About the Handiwork Ohm's Law Calculator
The Ohm's Law Calculator solves for voltage, current, resistance, and power from any two known values using V = I × R and the three power formulas (P = VI, P = I²R, P = V² ÷ R). Enter two fields and the other two fill in automatically; enter three or four to double-check that a set of measurements or a datasheet spec is internally consistent.
How to use the Handiwork Ohm's Law Calculator
- Enter any two of voltage, current, resistance, or power in the fields above.
- The other two values are calculated automatically, along with the formula used.
- Optionally enter a third or fourth value to verify it agrees with the other two — the tool flags any mismatch.
Ohm's law and the power formulas
Ohm's law states that voltage equals current times resistance: V = I × R. Combined with the definition of electrical power, P = V × I, that single relationship expands into three equivalent power formulas: P = VI, P = I²R (from substituting V = IR), and P = V² ÷ R (from substituting I = V ÷ R). Knowing any two of voltage, current, resistance, or power is enough to derive the other two algebraically, which is exactly what this calculator automates.
A worked example
A 12 V battery driving a 6 Ω resistive load draws I = V ÷ R = 12 ÷ 6 = 2 A of current, and dissipates P = V × I = 12 × 2 = 24 W of power. Enter voltage and resistance into the calculator and it returns the same 2 A and 24 W, along with the formula it used.
When this applies — and when it does not
Ohm's law in this simple form (V = IR) holds for purely resistive, DC circuits — resistors, incandescent bulbs, heating elements, and most wiring. It does not directly apply to reactive AC loads (motors, transformers, capacitors, inductors) where impedance replaces resistance and a phase angle enters the power calculation, or to non-ohmic components like diodes and LEDs whose resistance changes with current.
Checking three or four known values
If you already know three or four of the quantities — for example, from a multimeter reading and a component's rated specs — enter all of them. The calculator derives the missing or overlapping values from the first two it reads and compares them against what you entered, flagging a mismatch if they disagree by more than a small rounding tolerance. This is useful for catching a misread label or a faulty measurement before you rely on it.
Continue this workflow
Use the adjacent tool when the next step calls for a different input, output, or method.
Assumptions and limitations
- Assumes a purely resistive DC circuit; AC circuits with reactance (capacitors, inductors, motors) need impedance and power-factor calculations instead of plain Ohm’s law.
- Does not model non-ohmic components (diodes, LEDs, transistors) whose resistance varies with voltage or current.
- Inputs are treated as non-negative magnitudes; the tool does not track current direction or voltage polarity.
- The consistency check for three or four entered values uses a small tolerance (about 0.5%) to allow for rounding, not exact equality.
Sources and standards
These authoritative references were used to verify the method and guidance on this page.
Frequently asked questions
What is Ohm's law?
Ohm's law states that the voltage across a resistive load equals the current through it times its resistance: V = I × R. Rearranged, I = V ÷ R and R = V ÷ I.
How do I calculate power from volts and amps?
Power in watts equals voltage times current: P = V × I. If you know current and resistance instead, use P = I²R; if you know voltage and resistance, use P = V² ÷ R.
Why do I only need to enter two values?
Voltage, current, resistance, and power are linked by V = IR and P = VI, which together form a system with two independent equations. Any two known quantities are enough to solve algebraically for the other two.
Does this work for AC circuits?
Only for purely resistive AC loads at a single instant. Circuits with capacitors, inductors, or motors have reactance in addition to resistance, so you need impedance (Z) and power-factor calculations rather than plain Ohm's law.
Why does the calculator say my values are inconsistent?
If you enter three or four values, the calculator checks whether they all satisfy V = IR and P = VI within a small rounding tolerance. A mismatch usually means one of the entered numbers is wrong — for example, a mislabeled resistor or a misread meter.