Electrical & Electronics
Ohm’s Law & DC Power Calculator
Calculate voltage, current, resistance and power from two known values, with a live circuit diagram.
Your inputs
Use positive values. Zero voltage, current or power is supported when resistance is known. You can also type units, such as 250 mA or 4.7 kΩ.
Your results
- Voltage · V
- —
- Current · I
- —
- Resistance · R
- —
- Power · P
- —
V = I × R
P = V × IFor resistive DC circuits. Values are magnitudes. Component ratings and AC power factor are not included.
Calculate voltage, current, resistance and DC power
Use two known values to calculate the other quantities for an ideal resistive DC load. Choose voltage V, current I, resistance R or power P. All six distinct pairs are supported. This model does not account for AC reactance, power factor, temperature or component tolerances.
Step by step
- Choose which two quantities you know. Enter their values and select units; a recognized unit suffix such as 250 mA overrides the dropdown.
- Review all four results, the formula and circuit diagram. Results update as you type and use automatic SI prefixes; verify the units as well as the numbers.
- Print the calculation if needed. Treat calculated power as load dissipation, then consult the actual component's ratings and operating conditions separately.
Settings and limits
- Units and precision
- Inputs support prefixes from small units such as mV and µA to kV and MΩ. Numbers follow the page language. Results show up to six significant digits; calculations use ordinary floating-point arithmetic.
- Formula
- The base relations are V = I × R and P = V × I. The calculator rearranges them for your selected pair. For resistance and power, I = √(P/R) and V = √(P × R).
- Valid values
- Use nonnegative magnitudes and strictly positive resistance. Zero voltage, current or power is supported when resistance is known. Without known resistance, both inputs must be positive to determine a unique result.
Worked example
Choose voltage and resistance as the known quantities. Enter 12 V and 48 Ω. Check the results against the table below.
| V | R | I | P |
|---|---|---|---|
| 12 V | 48 Ω | 250 mA | 3 W |
I = 12/48 = 0.25 A, displayed as 250 mA. P = 12 × 0.25 = 3 W. That is calculated dissipation, not a recommendation to use a component with a 3 W rating.
Questions and troubleshooting
Why is the displayed number different after changing units?
The calculator may express the same quantity with another SI prefix: 0.25 A and 250 mA are equal. An explicit suffix in the typed value takes precedence over the selected unit.
Can I use this for a motor or mains AC circuit?
The simple resistive DC model does not include motor behavior or AC power factor. Use Watts to Amps & AC Power for the supported AC calculations and verify equipment-specific assumptions.
Why are the results blank?
Invalid, negative or ambiguous zero inputs clear the results and disable printing. Correct the value or unit, choose a suitable known pair, or reset the calculator.