Ohm's Law Calculator
Volts, amps, ohms, and watts from any two electrical values.
What it does
Georg Ohm found in 1827 that current through a conductor scales directly with voltage and inversely with resistance. That one proportion, V equals I times R, plus power as V times I, answers nearly every basic circuit question.
The tool above takes any known pair among volts, amps, and ohms. It returns all four values with power, so heater elements, LED resistors, and fuses all size from one pass.
Power grows with the square of volts at fixed resistance, which is why small voltage bumps heat elements fast. Doubling twelve volts to twenty four across the same resistor quadruples watts from twenty four to ninety six.
House mains demand respect beyond the math. This page teaches relationships; wiring work belongs to qualified people with the power off and verified dead.
How to use
- Pick which two values you know.
- Enter them with units as labeled.
- Read all four results, including power.
- Size parts above the computed power with margin.
Formula
- V
- Voltage pushing charge, in volts.
- I
- Current flow, in amps.
- R
- Resistance opposing flow, in ohms.
- P
- Power turned to heat or work, in watts.
The three power forms agree always: substitute V equals IR into VI for I squared R, or I equals V over R for V squared over R.
Worked example
Example: 12 V pushing 2 A
Known volts 12 and amps 2.
- Resistance: 12 / 2 = 6 Ω.
- Power: 12 x 2 = 24 W.
- Check with squares: 12²/6 = 144/6 = 24 W. Agrees.
- A 25 W resistor rating fits with a little margin.
Reference table
| Volts | Amps | Watts |
|---|---|---|
| 12 V | 0.12 A | 1.44 W |
| 24 V | 0.24 A | 5.76 W |
| 120 V | 1.2 A | 144 W |
| 230 V | 2.3 A | 529 W |
Limits
- Resistive DC and simple AC assumed. Reactance needs complex math.
- Resistors drift with heat, shifting real values.
- Mains AC uses RMS values, which this matches.
- Safety margins and codes sit outside the formulas.
Common mistakes
- Mixing milliamps with amps by factors of a thousand.
- Sizing resistors at exact watts with no margin.
- Forgetting LED forward voltage before the resistor.
- Measuring mains without rated probes and training.
- Assuming USB supplies hold voltage at full current.
Try it yourself
Quick checks. Answers sit tucked under each question.
9 V across 3 Ω. Current and power?
I = 9/3 = 3 A. P = 9 x 3 = 27 W.
2 A through 10 Ω. Volts and watts?
V = 2 x 10 = 20 V. P = 4 x 10 = 40 W.
Questions
What is Ohm's law in words?
Current equals voltage divided by resistance. More push or less opposition means more flow.
How do watts relate?
Power is volts times amps. At fixed resistance it also equals volts squared over ohms.
AC or DC?
Both for resistive loads with RMS values. Motors and capacitors need more theory.
Why do resistors get hot?
They convert electrical power to heat by design. Wattage ratings mark safe limits.
What kills in shocks?
Current through the body, aided by voltage and wet skin. Treat all unknown circuits as dangerous.
Why do cars use 12 V?
Safe to touch, enough for starters with thick cables, and standard worldwide.