V, I, R & power from any two

Ohm's Law Calculator: voltage, current, resistance, and power

Q: How many values do I need to enter?

The solver runs only when exactly two of the four fields hold valid numbers; leave the other two blank. Enter a third value and the Results panel clears until you are back to two inputs.

Q: What does the Work shown panel display?

After a valid pair is entered, Work shown renders the two formulas used (in KaTeX) and one substitution line with your values, units, and rounding. It updates when inputs, prefixes, or decimal places change.

Q: What happens when resistance is zero?

With R = 0 and a positive voltage (V + R inputs) or positive power (R + P inputs), the tool treats it as a short circuit: current and power show ∞, current is tagged (short) in the Results grid, and Work shown states that R = 0 drives current toward infinity.

Q: Does Ohm's law work on AC circuits?

Purely resistive AC loads can use the same formulas with RMS voltage and current. This page does not compute impedance (Z), reactance, or phase for inductors and capacitors.

Q: What inputs are rejected?

Only non-negative numbers parse; negatives and blanks are ignored as inputs. Placeholders (12, 0.5, 24, 6) are hints only. V = 0 and I = 0 together leave resistance undefined (the widget may show NaN). Clear wipes all four fields.

This calculator finds the two missing values among voltage (V), current (I), resistance (R), and power (P) when you enter any other pair, using V = I × R and P = V × I with SI prefixes (kV, mA, kΩ, mW) and a Work shown panel for the algebra. It does not model AC impedance or non-Ohmic devices such as bare diodes. Everything runs locally in your browser.

By Jeff Beem

Updated July 6, 2026

Circuit parameters

Enter any two values; the other two are calculated automatically. Use the unit dropdowns for SI prefixes.

Voltage (V)

Current (I)

Resistance (R)

Power (P)

Decimals

Results

Enter any two values

Voltage, current, resistance, or power, the remaining two are calculated.

Ohm's law wheel

Information hub

What Ohm’s law is

In 1827, Georg Ohm published the relationship after years of experiments with metal wires and voltaic cells. He found current through a conductor scaled with the potential difference across it and inversely with a property he named resistance. Critics at the time dismissed it; it’s now the backbone of circuit design.

The “pressure and flow” analogy fits well: voltage is electrical pressure, resistance is friction, and current is flow. Higher voltage pushes more charge; higher resistance chokes it. That mental model helps when sizing resistors or troubleshooting circuits.

The variables

Voltage (V): Electromotive force, the push that drives charge. Units: volts. In $V = I \times R$ it’s the “pressure” that sets current for a given resistance.
Current (I): Charge flow rate through a conductor. Units: amperes. One amp is one coulomb per second passing a point. Higher current means more electrons moving.
Resistance (R): Opposition to current. Units: ohms (Ω). Wires, resistors, and loads all contribute. Thinner or longer wires raise R; copper beats steel for the same gauge.
Power (P): Rate of energy conversion to heat, light, or motion. Units: watts. $P = V \times I$ , or $P = I^2 R$ and $P = V^2/R$ when resistance is known.

Real-world applications

LED resistor sizing
Typical LED: ~20 mA at 2 V. From 5 V: R = (5 − 2) / 0.02 = 150 Ω. Round up to the nearest standard value (150 or 220 Ω) to avoid frying the LED.
Home appliance amperage
1,200 W microwave on 120 V: I = 1200 / 120 = 10 A. Most circuits top out at 15–20 A. Stack too many high-wattage devices and you’ll trip the breaker.
Car battery diagnostics
A good 12 V battery has low internal resistance and delivers strong current to the starter. If voltage drops under load, internal R has climbed, often from age or sulfation.
Fuse and breaker sizing
Fuses protect wiring by limiting current. A short (R ≈ 0) sends current through the roof. Pick a fuse that opens before the wire overheats. Ohm’s law defines that boundary.

Ohmic vs non-ohmic · temperature

The law holds for Ohmic materials, resistance stays flat over voltage and current. Metals, carbon resistors, and most wires qualify. Diodes, LEDs, and filaments are non-Ohmic: their R shifts with V and I, so V = IR breaks down.

Temperature matters. Copper gains ~0.4% resistance per °C. A 25°C rise adds ~10% to R, enough to throw off precision work or high-power designs. NTC thermistors drop resistance as they heat; we use them for sensing and inrush limiting.

Reading voltage, current, resistance, and power

Solved values appear as read-only rows on the left and as Generated tiles in the dark Results panel; Work shown lists the algebra the solver used.

Example: V = 12 V, I = 0.5 A (widget placeholders)

Type 12 in Voltage (V) (unit V) and 0.5 in Current (I) (unit A). Leave resistance and power blank. The tool generates R = 24 Ω and P = 6 W. Work shown reads R = 12 V / 0.5 A = 24 Ω; P = 12 × 0.5 = 6 W, matching the placeholder hints in the empty form.

Formula wheel and Information hub

In the right column, the Ohm's law wheel image sits between the Results panel and Work shown and lists all twelve rearrangements of V, I, R, and P. The separate Information hub block on this page adds history, a variable glossary, application examples, and notes on Ohmic vs non-Ohmic behavior.

Ohm's law calculator: voltage, current, resistance, and power

Enter any two of V, I, R, or P with matching SI prefixes, then read the generated pair in the Results grid and the Work shown substitution line.

What this Ohm's law calculator returns

This Ohm's law calculator fills in whichever two of voltage (V), current (I), resistance (R), and power (P) you leave blank after a valid pair is entered. It handles all six input combinations and twelve formula rearrangements. For LED drops with known forward voltage, the LED resistor tool is a better fit.

Inputs:

Two non-negative values, each with an SI prefix dropdown.
Outputs:

The other two values in the left column and Results grid, plus Work shown.
Limits:

R = 0 with positive V or P flags a short circuit (∞ current and power). V = 0 and I = 0 together leave R undefined.

How the math works

Ohm's law relates voltage, current, and resistance; power is their product. Substituting

V = IR

into

P = VI

yields

P = I^2 R

and

P = V^2 / R

. The widget converts entries to base volts, amperes, ohms, and watts, applies the pair that matches your two inputs, then converts back to your chosen prefixes.

Controls on this page

Inputs sit in the left column; results and reference material sit on the right.

Circuit parameters:

Four rows (V, I, R, P). Generated rows become read-only with a bold outline.
Decimals / Clear:

Rounding selector (0–4, default 2) for every formatted number, plus a reset button.
Results:

Dark 2×2 summary with Generated tags on solved quantities.
Ohm's law wheel / Work shown:

Static formula diagram, then live KaTeX plus substitution after a valid pair is entered.
Information hub:

Optional glossary, applications, and pro tips on the same page.

FAQ

How many values do I need to enter?

The solver runs only when exactly two of the four fields hold valid numbers; leave the other two blank. Enter a third value and the Results panel clears until you are back to two inputs.

What does the Work shown panel display?

After a valid pair is entered, Work shown renders the two formulas used (in KaTeX) and one substitution line with your values, units, and rounding. It updates when inputs, prefixes, or decimal places change.

Which unit prefixes can I pick?

Voltage: V, kV, mV. Current: A, mA, µA. Resistance: Ω, kΩ, MΩ. Power: W, kW, mW. All math runs in base SI units first. Changing a prefix rescales the displayed number so the physical quantity stays the same, whether the field is one you typed or one the tool generated.

What happens when resistance is zero?

With R = 0 and a positive voltage (V + R inputs) or positive power (R + P inputs), the tool treats it as a short circuit: current and power show ∞, current is tagged (short) in the Results grid, and Work shown states that R = 0 drives current toward infinity.

Does Ohm's law work on AC circuits?

Purely resistive AC loads can use the same formulas with RMS voltage and current. This page does not compute impedance (Z), reactance, or phase for inductors and capacitors.

What inputs are rejected?

Only non-negative numbers parse; negatives and blanks are ignored as inputs. Placeholders (12, 0.5, 24, 6) are hints only. V = 0 and I = 0 together leave resistance undefined (the widget may show NaN). Clear wipes all four fields.

Electronics Reference Note

Educational Use: These tools use standard electrical formulas (e.g., Ohm's Law, NEC voltage drop) for learning, hobby projects, and general reference, not for licensed electrical work or safety-critical installations.

Verification Recommended: Wire sizing, voltage drop, and circuit design depend on local codes, ambient conditions, and load profiles. For real installations, consult a qualified electrician or engineer.

Not Professional Advice: This site does not provide electrical or engineering advice. All calculations run locally in your browser; no data is stored or transmitted.