Ohm's Law Calculator
Enter any two known electrical values. The calculator works out voltage, current, resistance and power using Ohm's Law and standard resistive power formulas.
Your values
Choose exactly two known values, or use the example below.
Based on the two values entered. The calculator uses ideal DC/resistive relationships.
Formula used
I = V ÷ R
I = 12.0 V ÷ 6.00 Ω = 2.00 A
P = V² ÷ R
P = (12.0 V)² ÷ 6.00 Ω = 24.0 W
Calculation summary
| Quantity | Base SI value | Display value | Status |
|---|---|---|---|
| Voltage (V) | 12.0 V | 12.0 V | Entered |
| Current (I) | 2.00 A | 2.00 A | Calculated |
| Resistance (R) | 6.00 Ω | 6.00 Ω | Entered |
| Power (P) | 24.0 W | 24.0 W | Calculated |
Results are theoretical DC/resistive values. Check real measurements, component ratings, heat, wiring and safety requirements separately.
How to use this calculator
- Choose two of the four quantity cards — Voltage, Current, Resistance or Power — as your known values.
- Enter each known figure and pick its SI prefix, from micro (µ) up to mega (M).
- Select "Calculate" to work out the other two values.
- Review the formula used, the calculation summary table, and any safety-relevant notes.
- Select "Change inputs" to edit the values or choose a different pair, or "Clear all" to start over.
How is Ohm's Law calculated?
Ohm's Law relates voltage, current and resistance in an ideal DC/resistive circuit, and combines with the power formula to relate all four quantities. This calculator always uses the direct formula for the pair you entered, rather than solving through a rounded intermediate value.
- Voltage + current known: R = V ÷ I; P = V × I
- Voltage + resistance known: I = V ÷ R; P = V² ÷ R
- Voltage + power known: I = P ÷ V; R = V² ÷ P
- Current + resistance known: V = I × R; P = I² × R
- Current + power known: V = P ÷ I; R = P ÷ I²
- Resistance + power known: V = √(P × R); I = √(P ÷ R)
For example, with a voltage of 12 V and a resistance of 6 Ω known: current is I = V ÷ R = 12 V ÷ 6 Ω = 2 A, and power is P = V² ÷ R = (12 V)² ÷ 6 Ω = 24 W.
What do the SI prefixes mean?
Each value can be entered with a prefix, so a reading from a component datasheet or a multimeter does not need converting by hand first. Every prefix is converted to the base unit — volts, amps, ohms or watts — before any formula runs.
| Prefix | Symbol | Multiplier |
|---|---|---|
| micro | µ | × 0.000001 (10⁻⁶) |
| milli | m | × 0.001 (10⁻³) |
| — (base unit) | — | × 1 |
| kilo | k | × 1,000 (10³) |
| mega | M | × 1,000,000 (10⁶) |
Lower-case m (milli, one thousandth) and upper-case M (mega, one million) are always kept distinct in this calculator — 5 mA and 5 MA differ by a factor of ten billion, so the two are never merged or guessed from a careless typo.
Formula reference
| Quantity | Formulas |
|---|---|
| Voltage (V) | V = I × R; V = P ÷ I; V = √(P × R) |
| Current (I) | I = V ÷ R; I = P ÷ V; I = √(P ÷ R) |
| Resistance (R) | R = V ÷ I; R = V² ÷ P; R = P ÷ I² |
| Power (P) | P = V × I; P = I² × R; P = V² ÷ R |
What does this calculator assume, and what does it not do?
This calculator assumes an ideal, purely resistive DC circuit: a fixed resistance, a constant voltage and current, and no reactive (inductive or capacitive) component. It does not model AC voltage, RMS or peak values, power factor, impedance, phase, three-phase supplies, temperature effects on resistance, or component tolerances. It does not make a wiring, fuse, battery or component-rating decision, and a result from it should never be used alone to make one — those decisions need a real measurement, the manufacturer's rated limits, and the relevant electrical safety standard.