Ohm's Law Calculator

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?

Ohm's Law states that the voltage across a conductor equals the current through it multiplied by its resistance: V = I × R. Combined with the power equation P = V × I, you can derive any of the four variables — voltage, current, resistance, or power — when two are known. It is the foundation of DC circuit analysis and applies to any resistive circuit element.

How do I know which two values to enter?

Enter whichever two values you already know from your circuit or component specifications. Common starting points: a battery voltage and a resistor value (V and R → solve for I and P); a device's power rating and supply voltage (P and V → solve for I and R); or a measured current and resistance (I and R → solve for V and P). The calculator handles all twelve possible two-input combinations.

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

In a DC resistive circuit, opposition to current is resistance R (a scalar). In an AC circuit, opposition is impedance Z (a magnitude with a phase angle θ). The AC calculator computes apparent power S in volt-amps (VA), real power P in watts (W), reactive power Q in volt-amps reactive (VAR), and power factor (cos θ). Use DC mode for batteries and basic electronics; use AC mode for mains circuits, motors, and capacitive or inductive loads.

What does the resistor power-rating check do?

Every resistor has a maximum power dissipation rating — the most heat it can handle without damage. After calculating, select a standard wattage rating (1/8 W, 1/4 W, 1/2 W, 1 W, 2 W, 5 W, or custom). The calculator compares the computed power to that rating and displays Safe (below 75% of rating), Borderline (75–100%), or Overload (above rating). As a rule of thumb, choose a resistor rated at least twice the computed dissipation for a safe margin.

Why does resistance change with temperature?

In metallic conductors, rising temperature increases atomic vibrations, which impede electron flow and raise resistance. The temperature coefficient α (in /°C) quantifies this change. Copper, the most common wiring material, has α ≈ 0.00393/°C — meaning its resistance rises roughly 0.4% per degree Celsius. This matters in high-current applications, motor windings, precision sensors, and any circuit where heat is a significant factor. The Temperature Coefficient panel in this calculator uses the formula R(T) = R₀ × [1 + α × (T − T₀)].

How to Use the Ohm's Law Calculator

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?

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