How the Ohms Law Calculator Works
Electricity in a simple circuit follows a few rules that are easy to use. Ohm's law says how the voltage, the current and the resistance are linked. A second rule links these to power. The Ohm's Law Calculator uses both, so that if you know any two of the four values, you get the other two.
The four quantities
| Quantity | Symbol | Unit | What it is |
|---|---|---|---|
| Voltage | V | volt (V) | The push that drives the current, like water pressure. |
| Current | I | ampere, or amp (A) | The flow of charge, like the flow of water. |
| Resistance | R | ohm (Ω) | How much the part opposes the flow, like a narrow pipe. |
| Power | P | watt (W) | The energy used each second, which turns into heat or light. |
- Quantity: Voltage
Symbol: V
Unit: volt (V)
What it is: The push that drives the current, like water pressure. - Quantity: Current
Symbol: I
Unit: ampere, or amp (A)
What it is: The flow of charge, like the flow of water. - Quantity: Resistance
Symbol: R
Unit: ohm (Ω)
What it is: How much the part opposes the flow, like a narrow pipe. - Quantity: Power
Symbol: P
Unit: watt (W)
What it is: The energy used each second, which turns into heat or light.
The formulas
Ohm's law is V = I x R. The power law is P = V x I. Put together, they give three ways to write each quantity:
| To find | From two of the others |
|---|---|
| Voltage (V) | I x R, or P / I, or the square root of (P x R) |
| Current (I) | V / R, or P / V, or the square root of (P / R) |
| Resistance (R) | V / I, or V x V / P, or P / (I x I) |
| Power (P) | V x I, or I x I x R, or V x V / R |
What you enter
- Which two you know: voltage and current, voltage and resistance, voltage and power, current and resistance, current and power, or resistance and power.
- The two values, each with a unit. You can use small and large units, such as millivolts, milliamps, kilohms and kilowatts.
The results are shown with the prefix that makes each number easy to read. A current of 0.0047 amps is shown as 4.7 mA, and 4,700 ohms is 4.7 kΩ.
A worked example
A circuit has a voltage of 12 V and a current of 2 A.
| Step | Result |
|---|---|
| Resistance (12 / 2) | 6 Ω |
| Power (12 x 2) | 24 W |
| Check: current from power and resistance, the square root of (24 / 6) | 2 A |
If you start from any other pair, for example 12 V and 6 Ω, or 6 Ω and 24 W, you get the same four values.
Power grows with the square
Resistance stays the same, and the voltage goes up. The current goes up in the same proportion, but the power goes up with the square of the voltage. For a resistance of 6 Ω:
| Voltage | Current | Power |
|---|---|---|
| 3 V | 0.5 A | 1.5 W |
| 6 V | 1 A | 6 W |
| 9 V | 1.5 A | 13.5 W |
| 12 V | 2 A | 24 W |
| 15 V | 2.5 A | 37.5 W |
| 18 V | 3 A | 54 W |
3 V █ 1.5 W 6 V ████ 6 W 9 V ██████████ 13.5 W 12 V ██████████████████ 24 W 15 V ████████████████████████████ 37.5 W 18 V ████████████████████████████████████████ 54 W
Doubling the voltage from 6 V to 12 V doubles the current, but makes the power four times bigger. This is why a part can be fine at one voltage and burn out at a higher one.
The graph
The calculator draws the current against the voltage for your resistance. It is a straight line, because current is proportional to voltage. The slope of the line is 1 / R, so a small resistance gives a steep line. The dot shows your circuit.
The same voltage, different resistors
The current for different resistances, at 12 V:
| Resistance | Current | Power |
|---|---|---|
| 1 Ω | 12 A | 144 W |
| 2 Ω | 6 A | 72 W |
| 3 Ω | 4 A | 48 W |
| 6 Ω | 2 A | 24 W |
| 12 Ω | 1 A | 12 W |
| 24 Ω | 0.5 A | 6 W |
1 Ω ████████████████████████████████████████ 12 A 2 Ω ████████████████████ 6 A 3 Ω █████████████ 4 A 6 Ω ███████ 2 A 12 Ω ███ 1 A 24 Ω ██ 0.5 A
Some practical examples
- An LED resistor. A supply of 5 V, an LED that drops 1.7 V, and a wanted current of 20 mA. The resistor has 5 - 1.7 = 3.3 V across it. With 3.3 V and 20 mA, the resistance is 165 Ω, and the power is 66 mW. A 1/4 W resistor is enough. A standard 180 Ω resistor is a good choice, and it gives about 18 mA.
- A kettle. A 2.3 kW kettle on 230 V takes 2,300 / 230 = 10 A. Its resistance is 230 / 10 = 23 Ω when hot.
- A pull-up resistor. 5 V across 10 kΩ gives a current of 0.5 mA, and 2.5 mW.
The power rating of a resistor
Every power in a resistor turns into heat. A resistor must be able to lose this heat, or it gets too hot. The calculator suggests a rating that is at least twice the power, so that the part does not work at its limit. The common ratings are 1/8, 1/4, 1/2, 1, 2, 3, 5, 10, 25, 50 and 100 watts. For the example of 24 W, the suggestion is a 50 W resistor.
Tips
- Use the same base units, or let the calculator change them for you. A common mistake is to use milliamps with volts and ohms.
- Remember the square. If you double the voltage or the current, the power is four times bigger.
- Choose a safe margin for the power rating of a part.
- Check with a meter. Real parts differ from their marked values.
What this calculator does not include
- Parts that do not follow Ohm's law, such as diodes, LEDs and transistors. For them, the resistance changes with the voltage.
- Alternating current circuits with coils and capacitors, which need impedance and power factor.
- The change of resistance with temperature.
- Several resistors in series or in parallel. Find the total resistance first, and enter it here.
This tool gives a general guide for simple direct-current circuits. For mains wiring or anything that can cause a fire or a shock, get the work done by a qualified electrician.