Use the Ohm’s Law & DC Power Calculator
Choose two known values to calculate voltage, current, resistance and power for a resistive DC circuit. This guide explains the input pairs, units, live results and checks, using a single resistor as the example.
Start with one resistor
Imagine an ideal 470 Ω resistor with 12 V across it. The calculator can find the current through the resistor and the power it dissipates as heat. Enter the values directly in the calculator; there is no sample file to open.
Step by step
Open Ohm’s Law & DC Power Calculator. Under Known values, choose V + R: voltage and resistance.
Enter 12 for Voltage and select V. Enter 470 for Resistance and select Ω. In the schematic, V and R are known; I and P are the values to calculate.
Known: 12 V and 470 Ω
The source provides 12 V across the 470 Ω resistor. The arrow shows conventional current direction; its magnitude and the resistor’s power are still to be calculated. Read the live Your results; they update as you edit, without a calculate button. Current should show 25.5319 mA and Power should show 306.383 mW. The Formula section shows V = I × R and P = V × I with the calculated values.
12 V · 470 Ω
For this ideal resistor, I = 12 / 470 A and P = 12 × I W. The labels use the calculator’s displayed results; 306.383 mW is approximately 0.306383 W. Change the resistance entry to 0.47 and its unit to kΩ. Both changes are needed: 0.47 kΩ equals 470 Ω. Current and power should remain the same.
12 V · 470 Ω
12 V · 0.47 kΩ
The two resistance entries describe the same resistor, so the calculated current and power are identical. Choose V + I under Known values. The tool fills the new inputs from the previous valid calculation, including a more precise current than the result tile displays. Read the calculated resistance: it should be about 470 Ω. The input-pair table below explains all six choices.
Return to V + R, keep resistance at 470 Ω and change voltage to 24 V. Compare the new current and power with the 12 V case below. Current doubles and power increases fourfold because resistance stays constant. Use Print results when you want to print the result.
12 V · 470 Ω
24 V · 470 Ω
With the same 470 Ω resistor, increasing voltage from 12 V to 24 V changes current to 51.0638 mA and dissipated power to 1.22553 W.
Choose the two known values
V is voltage across the resistor, I is current through it, R is resistance and P is dissipated power. Each pair calculates the other two quantities; all four appear in the results.
| Known values | Other values calculated |
|---|---|
| V + I Voltage + Current | Resistance and Power |
| V + R Voltage + Resistance | Current and Power |
| V + P Voltage + Power | Current and Resistance |
| I + R Current + Resistance | Voltage and Power |
| I + P Current + Power | Voltage and Resistance |
| R + P Resistance + Power | Voltage and Current |
Changing the known pair changes the input fields. After a valid calculation, the tool prefills them from that result, using up to ten significant digits. If the previous input was invalid, it uses its default values for the new fields. Check the values and units before continuing.
Choose units or type a unit suffix
| Quantity | Available units |
|---|---|
| Voltage · V | mV, V, kV |
| Current · I | nA, µA, mA, A, kA |
| Resistance · R | mΩ, Ω, kΩ, MΩ, GΩ |
| Power · P | µW, mW, W, kW, MW |
The number and selected unit work together. Changing Ω to kΩ while leaving 470 in the input means 470,000 Ω; it does not convert the number to 0.47. To keep the same resistance, change both the number and unit as shown in step 4.
You can also type a value with a unit, such as 250 mA or 4.7 kΩ. A supported typed suffix takes precedence over the dropdown: 470 Ω is read as 470 Ω even if the dropdown says kΩ. Keep the number and units consistent. The walkthrough uses plain numbers with the dropdowns.
Results choose an SI prefix automatically and show up to six significant digits. For example, 0.3063829787 W is displayed as 306.383 mW. This changes the presentation, not the calculated quantity.
Print results or reset
| Control | What it does |
|---|---|
| Print results | Opens the browser’s print dialog for the calculator page. Printing is disabled while the inputs are invalid. |
| Reset | Returns to V + I with 12 V and 250 mA. Those defaults give 48 Ω and 3 W; Reset does not return to this guide’s 470 Ω example. |
Check the calculation
- For 12 V and 470 Ω, confirm 25.5319 mA and 306.383 mW. Changing to 0.47 kΩ should leave both results unchanged.
- Check the equations in base units: I = V / R = 12 / 470 A, and P = V × I = 144 / 470 W. Use amperes when multiplying volts by current to obtain watts.
- At 24 V and the same 470 Ω, confirm 51.0638 mA and 1.22553 W. The higher voltage increases both current and heating.
- For a reverse check, switch known pairs using the prefilled values. If you retype a rounded result, expect a small difference in the reconstructed value. Compare the input units as well as the numbers.
Questions and troubleshooting
Why did my result change by a factor of 1,000?
Check the input units and any typed suffix. At 12 V, 470 kΩ is 470,000 Ω and gives 25.5319 µA, not 25.5319 mA. Ω and kΩ differ by 1,000; mA and A also differ by 1,000. When changing units, adjust the number if you intend to keep the same physical value.
Why are the inputs highlighted and results blank?
Check that both inputs contain finite, valid numbers and supported units for the selected quantities. Negative inputs are rejected because the tool works with magnitudes. Correct the highlighted fields; the results update automatically.
Can I enter zero?
Resistance must be greater than zero. When resistance is known, zero voltage, current or power is supported and produces zero voltage, current and power while retaining the known resistance. Pairs without resistance require both inputs to be greater than zero.
Why does a reverse check give a slightly different resistance?
The result tiles are rounded to up to six significant digits. Retyping 25.5319 mA with 12 V gives a resistance close to 470 Ω, with a small rounding difference. Switching the known pair uses the previous calculated values with up to ten significant digits, which reduces that difference.
Does calculated power tell me which resistor to buy?
The power result is the heat dissipated by the ideal resistor in this calculation. It does not select a component or verify a datasheet rating. Compare it with the manufacturer’s permitted dissipation at your operating temperature and mounting conditions, including any derating requirements.
Can I use this example for an LED or reactive AC load?
This calculator models a resistive DC circuit using magnitudes. It does not include an LED’s forward voltage, AC phase angle, power factor or reactive impedance. For an LED circuit, use Choose a series resistor for an LED string; for a single series or parallel resistor group, use Combine resistors or capacitors in one series or parallel group. More complex circuits need a model that includes their other components.