Ohm's Law states that the voltage across a conductor equals the current flowing through it multiplied by its resistance: V = I × R. Combined with the power relationship P = V × I, you can solve for any of the four variables — voltage, current, resistance, or power — when two are known.

How do I use this calculator?

Enter any two known values (V, I, R, or P) and leave the rest blank. Select the appropriate units for each field, then click Calculate. The calculator solves for all remaining unknowns and shows the exact formula used.

What is the temperature coefficient of resistance?

Resistance changes with temperature for most conductors. The temperature coefficient (α) describes how much resistance changes per degree Celsius. This calculator supports common materials like copper, aluminum, and nichrome, or you can enter a custom α value to find resistance at any target temperature.

Ohm's Law Calculator — Voltage, Current, Resistance & Power

Enter any two known values — leave the others blank. Select units to match your measurement scale.

Voltage (V)

Current (I)

Resistance (R)

Power (P)

Optional — Resistor Power-Rating Check

Solved For

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Voltage (V)

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Current (I)

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Resistance (R)

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Power (P)

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Power Breakdown

Computed P Rated P

Ohm's Law Formula Wheel

Results are for reference and estimation only. Verify all values before use in any electrical installation. See full disclaimer below.

Find how a conductor's resistance changes with temperature. R(T) = R₀ × [1 + α × (T − T₀)]

Material

Custom α (/°C)

Reference Resistance R₀ (Ω)

Reference Temp T₀ (°C)

Target Temp T (°C)

Find the equivalent resistance of 2–3 resistors in series or parallel. The result can be used directly in the main Ohm's Law calculator above.

R1 (Ω)

R2 (Ω)

R3 (optional, Ω)

⚡ Need more? This quick combinator handles series and parallel equivalent resistance for up to 3 resistors. For voltage divider ratios, loaded dividers, or multi-resistor network analysis, use the full-featured Voltage Divider Calculator → available in the Basic Calculators category.

How to Use the Ohm's Law Calculator

Choose DC or AC Mode

Select DC Mode for standard resistive circuits — batteries, LED circuits, basic electronics. Select AC Mode when working with impedance, mains voltage, motors, or any circuit where phase angle matters.

Enter Any Two Known Values

Type your known values into any two of the four fields — Voltage, Current, Resistance, and Power. Use the unit selectors to match your measurement scale (mV, V, kV / mA, A / Ω, kΩ, MΩ / mW, W, kW). Leave the unknowns blank.

Review Results and Formula

Click Calculate. All four values appear instantly in the result cards. The exact formula used — with substituted values — is shown below the cards. The power breakdown chart and Ohm's Law formula wheel update automatically.

Use the Optional Engineering Tools

Select a standard resistor wattage rating to check for overload. Expand the Temperature Coefficient panel to see how resistance changes with heat. Use the Series/Parallel Combinator to find equivalent resistance before running your main calculation.

Frequently Asked Questions

What is Ohm's Law?

How do I know which two values to enter?

What is the difference between DC and AC Ohm's Law?

What does the resistor power-rating check do?

Why does resistance change with temperature?

Ohm's Law Calculator

How to Use the Ohm's Law Calculator

Frequently Asked Questions

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