Ohms Law Calculator
Enter any two of voltage, current, resistance and power, and find the other two.
Results
| Item | Value |
|---|
Quick Answer
Ohm’s law says voltage equals current times resistance: V = I × R. Power is P = V × I. For example, 12 V at 2 A means a resistance of 6 Ω and a power of 24 W.
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Table of Contents
- Ohms Law Calculator
- Quick Answer
- Related Calculators
- How This Ohms Law Calculator Works
- How to Use the Ohms Law Calculator
- Core Formulas
- Key Terms
- Worked Example
- Real-Life Examples
- Volts vs Amps vs Watts
- Amps Drawn at 120 V
- Things to Keep in Mind
- Common Mistakes to Avoid
- Frequently Asked Questions
- References & Sources
How This Ohms Law Calculator Works
Ohm’s law links voltage, current and resistance. Add the power formula, and any two values give you the other two. Enter two values, leave the others blank, and press Calculate.
Quick explanation: V = I × R, and P = V × I.
- Voltage (V): electrical pressure, in volts.
- Current (I): flow of charge, in amps.
- Resistance (R): opposition to flow, in ohms.
- Power (P): rate of energy use, in watts.
How to Use the Ohms Law Calculator
- Enter any two values you know: voltage, current, resistance or power.
- Leave the other two boxes empty.
- Press Calculate.
- Read the two missing values.
- Check the units: volts (V), amps (A), ohms (Ω) and watts (W).
Core Formulas
- Voltage: V = I × R, or V = P ÷ I, or V = √(P × R)
- Current: I = V ÷ R, or I = P ÷ V, or I = √(P ÷ R)
- Resistance: R = V ÷ I, or R = V² ÷ P, or R = P ÷ I²
- Power: P = V × I, or P = I² × R, or P = V² ÷ R
Key Terms
- Voltage (V): the electrical push, measured in volts.
- Current (I): the flow of electric charge, measured in amps.
- Resistance (R): opposition to current, measured in ohms (Ω).
- Power (P): the rate of energy use, measured in watts.
- Load: the device that uses electricity in a circuit.
Worked Example
A 60 W device runs on 120 V.
- Current = 60 ÷ 120 = 0.5 A
- Resistance = 120 ÷ 0.5 = 240 Ω
Real-Life Examples
- An LED resistor: to run a 2 V LED at 20 mA (0.02 A) from a 9 V battery, the resistor must drop 7 V. R = 7 ÷ 0.02 = 350 Ω, so a 390 Ω standard resistor is a safe choice.
- A space heater: a 1,500 W heater on 120 V draws 1,500 ÷ 120 = 12.5 A. That is a lot for one household circuit, so avoid sharing it with other large loads.
- A car accessory: a 12 V device that draws 5 A uses 12 × 5 = 60 W.
- Resistor power: a 100 Ω resistor with 0.1 A through it gives off 0.1² × 100 = 1 W of heat, so it needs a rating above 1 W.
Volts vs Amps vs Watts
- Volts (V): electrical pressure that pushes current through a circuit.
- Amps (A): the rate of current flow.
- Watts (W): the rate of energy use, volts × amps.
- Ohms (Ω): how much a part resists current. More resistance means less current at the same voltage.
- Water analogy: voltage is like water pressure, current like flow, and resistance like a narrow pipe.
Amps Drawn at 120 V
For simple resistive loads such as heaters and incandescent bulbs.
| Power | Current at 120 V | Resistance |
|---|---|---|
| 60 W | 0.50 A | 240.0 Ω |
| 100 W | 0.83 A | 144.0 Ω |
| 500 W | 4.17 A | 28.8 Ω |
| 1,000 W | 8.33 A | 14.4 Ω |
| 1,200 W | 10.00 A | 12.0 Ω |
| 1,500 W | 12.50 A | 9.6 Ω |
| 1,800 W | 15.00 A | 8.0 Ω |
Things to Keep in Mind
- DC and simple AC loads: these formulas suit basic resistive loads.
- Motors and electronics: AC loads may need power factor, which is not included here.
- Safety: do not rely on this calculator for wiring or breaker sizing.
Common Mistakes to Avoid
- Mixing milliamps and amps. 20 mA is 0.02 A.
- Entering more than two values. Enter two, and let the calculator find the rest.
- Using it for motors without power factor. AC motors and electronics need more than Ohm’s law.
- Ignoring a resistor’s power rating. A resistor can overheat if its watt rating is too low.
- Working on live circuits. Turn off power and use a qualified electrician for home wiring.
Frequently Asked Questions
It says voltage equals current times resistance: V = I × R.
Divide voltage by resistance. For example, 12 V ÷ 6 Ω = 2 A.
Multiply volts by amps. For example, 120 V × 2 A = 240 W.
Two. The calculator finds the other two values.
No. It suits resistive parts best. Diodes, for example, do not follow it in a simple way.
At 120 V, about 8.33 A. At 240 V, about 4.17 A.
Subtract the LED voltage from the supply voltage, then divide by the LED current in amps. Round up to the next standard resistor.
At the same voltage, current goes down. Doubling the resistance halves the current, and the power drops too.
It is a memory aid showing P = V × I, so you can cover one value to see how to find it from the other two.
References & Sources
| Source | Used For | Link |
|---|---|---|
| Wikipedia, Ohm’s law | The relation between voltage, current and resistance. | https://en.wikipedia.org/wiki/Ohm%27s_law |
| Wikipedia, Electric power | Power formulas. | https://en.wikipedia.org/wiki/Electric_power |