---

Ohm’s Law Calculator: The Full V, I, R, P Wheel, Series/Parallel and Wire Resistance

Ohm’s law looks simple on paper, V equals I times R, but the moment you bring power into the picture, there are actually twelve different formulas connecting these four quantities, and it is easy to grab the wrong one. This tool solves the whole wheel for you. Fill in any two of voltage, current, resistance and power and it works out the other two automatically, whichever pair you happened to know. Beyond a single resistor, it also totals series and parallel resistor networks together, calculates a real conductor’s resistance from its material, length and thickness and checks whether a component’s power rating can actually handle the load you have calculated.

---

How to Use

This tool has three tabs at the top: V · I · R · P, Series / Parallel and Wire Resistance. Here is what each one does.

Using the V, I, R, P wheel

1. Fill in any two of the four fields, Voltage (V), Current (I), Resistance (R), and Power (P) and leave the other two blank.
2. If you want to check a component’s safety margin, scroll to Power Rating Check and enter its Component Wattage Rating (W), for example 0.25 for a quarter-watt resistor. This step is optional and can be left blank.
3. Tap Calculate. The tool solves the two values you left blank and shows all four together, along with which pair you originally entered.
4. If you filled in the wattage rating, the result also shows a clear badge, either within rating or exceeds rating, along with a suggested wattage that gives you a comfortable 2 times safety headroom above the actual calculated dissipation.
5. Use Clear all to reset every field and start over with a fresh set of values.

Using Series / Parallel mode

1. Switch to the Series / Parallel tab.
2. Tap Add Resistor for each resistor in your network, up to 10 resistors and enter its resistance value.
3. Optionally enter a Supply Voltage. If you do, the tool also works out the total current and total power for both the series and the parallel arrangement of the exact same set of resistors.
4. The result shows both totals side by side , the series sum and the parallel combination, so you can compare how differently the same resistors behave depending on how they’re wired.

Using Wire Resistance mode

1. Switch to the Wire Resistance tab.
2. Select a Material from the dropdown, copper, aluminium, silver, gold, iron, nichrome, tungsten or brass. Selecting a material automatically fills in its standard resistivity, though you can still edit the Resistivity ρ (Ω·m) field directly if you have a more specific value.
3. Enter the Length (m) of your conductor.
4. Choose whether your wire’s size is given as a Cross-section area (mm²) or a Diameter (mm, round wire), and enter that value. The tool converts a diameter into area automatically using the standard circular area formula.
5. Tap Calculate to see the conductor’s total resistance.

Exporting your result
Once you have a result in any tab, use Print / PDF for a clean printable copy or Copy to paste the figures elsewhere.

---

Key Features

• Solves the complete 12-formula Ohm’s law wheel from any of the six possible input pairs, not just the basic V = IR case
• Series and parallel resistor totals calculated together for the same set of up to 10 resistors, with optional total current and power for both configurations
• Wire resistance calculator covering 8 conductor materials, copper, aluminium, silver, gold, iron, nichrome, tungsten and brass, with editable resistivity
• Accepts wire size as either a cross-sectional area or a diameter, converting automatically between the two
• Power rating safety check comparing calculated dissipation against a component’s actual wattage rating
• Automatic 2 times derating suggestion, giving you a safe wattage figure to design toward instead of just a pass or fail result
• Export as PDF or copy the result.

---

Formula / Logic Used

The Six Solvable Pairs

Given any two of V, I, R, and P, the tool applies the matching pair of formulas from the twelve-formula wheel:

$$V,I \to R = \frac{V}{I},\ P = VI \qquad V,R \to I = \frac{V}{R},\ P = \frac{V^2}{R} \qquad V,P \to I = \frac{P}{V},\ R = \frac{V^2}{P}$$

$$I,R \to V = IR,\ P = I^2R \qquad I,P \to V = \frac{P}{I},\ R = \frac{P}{I^2} \qquad R,P \to V = \sqrt{PR},\ I = \sqrt{\frac{P}{R}}$$

Series and Parallel Totals

$$R_{series} = \sum_{i} R_i, \qquad R_{parallel} = \frac{1}{\sum_i \frac{1}{R_i}}$$

The parallel result is always smaller than the smallest individual resistor in the set, a useful sanity check that catches an accidental arithmetic slip.

Wire Resistance

$$R = \frac{\rho L}{A}, \qquad A = \frac{\pi d^2}{4} \ \text{(when size is given as diameter)}$$

Where $\rho$ is the material’s resistivity, $L$ is the conductor’s length and $A$ is its cross-sectional area. Copper’s resistivity is taken as roughly 1.68 × 10⁻⁸ Ω·m at 20°C.

Power Rating Check

A ratio above 1 means the component will overheat and likely fail; a 2 times headroom above the actual calculated dissipation is treated as the normal, comfortable design margin.

---

Who Should Use This Tool

Anyone studying or working with basic electrical and electronic circuits who needs to move quickly between voltage, current, resistance and power without hunting for the right formula each time. Also useful for diploma and B.Tech Electrical or Electronics students learning the full Ohm’s law relationships, resistor network and wire resistance calculations for their coursework.

---

Frequently Asked Questions (FAQs)

---

Related Tools

• Power Calculator – for a broader, unit-flexible power calculation beyond the electrical wheel
• RLC Color Code Calculator – decode the actual resistor value you’re plugging into this calculator
• Cable Size Calculator – for a full ampacity and derating based wire sizing, beyond simple resistance

Read More>>>