Ohm's Law Calculator
Enter any two of voltage, current, resistance, and power, and the page fills in the other two.
🔒 Processed entirely in your browser — nothing you enter here is ever uploaded.
The result appears on this page after you run the tool. Nothing you enter is uploaded.
Result
About this tool
The form has four numbers: voltage in volts, current with a unit of amps, milliamps, or microamps, resistance with a unit of ohms, kilohms, or megohms, and power with a unit of watts, milliwatts, or microwatts. A value counts as known only when it is strictly positive. You need at least two known values. One number, or a box left blank, is not enough, and the page throws the two-values error. Zero is not treated as a known value, because the positive check rejects it. Negative numbers are rejected the same way.
The solver fills the gaps in a fixed order. If voltage and current are both known, resistance becomes V/I. If voltage and resistance are known, current becomes V/R. If current and resistance are known, voltage becomes I×R. If power and voltage are known, current becomes P/V. If power and current are known, voltage becomes P/I. If power and resistance are known, current becomes the square root of P/R and voltage becomes the square root of P×R. After those steps, a missing power is set to V×I, and a missing resistance is set to V/I. If any of the four is still not positive, the same two-values error is thrown. The report then prints all four with the engineering formatter.
A 12 volt source and a 4 ohm resistor are a clean check. Current is 12/4 = 3 amps, and power is 12×3 = 36 watts. The resistance field defaults to kilohms, so the number 4 in that field with the kilohm unit still selected is 4000 ohms, not 4 ohms. Switch the unit to ohms before you type 4 if you want the 3 amp result. Another check: 9 volts and 20 milliamps. Twenty with the milliamp unit is 0.02 amps, resistance is 9/0.02 = 450 ohms, and power is 9×0.02 = 0.18 watts, which prints as 180 mW. The arithmetic is DC and ideal. It does not know about reactance, temperature rise, resistor tolerance, or whether the power exceeds the part you actually have. If you fill in three or four boxes, later rules can overwrite an earlier result. Power-and-resistance runs after the V-and-I rules, so a power entry changes the current and the voltage even when those were also filled. The practical habit is to type the two quantities you measured and leave the other boxes empty. The formatter prints 3000 ohms as 3 kΩ and 0.02 amps as 20 mA.
Example. Voltage 12 V and resistance 4 Ω return current 3 A and power 36 W. Voltage 9 V and current 20 mA return resistance 450 Ω and power 180 mW.
Limit. At least two of voltage, current, resistance, and power must be strictly positive. A later power-and-resistance step overwrites current and voltage when power is also filled in.
What do 12 volts and 4 ohms give?
Current is 12/4 = 3 A and power is 12×3 = 36 W. The resistance unit menu defaults to kilohms, so type 4 only after you switch that menu to ohms. Otherwise 4 is read as 4 kΩ.
Can I start from power and resistance?
Yes. Current becomes the square root of power divided by resistance, and voltage becomes the square root of power times resistance. Power of 36 W and resistance of 4 Ω return 3 A and 12 V. You still need two positive values. One box is rejected.
What if I fill in more than two boxes?
The solver applies its rules in order, and the power rules run after the voltage and current rules. A power value will replace the current or the voltage those earlier rules just computed. Leave the unknowns empty.
Which units are multiplied in?
Current is amps, milliamps (×0.001), or microamps (×0.000001). Resistance is ohms, kilohms (×1000), or megohms (×1000000). Power is watts, milliwatts, or microwatts. Voltage is plain volts. The report prints engineering prefixes, so 0.18 W is shown as 180 mW.