Ohm's Law Calculator
Calculate voltage, current, resistance, or power using Ohm's Law. Enter any two values to find the other two instantly.
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How to use this calculator
V = voltage (volts), I = current (amperes), R = resistance (ohms), P = power (watts). Enter any two values; the other two are derived.
- 1
Enter two of the three values: voltage in volts, current in amperes, resistance in ohms.
- 2
Leave the unknown one at 0, or empty the box, since empty counts as 0.
- 3
Convert milliamps and kilohms to base units before typing them.
- 4
Read the solved value and the power in watts.
Frequently asked questions
What is Ohm’s law?
The voltage across a resistor equals the current through it times its resistance: V = I × R. Rearranged, I = V ÷ R and R = V ÷ I, so any two of the three give the third. The tool applies these three rearrangements.
How is power calculated?
The tool computes P = V × I after the missing value is filled in. That equals I² × R and V² ÷ R as well. For 3 V and 0.02 A the power is 0.06 W, and for 12 V across 470 Ω it is 0.3064 W.
How do I size an LED resistor?
Subtract the LED’s forward voltage from the supply and divide by the current you want. A 5 V supply, a 2 V LED and 20 mA give (5 − 2) ÷ 0.02 = 150 Ω. Entering 3 V and 0.02 A with resistance at 0 confirms 150 Ω and 0.06 W of heat in the resistor.
Why must one field be zero?
A field at 0 is treated as the unknown. With exactly one zero the tool solves for it. With two or three zeros it shows “Enter at least two values”. An emptied box is read as 0, so clearing a box marks it unknown.
What if I fill in all three fields?
The tool solves nothing and does not check that they agree. It keeps your V, I and R as typed and shows power as V × I. Entering 12 V, 2 A and 10 Ω prints 24 W alongside 10 Ω, even though 12 V at 2 A implies 6 Ω.
What units are expected, and how are results shown?
Volts, amperes, ohms and watts, with no prefix conversion: 25 mA is 0.025, 4.7 kΩ is 4700. Values of 1 or more show four decimals; values below 1 show four significant digits, so 5 V across 1 MΩ gives 0.000005000 A instead of 0.0000.
Does it work for AC or non-ohmic parts?
Only for resistive loads using RMS values. Coils and capacitors need impedance and power factor, which are not inputs here. Diodes, LEDs and filament lamps change resistance with voltage and heat, so V ÷ I describes one operating point only.
Can a result be negative?
Yes. Signs are carried through, so −12 V across 4 Ω gives −3 A while power stays positive at 36 W. The tool does not reject negative inputs or a negative resistance.
Ohm’s law calculator — voltage, current, resistance and power
The formulas the tool applies
The core relationship is V = I × R, with I = V ÷ R and R = V ÷ I. Power follows from P = V × I, also written I² × R and V² ÷ R. Type two of the three quantities and the tool fills in the third, then prints all four lines. The table lists which expression serves each pair. A 6 Ω load on 12 V, for instance, draws 2 A and dissipates 24 W.
| To find | Know V and I | Know V and R | Know I and R |
|---|---|---|---|
| Voltage V | — | — | I × R |
| Current I | — | V ÷ R | — |
| Resistance R | V ÷ I | — | — |
| Power P | V × I | V² ÷ R | I² × R |
How the zero convention works
The tool counts how many boxes are non-zero. A zero marks the unknown, and an emptied box also reads as zero. Fewer than two non-zero entries produce an error message. If all three are filled, nothing is recalculated: the values stay as typed and power is V × I, so inconsistent inputs are not flagged. Signs are carried through: a negative voltage over a positive resistance gives a negative current, while power stays positive because the two negatives cancel. A true zero, such as a 0 Ω short, cannot be entered.
Five runs of the calculator
Each case was run through the tool, and the results are exactly as it prints them.
| Situation | Inputs | Result |
|---|---|---|
| LED resistor, 5 V supply, 2 V LED, 20 mA | 3 V, 0.02 A | 150 Ω, 0.06 W |
| 12 V across a 470 Ω resistor | 12 V, 470 Ω | 0.02553 A, 0.3064 W |
| 120 V heater element of 8 Ω | 120 V, 8 Ω | 15 A, 1800 W |
| 2 A flowing through 6 Ω | 2 A, 6 Ω | 12 V, 24 W |
| 5 V across 1 MΩ | 5 V, 1000000 Ω | 0.000005000 A, 0.00002500 W |
Display format and unit prefixes
The tool never converts prefixes, so enter base units: 25 mA as 0.025 and 2 MΩ as 2000000. Results of 1 or more show four decimals, such as 1800.0000 W. Results below 1 show four significant digits, which stops microamp values from collapsing to 0.0000. That significant-digit rule is a correction; the earlier four-decimal format hid currents below 0.00005 A. Resistance is formatted the same way, so 5 V with 0.0003 A returns 16666.6667 Ω.
Where the formulas stop applying
Ohm’s law holds for components whose resistance stays constant. Diodes, LEDs and filament lamps change resistance with voltage and temperature, and AC circuits with coils or capacitors need impedance and power factor. A resistor’s power rating should exceed the dissipation shown here with some margin, so the 0.3064 W case calls for more than a quarter-watt part. Treat results for non-ohmic parts as one operating point and check the datasheet. High-wattage results, such as the 1800 W heater case, are a reminder that parts and wiring must be rated for the dissipation, since the tool only does the arithmetic.
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