This Ohms law calculator solves relationships among voltage, current, and resistance. Enter any two known values and set the unknown to 0. The tool fills the missing quantity using V = I * R style relationships and reports voltage after solving.
Students and hobbyists use it for quick DC checks on simple resistive circuits before breadboarding. It is not a full circuit simulator.
How the solver works
If voltage is missing and current and resistance are known, voltage becomes I * R. If current is missing, current becomes V / R. If resistance is missing, resistance becomes V / I. At least two positive knowns are required.
Worked example
Current 2 A, resistance 6 ohm, voltage unknown (0). Voltage = 2 * 6 = 12 V.
| Input | Value |
|---|---|
| Voltage V | 0 (solve) |
| Current I | 2 A |
| Resistance R | 6 ohm |
| Resulting voltage | 12 V |
Default style check
With catalog defaults V = 12, I = 0, R = 4, current solves to 3 A and the reported voltage is 12 V. That is a useful sanity check that V / R behaves as expected.
How to use the fields
- Voltage in volts.
- Current in amperes.
- Resistance in ohms.
- Set exactly the unknown you want filled to 0 when you are solving for it.
Power intuition
Once V and I are known, power is V * I. This page does not print power, but you can compute it from the solved pair when sizing resistors or supplies. Watch resistor watt ratings so heat stays safe.
Common mistakes
- Mixing milliamps with amps without converting
- Using AC meter readings on a DC only formula without care
- Ignoring series and parallel combinations before applying Ohm’s law to a single equivalent R
- Entering only one known value
Lab tips
Measure two quantities when you can and predict the third before you trust a single meter reading. If measured voltage disagrees with I * R, check wiring, meter range, and whether the resistor value drifted with heat.
Series and parallel prep
Ohm’s law applies to a single resistance value. If you have series resistors, add them first. If you have two parallel resistors, compute the equivalent resistance first, then solve for current or voltage on that equivalent. Jumping straight to V = I * R on one branch without a clear circuit model produces wrong expectations.
For a supply feeding one resistor, the defaults style check (12 V across 4 ohm) predicts 3 A. Confirm your power supply can deliver that current safely before you close the circuit.
Meter discipline
- Measure voltage in parallel with the load, current in series, and resistance with power removed.
- Start on a higher meter range, then refine.
- Watch polarity on DC circuits so signs match your notes.
- Recalculate after any component swap so the predicted V, I, or R stays current.
Those habits turn the solver into a rapid lab companion instead of a source of mystery numbers.
Worked current and resistance cases
Known voltage 12 V and current 3 A with resistance unknown yields R = 12 / 3 = 4 ohm. Known voltage 9 V and resistance 3 ohm with current unknown yields I = 9 / 3 = 3 A, and the reported voltage remains 9 after solving.
Keep a small notebook of predicted versus measured values. When they diverge, the fault is often a wiring mistake, a wrong meter mode, or a resistor that is not the labeled value.
For battery projects, confirm continuous current limits. A calculation that asks for more amps than the cell can deliver safely is a design problem, not a calculator problem.
Safety margins
Electronics get hot. After you solve for current, check resistor wattage with P = V * I or I squared times R. Choose parts rated above the calculated power so they stay cool. Fuse protection belongs in the design when fault currents could be high.
Never probe a live circuit with a meter set to resistance mode. Power down before measuring R, then power up to confirm V and I against the prediction.
Limitations
Results assume a simple resistive relationship. Reactive impedance, nonlinear devices, and multi node networks need more than this solver. Use it for first pass V, I, R checks only.
For related unit and percent checks while you plan materials, try the unit converter or the percentage calculator.