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Ohm's Law Calculator

Know two of voltage, current, resistance and power? Enter them, and this finds the other two for you.

Level: Grades 8–9

Printable worksheet

Ohm's Law Calculator

Know the answer you need? Pick an input to work backward.

Inputs

In volts. For millivolts and similar, use e-notation: 3.3e-3 means 3.3 × 0.001, which is 3.3 mV.

In amperes. 250 mA is 0.25 A.

Result

Resistance

6 Ω

Resistance is 6 Ω and power is 24 W.

Power
24 W
Voltage
12 VGiven
Current
2 AGiven

See the step-by-step solution ↓

Step-by-step solution

  1. Find the resistance with Ohm's law: divide the voltage by the current

    R=122=6 Ω

  2. Find the power: multiply the voltage by the current

    P=12×2=24 W

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

What this calculator does

Find the missing values in a simple electric circuit from the two you already know. Voltage (V) is the electrical push, measured in volts. Current (I) is how much electricity flows, in amperes (amps). Resistance (R) is how hard a part resists that flow, in ohms (Ω). Ohm's law ties them together: V = I × R.

Power (P), in watts, is how fast energy is used, and it equals voltage times current: P = V × I. Put the two rules together, and any two of these four values are enough to find the other two.

Choose which pair you know and enter the values in volts, amps, ohms or watts. The calculator gives you the missing pair, with each formula shown. It's handy for choosing resistors, or checking how much current a heater or bulb draws. It also shows how much power a resistor has to get rid of as heat.

Where it's used

Why this math matters outside the classroom:

  • LED resistors. Hobbyists pick an LED resistor on 5 V by dividing the 3 V left over after the LED by 0.02 A, which gives 150 Ω.
  • Appliances. A 1,200 W toaster on 120 V draws 10 A, so its heating element has a resistance of 12 Ω.
  • Troubleshooting. Technicians measure the voltage across a part and the current through it to confirm its resistance hasn't drifted.
  • Lab circuits. Students find a 9 V battery across a 450 Ω resistor drives 0.02 A and turns 0.18 W into heat.

Grade level and standards: Grades 8–9; Common Core State Standards for Mathematics HSA.CED.A.4. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.

How to use it

  1. Choose the pair of values you already know, for example voltage and resistance.
  2. Enter those two values in the basic units: volts, amps, ohms and watts.
  3. Read the two values the calculator found. The two you entered are listed too, marked as given.

What the inputs mean

Voltage (V)
There are no separate options for millivolts or kilovolts. Convert first, or use e-notation, a short way to write powers of 10: 5 mV is 0.005 or 5e-3, and 11 kV is 11000 or 11e3.
Current (I)
To turn milliamps into amps, divide by 1,000: 20 mA is 0.02 A.
Resistance (R)
For kilohms, multiply by 1,000; for megohms, by 1,000,000. So 4.7 kΩ is 4700, and 1 MΩ is 1e6.
Power (P)
Power is how fast energy is used. A 60 W bulb uses 60 joules (units of energy) every second.

Formula and method

V = I × R      I = V ÷ R      R = V ÷ I
P = V × I = I² × R = V² ÷ R
From R and P:  V = √(P × R),  I = √(P ÷ R)
From I and P:  V = P ÷ I,  R = P ÷ I²
From V and P:  I = P ÷ V,  R = V² ÷ P

The first line is Ohm's law, rearranged three ways to find each value. The second line is the power rule. It swaps in Ohm's law to remove whichever value you don't know.

When you know resistance and power, voltage and current come from square roots. That's because power grows with the square of either one (the value times itself). All values are treated as positive sizes, with no minus signs.

Worked example

A 12 V supply drives a current of 2 A through a resistive load (a part, like a heater, that only resists the flow).

  1. Resistance: R = V ÷ I = 12 ÷ 2 = 6 Ω.
  2. Power: P = V × I = 12 × 2 = 24 W.

Result: The load has a resistance of 6 Ω and turns 24 W into heat.

Understanding the result

The power figure tells you how much heat a resistor must get rid of. Choose a resistor rated comfortably above it. A common rule of thumb is about double.

If the current is higher than a wire, fuse or power supply is rated for, the circuit needs a bigger rating or a higher resistance.

Assumptions and limitations

  • The part is ohmic, meaning its resistance stays the same at any voltage or current.
  • Values are steady, as from a battery (DC). For household-style AC, they're RMS values (the usual effective averages) in a circuit with only resistance.
  • All inputs are positive sizes.
  • It doesn't work for parts such as diodes, LEDs and transistors, whose current doesn't rise in step with voltage.
  • AC circuits with capacitors or inductors (coils) need impedance and phase, which this calculator doesn't handle.
  • Resistance changes with temperature. A bulb filament's resistance when cold is much lower than when it's lit.

Frequently asked questions

What does Ohm's law say?

The current through a wire or part rises in step with the voltage across it, and resistance sets how much: I = V ÷ R. Double the voltage across a fixed resistor, and the current doubles too.

How do I find the current a device draws from its wattage?

Divide the power by the voltage. A 1,000 W heater on a 230 V supply draws 1000 ÷ 230 ≈ 4.35 A. Its resistance while running is 230² ÷ 1000 = 52.9 Ω.

Can I enter kilohms or milliamps?

Convert them to the basic units first, or type the value in e-notation. So 4.7 kΩ is 4.7e3, and 15 mA is 15e-3. The results are always in volts, amps, ohms and watts.

Why are negative values not accepted?

In a circuit, a minus sign only shows direction. In a resistor, voltage and current always point the same way, so their sizes are enough. Resistance and the power turned into heat are always positive.

How this calculator is checked

Method: Ohm's law (V = IR) and electrical power (P = VI), algebraically rearranged. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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