Calculated
Ohm's Law Calculator
Sign up to favourite this calculatorEnter 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.
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Assumptions and limits
- An estimate from the numbers you entered, for guidance only. It is not financial, tax or medical advice.
- Calculator version 1, last updated 25 October 2026.
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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.
What do I need to enter?
- Two known quantities — pick exactly two of Voltage (V), Current (I), Resistance (R) and Power (P). The other two are calculated.
- Value — a number above zero and up to 1,000,000, before its prefix. Blank, zero and negative entries are rejected.
- Unit prefix — micro (µ), milli (m), the base unit, kilo (k) or mega (M), chosen separately for each known quantity.
What do the results show?
- The two calculated quantities — shown on the cards with a practical prefix and three significant figures. Values outside micro to mega appear in scientific notation.
- Formula used — the two formulas applied to your pair, with your values substituted in.
- Calculation summary — a table of all four quantities with the exact base SI value, the display value, and whether each was entered or calculated.
- Notes — a reminder that the result assumes an ideal DC or resistive circuit, plus any warning the values trigger.
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.
V = I × R
- 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.
Frequently asked questions
What is Ohm's Law?
Ohm's Law states that the voltage across a resistive DC circuit equals the current through it multiplied by its resistance: V = I × R. From that one relationship, current is I = V ÷ R and resistance is R = V ÷ I — this calculator solves whichever of the two you have not entered.
How do I calculate power in a DC circuit?
Power is P = V × I. Because V = I × R, that same power can also be written as P = I² × R or P = V² ÷ R — three equivalent formulas for the same figure. This calculator picks whichever formula uses the two values you entered and shows the substitution.
Can I enter any two of the four values?
Yes. Enter any two of voltage, current, resistance and power and the calculator works out the other two, using the direct formula for that pair rather than solving in stages through a rounded intermediate result.
What is the difference between volts, amps, ohms and watts?
Volts (V) measure voltage, the electrical push driving current around a circuit. Amps (A) measure current, the rate charge flows. Ohms (Ω) measure resistance, how much a component opposes that flow. Watts (W) measure power, the rate energy is delivered or dissipated — the product of voltage and current.
Does this calculator work for AC or DC circuits?
DC only. It uses the ideal, resistive form of Ohm's Law — a straight multiply or divide between voltage, current and resistance. An AC circuit needs RMS or peak values, power factor and impedance (resistance plus reactance), none of which this calculator models.
Why are the units the same for UK and US readers?
Volts, amps, ohms and watts are the same SI units worldwide — unlike length, weight or fuel volume, there is no separate US customary electrical unit to convert between. That is why this calculator has no Metric/Imperial switch: the only choice is the SI prefix (micro, milli, kilo, mega) attached to each unit.
Can I use this to choose a resistor, cable or fuse?
No. This calculator only does the ideal DC arithmetic on the figures you enter — it does not account for component tolerances, temperature effects, cable derating, ambient conditions or the safety margins a real wiring, fuse, battery or component-rating decision needs. Use it to understand the relationship between values, not to make that decision.
Why does changing mA to A (or kΩ to Ω) change the answer?
It shouldn't change the real quantity, only how it's typed. Every value is converted to its base SI unit (volts, amps, ohms or watts) before it's used in a formula, so 500 mA and 0.5 A calculate identically. If switching a prefix seems to change the result, check the number was adjusted to match — 500 in milliamps is not the same current as 500 in amps.
Last updated 25 October 2026.