Ohm's Law Calculator
It solves any two of voltage, current, resistance and power to give you the other two, using Ohm's law together with the power formula.
Power dissipated
24.000 W
Voltage (V)
12.0000 V
Current (I)
2.0000 A
Resistance (R)
6.0000 Ω
Power (P)
24.0000 W
Current in mA
2,000.0 mA
Suggested rating
48.00 W
Choose a component rated at roughly twice the calculated power for headroom. Ohm's law applies to resistive, linear parts — diodes, LEDs and transistors need their datasheet curves instead.
What it calculates
It solves any two of voltage, current, resistance and power to give you the other two, using Ohm's law together with the power formula.
Why it matters
These four quantities pin down a circuit completely. Knowing any two tells you the rest, which is what sizing a resistor, a fuse or a supply depends on.
Who it's for
Electronics hobbyists, electricians, students, and anyone sizing components or checking a power budget.
Formula
- V
- Voltage in volts
- I
- Current in amps
- R
- Resistance in ohms
- P
- Power in watts
Worked example
12 V across a 6 Ω resistor
- 1I = V ÷ R = 12 ÷ 6 = 2 A
- 2P = V × I = 12 × 2 = 24 W
2 amps and 24 watts — so the resistor needs a rating above 24 W
How the ohm's law calculator works
Two relations do all the work: V = I × R and P = V × I. Everything else is substitution — P = I²R and P = V²/R follow directly. Doubling the voltage across a fixed resistance doubles the current but quadruples the power, which is why components fail on voltage rises far faster than intuition suggests, and why power rating is the specification that usually matters most.
Two relations do all the work: V = I × R and P = V × I. Everything else is substitution — P = I²R and P = V²/R follow directly from them.
Doubling the voltage across a fixed resistance doubles the current but quadruples the power, which is why components fail on voltage rises far faster than intuition suggests, and why power rating is usually the specification that matters most.
Common mistakes
- Sizing a resistor on resistance alone and ignoring its power rating.
- Mixing units — milliamps with amps, or kilohms with ohms.
- Assuming Ohm's law holds for every component; diodes, LEDs and filament lamps are not linear.
Tips and best practice
- Choose a resistor rated at roughly twice the calculated power for headroom.
- Power scales with the square of voltage, so a small voltage rise is a large heat rise.
- Ohm's law applies to resistive elements; semiconductors need their own datasheet curves.
Frequently asked questions
What is Ohm's law?
Voltage equals current times resistance, V = I × R. Combined with P = V × I, it lets any two of voltage, current, resistance and power give you the other two.
How do I calculate watts from volts and amps?
Multiply them: P = V × I. 12 volts at 2 amps is 24 watts.
How do I work out the resistor I need?
Divide the voltage you need to drop by the current you want: R = V ÷ I. Then check the power it will dissipate, P = I² × R, and pick a rating above it.
Does Ohm's law apply to everything?
Only to resistive, linear components. Diodes, LEDs, transistors and filament lamps all have non-linear behavior and need their datasheet curves instead.
Why does power rise so fast with voltage?
Because P = V² ÷ R for a fixed resistance. Doubling the voltage quadruples the power, which is why components overheat quickly when the supply runs high.
Related calculators
Methodology & trust
- Formula source
- Ohm's law (V = IR) and the electrical power relations.
- Last updated
- 2026-07-28
- Privacy
- Every calculation runs in your browser. No inputs are sent to a server or stored.
- Accessibility
- Keyboard navigable, labeled inputs and WCAG AA color contrast.