How It Works

Ohm’s law links three core electrical quantities: voltage (V), current (I), and resistance (R). In a purely resistive circuit, these values move together in a predictable way: higher resistance reduces current for a given voltage, and higher voltage increases current for a given resistance. If you know two of them, the third is determined by simple algebra.

This tool follows the rule exactly, adds safeguards for edge cases (like division by zero), and formats the output in the style you prefer. It also provides a clear text summary you can copy or download, which is handy for lab notes, design logs, or classroom work.

Step-by-step logic

• 1) Pick a solving mode: Auto (detect the missing value) or explicitly solve for V, I, or R.
• 2) Enter known values: Provide at least two of the three quantities. You can still type the third value if you want the tool to compare and warn about inconsistencies.
• 3) Compute the result: The calculator applies V = I × R or the rearranged forms I = V ÷ R and R = V ÷ I.
• 4) Optional checks: If all three are provided, the tool verifies whether the values agree within a small tolerance and shows a friendly warning when they don’t.
• 5) Output formatting: Choose decimal places and standard vs scientific notation for clean reporting.

Formula rearrangements you’ll see

Many learners memorize V = I × R, but the key skill is rearranging it depending on what you need. The calculator uses these equivalents:

• Voltage: V = I × R
• Current: I = V ÷ R
• Resistance: R = V ÷ I

As an added convenience, the result panel also reports electrical power for the computed state (P = V × I). Power is not required for Ohm’s law itself, but it is often the next number you need when selecting resistor wattage ratings or estimating heat.

Quick example

Suppose you have a 12 V supply and a 6 Ω resistor. The current is I = 12 ÷ 6 = 2 A. The power dissipated in the resistor is P = 12 × 2 = 24 W. The same numbers can be reversed: if you measure 2 A through a 6 Ω load, you should expect about 12 V across it. This is exactly the kind of sanity check this calculator is built to make painless.

Key Features

Auto-detect missing variable

Leave one field blank and select Auto. The tool will infer what to solve for based on the missing value and compute it immediately. This mode is ideal when you’re working from a worksheet or a bench measurement and you naturally have two known numbers and one unknown.

Explicit solve mode

If you prefer a deterministic workflow, select exactly what you want to calculate (V, I, or R). This is useful when you want to keep a third value as a reference for consistency checks, or when you are iterating through “what-if” scenarios and want the same variable solved each time.

Consistency warnings

When you enter all three quantities, the tool compares V against I × R and warns you if the values don’t match. This can happen due to rounding, measurement tolerance, heating effects (resistance drift), or a unit mix-up such as entering milliamps as amps. The warning helps you catch issues before they turn into confusing conclusions.

Precision & notation controls

Pick the number of decimal places for reporting and optionally switch to scientific notation. Scientific notation is especially helpful when dealing with microamp currents, megaohm resistors, or when you want to paste the result into a report that expects engineering-style formatting.

Copy & download

Copy a plain-text summary of the calculation to your clipboard or download it as a TXT file. The output includes the solved variable, the final V, I, and R values, plus power. It’s perfect for documentation, sharing in a chat, or attaching to a ticket without reformatting.

Use Cases

• Electronics basics: Verify a breadboard circuit by checking that your measured voltage drop and resistor value imply a realistic current. If the calculated current is far above what a component can handle, you can adjust the design immediately.
• LED and resistor sanity checks: If you know the voltage across a resistor and the desired LED current, compute the required resistance quickly, then choose the nearest standard value (E12/E24 series) and validate the current again.
• Battery-powered projects: Estimate current draw from a supply voltage and load resistance to anticipate runtime, fuse sizing, connector limits, and cable thickness. Even a rough estimate can prevent undervaluing heat and losses.
• Resistor wattage selection: After computing current, check power (P = V × I) to choose a resistor with an adequate watt rating and some safety margin for airflow, ambient temperature, and enclosure constraints.
• Education & homework: Practice rearranging formulas and confirm answers without getting stuck on algebra. Because the tool can solve in multiple directions, it’s useful for exercises that ask you to find V, I, or R under different givens.
• Troubleshooting: Compare expected current (from V and R) against observed current to spot wiring mistakes, short circuits, wrong resistor bands, or measurement setup issues such as an ammeter in the wrong range.
• Sensor pull-ups and dividers: For simple pull-up resistors or resistive sensor circuits, quickly compute the current through a resistor at a given voltage to ensure you’re not loading a microcontroller pin or sensor output too heavily.
• Automotive and hobby repairs: Estimate current for a small accessory load or test resistor packs. While vehicle systems have their own complexities, basic V–I–R checks are common when diagnosing simple resistive loads.
• Documentation and communication: Create a consistent, shareable summary for a report, lab notebook, or build log. Copying the result text avoids transcription mistakes and keeps your notes readable.

Whether you’re a student learning fundamentals or a maker validating a quick prototype, having a reliable V–I–R calculator reduces mistakes and speeds up decisions. It can also act as a quick “reasonableness check” when you’re looking at multimeter readings and wondering whether a number makes sense for the components on your bench.

Optimization Tips

Use consistent units

This calculator expects V in volts, I in amps, and R in ohms. If your values are in mA or kΩ, convert them first (for example, 25 mA = 0.025 A and 4.7 kΩ = 4700 Ω). Consistent units prevent results that look “off” by factors of 10, 100, or 1000. If you routinely work in milliamp ranges, consider writing the conversion next to your measurement to avoid accidental copy-paste errors.

Choose a practical precision

For most resistor calculations, two to three decimal places is enough because real components have tolerances and temperature effects. If you’re entering values from a datasheet or from a measurement that is only accurate to a few digits, excessive decimals can give a false sense of precision. When dealing with very small currents or very large resistances, scientific notation can improve readability while keeping accuracy.

Watch out for zero in the denominator

If you try to calculate current with R = 0, or resistance with I = 0, the math becomes undefined (division by zero). In practice, a true 0 Ω path means a short circuit and current is limited by the source, internal resistance, wiring, and protection components. Treat zero values as special cases and think in terms of real-world limits, especially when working with power supplies, batteries, and heating elements.

FAQ