Ohm’s Law Calculator
Volts, amps, ohms and watts: pick any two and get the other two, the nearest standard resistor with its color bands, and a safe wattage. LED resistor mode included.
By Manzoor Ahmad, pharmacist · Updated What is the Ohm’s law formula?
V = I × R: voltage equals current times resistance. Rearranged, I = V ÷ R and R = V ÷ I, and power is P = V × I. A 12 V supply pushing 2 A through a load means R = 12 ÷ 2 = 6 Ω and P = 12 × 2 = 24 W.
Tap the letter tiles to choose the two values you know.
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All 12 Ohm’s law and power formulas
Combine V = IR with P = VI and every quantity can be written three ways. The wheel in the calculator lights up the two formulas used for your inputs.
| To find | From V and I / R | From power | Third form |
|---|---|---|---|
| Voltage V (volts) | V = I × R | V = P ÷ I | V = √(P × R) |
| Current I (amps) | I = V ÷ R | I = P ÷ V | I = √(P ÷ R) |
| Resistance R (ohms) | R = V ÷ I | R = V² ÷ P | R = P ÷ I² |
| Power P (watts) | P = V × I | P = I² × R | P = V² ÷ R |
Worked examples
- Kettle: 2,000 W on 230 V draws I = 2000 ÷ 230 = 8.70 A, so its element is R = 230² ÷ 2000 = 26.5 Ω.
- Sensor pull-up: 3.3 V across 10 kΩ passes 0.33 mA and dissipates only 1.09 mW.
- Car headlight: a 55 W bulb on 12 V draws 4.58 A; its hot filament is about 2.6 Ω.
Resistor color code (4-band)
Bands 1 and 2 are digits, band 3 is the multiplier, band 4 is tolerance (gold ±5%, silver ±10%).
| Color | Digit | Multiplier |
|---|---|---|
| Black | 0 | ×1 |
| Brown | 1 | ×10 |
| Red | 2 | ×100 |
| Orange | 3 | ×1 k |
| Yellow | 4 | ×10 k |
| Green | 5 | ×100 k |
| Blue | 6 | ×1 M |
| Violet | 7 | ×10 M |
| Grey | 8 | ×100 M |
| White | 9 | ×1 G |
| Gold | – | ×0.1 |
| Silver | – | ×0.01 |
Ohm’s law FAQs
Ohm's law states that the current through a conductor is proportional to the voltage across it: V = I × R, where V is voltage in volts, I is current in amperes and R is resistance in ohms.
Divide voltage by resistance: I = V ÷ R. A 12 V supply across a 6 Ω resistor drives 12 ÷ 6 = 2 A.
Divide voltage by current: R = V ÷ I. If 9 V produces 30 mA, R = 9 ÷ 0.030 = 300 Ω. From power, R = V² ÷ P or R = P ÷ I².
Multiply voltage by current: P = V × I. Equivalent forms are P = I²R and P = V² ÷ R. A 2 A current at 12 V dissipates 24 W.
It is a memory aid with V at the top and I and R at the bottom. Cover the value you want: V = I × R, I = V ÷ R, R = V ÷ I. The Ohm's law wheel adds power, giving 12 formulas.
Use R = (supply voltage − LED forward voltage) ÷ LED current. For a red LED (2.0 V) at 20 mA on 5 V: R = (5 − 2) ÷ 0.020 = 150 Ω. Round up to the next standard value.
Calculate P = I²R, then choose a resistor rated at least twice that. A resistor dissipating 0.06 W fits a 1/8 W or 1/4 W part; one dissipating 0.3 W needs a 1 W part for a full 2× safety margin.
They are standard series of preferred values. E12 has 12 values per decade (10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82) for 10% tolerance; E24 has 24 values for 5% tolerance.
On a 4-band resistor, the first two bands are digits, the third is the multiplier and the fourth is tolerance. Yellow-violet-brown-gold is 47 × 10 = 470 Ω, ±5%.
No. It holds for ohmic conductors such as metal resistors at constant temperature. Diodes, LEDs, transistors and filament bulbs are non-ohmic: their resistance changes with voltage or temperature.
For purely resistive AC loads, yes, using RMS voltage and current. With capacitors or inductors you replace resistance with impedance, Z, and power depends on the power factor.
One ohm is the resistance that lets one ampere flow when one volt is applied. It equals one volt per ampere (V/A).
Sources
For learning and low-voltage hobby circuits. Mains voltage can kill: leave mains wiring to a qualified electrician. LED forward voltages vary by part, so check the datasheet.