Ohm's Law Calculator
Use this Ohm's law calculator to find the missing values in a DC or purely resistive circuit. Enter any two of voltage, current, resistance and power, and it works out the other two. Each quantity has its own unit, from millivolts to kilowatts, and a table runs all 12 Ohm's law and power formulas with your numbers so you can check the answer.
Updated
Results
- Formulas used
| Quantity | Formula | Result |
|---|
Results are estimates. See the methodology below.
How to use the calculator
- Fill in exactly two values out of voltage (V), current (I), resistance (R) and power (P). Leave the other two fields empty. Each value must be greater than zero.
- Pick a unit for each quantity: mV, V or kV for voltage; mA or A for current; Ω, kΩ or MΩ for resistance; mW, W or kW for power. Set the units of the empty fields too, since the results are shown in them.
- Read the results: all four quantities, with up to 6 significant digits, and the two formulas used. The table shows every formula evaluated with the solved values.
If fewer or more than two fields are filled, the calculator asks you to add or clear a value.
The formulas
Ohm's law says that, for an ohmic material, current is proportional to voltage: I = V / R, or amperes = volts ÷ ohms. Electrical power is P = V × I, watts = volts × amperes. Combining the two gives the full set:
| 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, I² × R, V² / R |
The calculator converts your inputs to volts, amperes, ohms and watts using the SI prefixes (milli = 10⁻³, kilo = 10³, mega = 10⁶), picks the pair of formulas that matches the two values you entered, and converts the results back to the units you chose.
Assumptions: direct current or a purely resistive load, and a resistance that stays constant. AC loads with reactance (motors, transformers), power factor, and parts that don't follow Ohm's law, such as LEDs and diodes, are not modeled.
Worked examples
| You enter | Results |
|---|---|
| 12 V across 6 Ω | 2 A, 24 W |
| 9 V battery, 2 mA (result in kΩ) | 4.5 kΩ, 0.018 W |
| 8 Ω speaker at 100 W | 28.2843 V, 3.53553 A |
| 120 V, 60 W bulb | 0.5 A, 240 Ω |
Resistor for an LED
The LED itself isn't ohmic, but its series resistor is. Enter the voltage the resistor has to drop, which is the supply voltage minus the LED's forward voltage from its datasheet, and the current you want. If the resistor must drop 5 V at 20 mA, you need 250 Ω, and it dissipates 0.1 W (100 mW). If it drops 3 V at 20 mA, the answer is 150 Ω and 0.06 W.
A 60 W bulb on 120 V
U.S. household outlets supply a nominal 120 V AC. For a purely resistive load such as an incandescent bulb, the formulas work with that effective voltage: 60 W at 120 V means 0.5 A and 240 Ω. That resistance applies while the filament is hot; a metal filament's resistance changes with temperature.
Reading the results
All twelve formulas in the table give the same answer for each quantity. If they didn't, the inputs would be inconsistent. The power result tells you how much heat a resistor turns out, which you can compare with its wattage rating. The results describe an ideal circuit; real batteries, wires and parts have tolerances and some internal resistance.
Frequently asked questions
What is Ohm's law?
It states that the current through a conductor is directly proportional to the voltage across it, so the ratio V / I, the resistance, stays constant. Materials that behave this way over a wide range of voltages are called ohmic. In formulas: V = I × R.
Why do I have to enter exactly two values?
The four quantities are linked by two equations, V = I × R and P = V × I. Two known values fix the other two. With a third value the inputs could contradict each other, so the calculator asks you to clear one field.
Does it work for household AC circuits?
Only for purely resistive loads, such as heaters or incandescent bulbs, using the effective voltage. Standard U.S. home wiring supplies a nominal 120 V between a hot wire and neutral, and a nominal 240 V across both hot wires for large appliances. Motors, electronics and anything with reactance need impedance and power factor, which this calculator doesn't model.
Can I use it to choose a wire size or breaker?
No. Choosing wires and breakers involves electrical codes and factors that Ohm's law doesn't cover. For household wiring, consult a licensed electrician.
What do mA, kΩ and MΩ mean?
They are SI prefixes: milli means one thousandth, kilo one thousand and mega one million. So 20 mA = 0.02 A, 4.5 kΩ = 4,500 Ω and 1 MΩ = 1,000,000 Ω. Choose the unit you need for each quantity; the calculator does the scaling.
How much current does a 12 V device draw?
Divide power by voltage. A 60 W load on a 12 V supply draws 5 A, and its resistance is 2.4 Ω. Enter 12 V and 60 W to see all four values.
Sources
This calculator is for learning and low-voltage hobby projects. Household and higher voltages can cause serious shock, burns or fire; the results are not a substitute for a licensed electrician, so consult one for any household wiring.