Ohm's Law Calculator

Ohm's Law & Power Calculator

Fill in exactly 2 fields. The calculator will find the other 2 values.

Ohm's Law Wheel & Triangle

The Ohm's Law wheel shows all 12 formulas organized by what you find. Click any segment to auto-fill the calculator above.

All 12 Ohm's Law Formulas

Electrical Unit Converter

Convert between common electrical unit prefixes for voltage, current, resistance, and power.

Series & Parallel Resistance Calculator

R_total = R1 + R2 + R3 +.

AWG Wire Gauge Reference Table

American Wire Gauge (AWG) reference with diameter, resistance per foot, and maximum current capacity at 60 degrees C insulation rating.

Common Resistor Values (E12 & E24 Series)

Standard preferred resistance values used in electronics. These values repeat in each decade (multiply by 10, 100, 1k, 10k, 100k, 1M).

E12 Series (10% tolerance)

E24 Series (5% tolerance)

Common Capacitor Values

Practical Circuit Examples

Electrical Safety Guide

Voltage Safety Classifications

How Ohm's Law Works A

The Basic Relationship

Ohm's Law, discovered by Georg Simon Ohm in 1827, describes the linear relationship between voltage, current, and resistance in an electrical conductor. The law states that the current flowing through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance. V = I × R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms.

This relationship assumes a linear (ohmic) conductor at constant temperature. Real-world components like diodes, transistors, and thermistors are non-ohmic, meaning their resistance changes with applied voltage or temperature., for resistors, wire, and most passive components, Ohm's Law is accurate enough for practical engineering work.

Understanding Voltage

Voltage (V), also called electromotive force or potential difference, is the electrical pressure that pushes current through a circuit. Think of it as the height of a waterfall. One volt is defined as the potential difference that will move one coulomb of charge and transfer one joule of energy. Common voltage sources include batteries (1.5V, 3.7V, 9V, 12V), household mains (120V in North America, 230V in Europe), and solar panels (typically 18V for a 12V system).

Understanding Current

Current (I), measured in amperes (A), is the flow rate of electrical charge through a conductor. One ampere equals one coulomb per second, or roughly 6.24 × 10¹&sup8 electrons passing a point every second. Current flows from high potential to low potential in conventional current flow (opposite to actual electron flow). Current can be dangerously low and still be lethal: as little as 100mA through the heart can cause ventricular fibrillation.

Understanding Resistance

Resistance (R), measured in ohms (Ω), is the opposition to current flow. Every material has some resistance. Conductors like copper have very low resistance (about 1.68 × 10&sup-&sup8 Ω·m), while insulators like glass have extremely high resistance (10¹&sup0 to 10¹&sup4 Ω·m). Resistance depends on material, length, cross-sectional area, and temperature. R = ρL/A, where ρ is resistivity, L is length, and A is cross-sectional area.

Understanding Electrical Power

Power (P), measured in watts (W), is the rate at which electrical energy is converted to other forms (heat, light, motion, sound). The basic power equation is P = V × I. Combined with Ohm's Law, this gives P = I² × R and P = V² / R. Power dissipation is crucial for component selection: a resistor rated at 0.25W that dissipates 0.5W will overheat and potentially fail catastrophically. Always choose components rated for at least twice the expected power dissipation.

AC vs DC Considerations

Ohm's Law applies directly to DC circuits. In AC circuits, impedance (Z) replaces resistance and includes both resistive and reactive components. Impedance is a complex number: Z = R + jX, where R is resistance and X is reactance (from inductors and capacitors). The magnitude |Z| = √(R² + X²). For purely resistive AC loads (like heating elements), Ohm's Law works identically to DC using RMS voltage and current values.

Temperature Effects

Resistance changes with temperature. For most metals, resistance increases with temperature (positive temperature coefficient). For semiconductors and carbon, resistance decreases with temperature (negative temperature coefficient). The relationship is approximately R(T) = R0 × [1 + α(T - T0)], where α is the temperature coefficient. Copper has α = 0.00393 per degree C. A copper wire that is 100Ω at 20 degrees C will be about 139.3Ω at 120 degrees C.

Browser Compatibility

I've tested this Ohm's Law calculator across all major browsers to ensure reliable operation. The calculator uses standard JavaScript math operations and DOM manipulation, which are universally supported. Here are the specific versions I tested on:

Frequently Asked Questions

How to Use This Ohm's Law Calculator

Step 1 Identify Your Known Values

Determine which two electrical quantities you already know. You can enter any combination: Voltage and Current, Voltage and Resistance, Voltage and Power, Current and Resistance, Current and Power, or Resistance and Power. Each combination uses different formulas to find the unknowns.

Step 2 Select the Correct Units

Use the dropdown next to each input to select the right unit prefix. For example, if your current is 20 milliamps, enter 20 and select mA. The calculator handles all conversions internally. Available prefixes include microamps, milliamps, amps for current, and millivolts, volts, kilovolts for voltage.

Step 3 Enter Values and Calculate

Type your two known values into the corresponding fields and click Calculate. The calculator will instantly compute the remaining two values and show which formula was used. If the calculated values involve dangerous levels (high voltage or current), a safety warning will appear automatically.

Step 4 Review Results and Formulas

Each result shows the calculated value in the most readable unit, plus the formula used to derive it. This helps you understand the underlying math and verify the calculation. You can use these formulas to hand-check the results or apply them in your own spreadsheets and design documents.