12 V across a load drawing 2 A
R = V ÷ I
P = V × I
Resistance is 12 ÷ 2 = 6 Ω. Power is 12 × 2 = 24 W.
Electricity & circuits
Ohm's Law Calculator
Start with two known quantities to find voltage, current, resistance, and power for an ideal resistive DC load.
Calculate electrical quantities
Voltage, current, resistance & power
Enter two positive values, then select Calculate.
- Voltage (V)
- Current (I)
- Resistance (R)
- Power (P)
Calculation steps
≈ marks display rounding. Calculations run in your browser. Press Enter in an input to calculate. Successful results appear in this tab's Recent calculations.
Formulas and worked examples
5 V across 2 kΩ
I = V ÷ R
P = V² ÷ R
Convert 2 kΩ to 2,000 Ω. Current is 5 ÷ 2,000 = 0.0025 A (2.5 mA); power is 25 ÷ 2,000 = 0.0125 W (12.5 mW).
8 Ω dissipating 2 W
V = √(P × R)
I = √(P ÷ R)
Voltage is √16 = 4 V. Current is √0.25 = 0.5 A. The positive roots match this tool's magnitude convention.
| Known | First unknown | Second unknown |
|---|---|---|
| V and I | R = V ÷ I | P = V × I |
| V and R | I = V ÷ R | P = V² ÷ R |
| V and P | I = P ÷ V | R = V² ÷ P |
| I and R | V = I × R | P = I² × R |
| I and P | V = P ÷ I | R = P ÷ I² |
| R and P | V = √(P × R) | I = √(P ÷ R) |
Model, units, and rounding
The model combines V = IR with P = VI for an ideal ohmic load. This tool is limited to positive DC magnitudes with constant resistance. It does not model temperature changes, reactive components, AC phase or power factor, or the behavior of nonohmic devices such as diodes.
Inputs must be greater than zero and can contain at most 64 characters, 15 digits before the decimal point, and 10 digits after it. Expressions, e notation, and unit symbols inside numeric fields are not accepted. Choose the unit in its separate selector. Only the two quantities selected in the mode are used.
Prefixes convert exactly: milli is 1/1,000; micro is 1/1,000,000; kilo is 1,000; mega is 1,000,000. Results always use volts, amperes, ohms, and watts, even when the inputs use other prefixes.
Input fractions and unit conversions stay exact. Square-root results retain their exact radical until display. Values round only for display, with halfway cases rounded away from zero. The ≈ symbol marks a changed value. If a positive result rounds to zero, a notice appears; try more decimal places. Precision changes do not add a recent-calculation entry.
The calculated power is the ideal load's dissipation at the entered operating point. It is not a component power-rating recommendation: this calculation does not include tolerances, thermal limits, transient conditions, or a design margin.
Physics references: OpenStax — Ohm's law and OpenStax — electrical energy and power.
Common questions
Why can I enter only two values?
Two independent positive quantities determine the other two in this model. For example, voltage and current determine both resistance and power. Limiting input to a selected pair avoids silently accepting three or four measurements that disagree.
Why are zero and negative inputs rejected?
This calculator deliberately uses strictly positive magnitudes. Zero-voltage, zero-current, open-circuit, and short-circuit cases need separate treatment and may not determine a unique answer from every pair. Negative voltage or current can describe circuit direction, but signed analysis is outside this tool's scope.
Can I use this for AC?
This tool models DC. Ohm's law can also apply to an ideal pure resistor under AC, but a general AC calculation may require impedance, phase, RMS values, and power factor. Those quantities are not modeled here.
Does 0.000000 A mean there is no current?
Not if it is marked ≈. It means a positive current is too small to display at the chosen number of decimal places. The unrounded value is retained, and the other outputs are computed from the original inputs.