Ohm's Law Calculator
Solve for voltage, current, resistance, or power when you know two values.
Ohm's Law Calculator
Ohm's law relates voltage, current, and resistance in electrical circuits: voltage pushes current through resistance. Power (watts) links to those three as P = V × I. Given any two of the four quantities, you can solve for the others.
This calculator lets you choose what to solve—voltage, current, resistance, or power—and enter the known values. It applies the standard DC Ohm's law formulas, including alternate forms like P = V²/R and P = I²R when those pairs are what you have.
Use it for quick homework checks, bench troubleshooting, and understanding how changing resistance or voltage affects current in DC loads. For AC circuits with motors, also consider power factor via the watts/amps calculators.
How this calculator works
- Select solve for: voltage (V), current (I), resistance (R), or power (P).
- Enter the two (or more) known values for the other variables—leave the unknown at 0 or empty as appropriate.
- Core relation: V = I × R (Ohm's law).
- Power relation: P = V × I, also P = V²/R and P = I²R.
- The tool picks the correct rearrangement based on what you are solving and which inputs you provided.
Core Ohm's law relationships
V = I × R | P = V × I | P = V² ÷ R | P = I² × R
Voltage in volts equals current in amps times resistance in ohms. Power in watts is voltage times current. When you know voltage and resistance without current, P = V²/R. When you know current and resistance without voltage, P = I²R. These are equivalent for resistive DC circuits.
Formulas used by this tool
- Core relations: V = I × R and P = V × I.
- Pick what to solve; enter the other known values.
- Missing values are derived from standard Ohm's law formulas.
Worked example: find current
A 12 V DC load connects through a 4 Ω resistance (solve for current).
- Select solve for: current (I).
- Enter V = 12 and R = 4.
- I = V ÷ R = 12 ÷ 4 = 3 A.
- Power P = V × I = 12 × 3 = 36 W (also P = I²R = 9 × 4 = 36 W).
Practical tips
- Ohm's law in this form assumes DC or resistive AC—motors and transformers need power factor.
- Resistance includes wire, connectors, and load— not just a discrete resistor component.
- Doubling voltage doubles current if resistance stays fixed (P quadruples).
- Use consistent units: volts, amps, ohms, watts—milliamps require converting to amps.
- Zero resistance is a short circuit in theory; real wires always have some R.
Frequently asked questions
What is Ohm's law?
Ohm's law states V = I × R: voltage across a conductor equals current through it times resistance. It is the fundamental relationship for DC and resistive circuits. Georg Ohm defined it for metallic conductors at constant temperature.
How do I solve for each variable?
Voltage: V = I × R, or V = P/I, or V = √(P × R). Current: I = V/R, or I = P/V, or I = √(P/R). Resistance: R = V/I, or R = P/I², or R = V²/P. Power: P = V × I, or P = V²/R, or P = I²R. This calculator selects the right form from your inputs.
Does Ohm's law apply to solar and batteries?
Yes for DC portions: battery to load, MPPT to battery, within resistive elements. Battery internal resistance and cable drop follow V = I × R. Whole-system behavior also includes chemistry and controllers beyond pure Ohm's law.
What is the difference between V, I, R, and P?
V (volts) is electrical pressure. I (amps) is current flow. R (ohms) opposes flow. P (watts) is power—the rate energy is used or delivered. P = V × I ties power to the other three.
Can resistance change?
Yes. Many loads are not fixed resistors—LEDs, motors, and heaters change effective resistance as they warm up or speed up. Ohm's law still holds instantaneously; the R you calculate is an equivalent value at that operating point.
How is this related to watts-to-amps?
Watts-to-amps is P and V (with optional PF) → I. Ohm's law is the fuller framework: any two of V, I, R, P determine the rest. Use watts-to-amps for quick AC/DC conversions; use Ohm's law when resistance is known or you need all four quantities.
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