Enter any two of Voltage, Current, Resistance or Power — get all four instantly, with unit prefixes and a power-rating check.
| Find | Formula | Given |
|---|---|---|
| Voltage (V) | V = I × R | Current, Resistance |
| Voltage (V) | V = P / I | Power, Current |
| Voltage (V) | V = √(P × R) | Power, Resistance |
| Current (I) | I = V / R | Voltage, Resistance |
| Current (I) | I = P / V | Power, Voltage |
| Current (I) | I = √(P / R) | Power, Resistance |
| Resistance (R) | R = V / I | Voltage, Current |
| Resistance (R) | R = V² / P | Voltage, Power |
| Resistance (R) | R = P / I² | Power, Current |
| Power (W) | P = V × I | Voltage, Current |
| Power (W) | P = I² × R | Current, Resistance |
| Power (W) | P = V² / R | Voltage, Resistance |
Enter any two values. You can use any combination of Voltage, Current, Resistance and Power. The calculator solves all 12 possible formula combinations automatically — you don't need to pick which formula to use.
Pick the right unit prefix. Working with milliamps? Choose mA. Dealing with a megaohm bleed resistor? Switch to MΩ. The dropdowns convert everything to base units (V, A, Ω, W) before calculating.
Read the four results. Voltage in V, Current in A, Resistance in Ω and Power in W are displayed with automatic prefix scaling (e.g. 0.02 A is shown as 20 mA). Hit Copy to grab any value.
Check your resistor rating. Once power is known, the power-rating checker appears. Choose a standard wattage (⅛ W, ¼ W, etc.) to instantly see if your resistor can handle the dissipation — with a visual bar that turns red when over-spec.
Ohm's Law states that the voltage (V) across a conductor is directly proportional to the current (I) flowing through it, with resistance (R) as the constant of proportionality: V = I × R. Published by Georg Simon Ohm in 1827, it's the foundation of circuit analysis. Combine it with the power formula P = V × I to get the full "power wheel" that this calculator implements.
Yes — this calculator lets you use any two of the four quantities (V, I, R, P) as inputs. For example entering Power = 1 W and Resistance = 100 Ω yields I = √(P/R) = 100 mA and V = 10 V. The formula used is shown below the Calculate button so you always know which path the solver took.
Use the unit dropdown next to each field. Type 20 in the Current box and choose mA from its dropdown; type 4.7 in Resistance and choose kΩ. The calculator converts to base SI units (amps, ohms) before solving, so you never need to manually shift decimals.
A safe rule of thumb is to choose a resistor rated at least 2× the calculated power dissipation. For example, if your resistor dissipates 0.1 W, pick a ¼ W (0.25 W) part at minimum — but ½ W or 1 W gives a comfortable thermal margin. The power-rating bar turns amber above 50 % load and red above 80 % load.
When a value is very small (below 0.001) or very large (above 999,999), the calculator displays it in engineering-style prefix notation (e.g. 4.7 µA) or falls back to scientific notation (4.7e-9). This keeps the display readable across the full range from picoampere signals to kilowatt power supplies.