Ohms Calculator — Ohm's Law V=IR (Free Online)
Solve voltage, current, resistance, or power — enter any two values
Required Parameters
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Quick Answer
Ohm's law: V = I × R. Enter any two of voltage (V), current (I), and resistance (R) to find the third. Power: P = V × I = I²R = V²/R. This ohms calculator handles unit conversion automatically.
Ohms Calculator
Use this free ohms calculator to solve Ohm's law instantly: enter any two of voltage (V), current (I), and resistance (R) — the tool computes the third plus power (P) when relevant.
Quick start: Pick what you want to find (Voltage, Current, Resistance, or Power), enter the other two values, and read the result.
Ohms calculator formulas
| Find | Formula |
|---|---|
| Voltage | V = I × R |
| Current | I = V / R |
| Resistance | R = V / I |
| Power | P = V × I = I²R = V²/R |
The Ohm's law triangle
Cover the value you want:
- V covered → multiply I × R
- I covered → divide V ÷ R
- R covered → divide V ÷ I
Worked example
12 V supply, 470 Ω resistor:
- I = 12 / 470 = 25.5 mA
- P = 12 × 0.0255 = 0.31 W → use a 1/2 W resistor minimum
Unit tips
- mA → A: divide by 1,000 (250 mA = 0.25 A)
- kΩ → Ω: multiply by 1,000 (4.7 kΩ = 4700 Ω)
- Engineering shorthand: 4k7 = 4700 Ω, 10u = 10 µA
When Ohm's law does not apply
- LEDs and diodes — non-linear Vf, not constant R → LED resistor calculator
- AC with L or C — use impedance Z, not R → Reactance calculator
Related tools
- Ohm's Law Calculator — same engine, Ohm's law focused entry
- Voltage divider
- Parallel resistor calculator
- Electrical power calculator
Design Notes
Ohm's Law applies precisely to Ohmic materials where resistance remains constant regardless of current or voltage. In practical engineering, resistors heat up when dissipating power (P = I²R), which can alter their resistance (Temperature Coefficient of Resistance). Always verify that your calculated power dissipation is below the resistor's actual wattage rating (typically leave a 50% margin).
Common Mistakes
- 1
Mixing up units: Forgetting to convert milli-amps (mA) to Amps (A) before calculating.
- 2
Ignoring power dissipation: A 1/4W resistor will burn up if your calculation yields 0.5W, even if the resistance is correct.
- 3
Applying it to non-linear components: Ohm's law does not directly apply to diodes, LEDs, or transistors which have dynamic resistance.
Engineering Handbox
1. Identify knowns: V = 12, R = 470 2. Use Formula: I = V / R 3. Calculate: I = 12 / 470 = 0.02553 A (25.53 mA) 4. Calculate Power: P = 12 × 0.02553 = 0.306 W
Knowledge Base
What is an ohms calculator?
An ohms calculator solves Ohm's law relationships between voltage (volts), current (amps), and resistance (ohms). Enter any two known values to compute the third, plus power dissipation when relevant.
What is the ohms calculator formula?
V = I × R (voltage equals current times resistance). Rearranged: I = V/R and R = V/I. For power: P = V × I, P = I²R, or P = V²/R.
How is an ohms calculator different from Ohm's law calculator?
They solve the same physics — this page targets the search term 'ohms calculator' with the same V=IR engine. Use whichever entry point you prefer; both use identical math.
Can I use milliamps (mA) in the ohms calculator?
Yes. Enter values with units or engineering suffixes — e.g. 250 mA, 4k7 (4700 Ω), 12 V. The calculator converts to base SI units before applying V = IR.
Does the ohms calculator work for AC circuits?
Only for purely resistive AC loads. For capacitors or inductors, use impedance Z instead of R: V = I × Z. See our reactance calculator for Xc and XL at frequency.
Why doesn't Ohm's law work for LEDs?
LEDs are non-linear — they have a fixed forward voltage drop, not a constant resistance. Use our LED resistor calculator to size series resistors correctly.
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