about 23 mA. With an LED in the circuit the current is lower — the LED Resistor Calculator works that out.
Units and prefixes
| Prefix | Means | Example |
|---|---|---|
| µ (micro) | ÷ 1,000,000 | 10 µA = 0.00001 A |
| m (milli) | ÷ 1,000 | 20 mA = 0.02 A |
| k (kilo) | × 1,000 | 4.7 kΩ = 4,700 Ω |
| M (mega) | × 1,000,000 | 1 MΩ = 1,000,000 Ω |
Mind the case: m is milli (one thousandth) and M is mega (one million). On resistors, 4k7 means 4.7 kΩ — the letter takes the place of the decimal point. You can type it that way here too.
Power and the resistor's rating
A resistor turns power into heat. Each one has a power rating: 1/8 W, 1/4 W, 1/2 W, 1 W and more. Choose a rating of at least twice the power it uses, so it stays cool. The calculator shows that size. Example: 5 V across 220 Ω is 113.6 mW, so a 1/4 W resistor is fine (2 × 113.6 mW = 227 mW, under 250 mW).
Frequently asked questions
What is Ohm's law?
The current through a resistor is the voltage across it divided by its resistance: I = V ÷ R. Turned around, the same law gives V = I × R and R = V ÷ I.
How do I calculate watts from volts and amps?
Multiply them: P = V × I. For example, 12 V × 5 A = 60 W.
How do I find amps from watts and volts?
Divide: I = P ÷ V. A 1500 W heater on 120 V draws 1500 ÷ 120 = 12.5 A.
Does Ohm's law work for LEDs?
For the resistor, yes. The LED itself does not follow Ohm's law: its voltage stays almost the same when the current changes. That is why an LED needs a resistor in series.
Does it work for AC?
For heaters, bulbs with a filament and other simple resistive loads, yes, with the usual RMS values (120 V, 230 V). Motors, chargers and LED lamps have a power factor, so P = V × I is only an estimate for them.
Why do two boxes say "worked out"?
Those two boxes are the answers. Type into one of them and it becomes one of your two values. The older of your two values is then worked out instead.
Related tools
- LED Resistor Calculator — the resistor for an LED, with its wattage
- Resistor Color Code Calculator — read the value from the color bands
- Voltage Divider Calculator — Vout from two resistors
- Battery Life Calculator — hours and days from mAh and current
- Electricity Cost Calculator — what your devices cost to run
- Unit Converter — power and energy units, and many more
How to use it
- Type two values, for example 5 V and 220 Ω
- Pick the unit next to each box (mV, V, mA, kΩ …), or type 4k7 or 20m
- Read the other two values in the boxes marked worked out and in the result
- Type into another box to change which two values you know
- Or tap one of the examples
