Ohm's Law Calculator

How to use this calculator

1. Enter the voltage across the component or circuit branch you’re analyzing.
2. Enter the resistance in ohms of the load (for example, a resistor or the approximate resistance of a device).
3. We compute the current I = V ÷ R and the power P = V × I.
4. Use the results to check whether your components and power supply can handle the load safely.

Inputs explained

Voltage (V)

The potential difference across the component or circuit branch, measured in volts. For DC circuits, this is the supply voltage; for AC, use the RMS voltage for typical power calculations.

Resistance (Ω)

The resistance of the load in ohms. This might be a resistor value from a schematic, a measured value with a multimeter, or an approximate resistance of a device or filament.

When to use it

• Estimating current draw to ensure your power supply and wiring are appropriately sized.
• Sizing resistors and checking expected wattage before purchasing parts for a new design or repair.
• Spot‑checking bench experiments when it’s easier to measure voltage and resistance than current directly.
• Teaching or learning basic electronics by exploring how changes in resistance and voltage affect current and power.

Tips & cautions

• Use RMS values for AC sources (like household mains) when plugging voltages into Ohm’s law for power estimates.
• Select resistors with a wattage rating comfortably above the calculated power—often 2× or more for safety and longevity.
• Double‑check units: mixing ohms (Ω) and kilo‑ohms (kΩ) without conversion will produce incorrect currents and powers.
• Remember that real components have tolerances; if precision matters, consider the worst‑case resistance within the tolerance band.
• Models purely resistive loads; reactive circuits with inductance or capacitance require complex impedance calculations, not just Ohm’s law in its simplest form.
• Does not account for temperature effects, resistance drift, or non‑linear devices such as diodes, LEDs, or transistors.
• Assumes steady‑state conditions and ignores transient spikes, inrush currents, and non‑sine waveforms.

Worked examples

Deep dive

FAQs

Can I solve for resistance or voltage instead?

Yes, algebraically. If you know current and voltage, resistance R = V ÷ I. If you know current and resistance, voltage V = I × R. A future version of this tool may allow entering any two variables and solving for the third.

Is this valid for AC circuits?

For purely resistive AC loads and RMS voltages, yes. For circuits with significant inductance or capacitance, you must use impedance and phasor analysis; simple Ohm’s law with DC‑style values will not capture phase and reactive effects.

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