Ohm's Law Calculator — Voltage, Current, Resistance & Power
Select which electrical quantity to solve for, enter the other two values, and instantly get voltage (V), current (I), resistance (R), and power (P). Supports milliamperes, microamperes, kilohms, megaohms, milliwatts, and kilowatts. The applied formula is shown with every result.
Enter the two known values to calculate the unknown.
How it works
What is Ohm's Law?
Ohm's Law states that the voltage across a conductor is directly proportional to the current flowing through it, provided the temperature remains constant. Formulated by German physicist Georg Simon Ohm in 1827, it is expressed as V = I × R, where V is voltage in volts (V), I is current in amperes (A), and R is resistance in ohms (Ω). Ohm derived this relationship by experimenting with wires of different lengths and thicknesses, noticing that doubling the resistance halved the current for a fixed voltage.
Ohm's Law applies to most conductors made of metals and many resistive materials at constant temperature — these are called ohmic materials. Non-ohmic devices like diodes, transistors, and incandescent bulbs do not follow a linear V-I relationship, so Ohm's Law is only an approximation for them. In circuit design, V = IR is the single most-used equation: it determines wire gauge, resistor values, voltage drop across components, and safe operating limits.
Power in electrical circuits
Electrical power is the rate at which energy is transferred or consumed, measured in watts (W). Three equivalent formulas relate power to the basic electrical quantities: P = V × I (power equals voltage times current), P = I² × R (useful when current and resistance are known), and P = V² / R (useful when voltage and resistance are known). All three follow directly from Ohm's Law and the definition of power.
Power dissipation matters practically because resistors, wires, and other components turn electrical energy into heat. Every resistor has a power rating — typically 0.125 W, 0.25 W, 0.5 W, or 1 W for common through-hole types — and exceeding this rating causes the component to overheat and fail. For example, a 100 Ω resistor in a 5 V circuit carries I = 5/100 = 50 mA and dissipates P = 0.05² × 100 = 0.25 W, so a quarter-watt resistor is the minimum safe choice. Always add a safety margin of at least 50%.
Series and parallel circuits
In a series circuit, resistors are connected end-to-end and the same current flows through all of them. The total resistance is simply the sum: R_total = R1 + R2 + R3 + ... Voltage divides across each resistor in proportion to its resistance (V_n = I × R_n), which is the principle behind voltage divider circuits used to set bias points or scale sensor outputs.
In a parallel circuit, resistors share the same voltage but the current splits between branches. The total resistance follows the reciprocal rule: 1/R_total = 1/R1 + 1/R2 + 1/R3 + ... For two resistors, the shortcut is R_total = (R1 × R2) / (R1 + R2). Parallel combinations always produce a total resistance lower than the smallest individual resistor. Understanding both configurations lets you design circuits that deliver the right voltage and current to every component, size fuses and wires correctly, and troubleshoot faults efficiently.
Frequently asked questions
›What is Ohm's Law in simple terms?
Ohm's Law says that if you increase the voltage pushing electricity through a wire, more current flows — and if you increase the resistance, less current flows. The exact relationship is V = I × R: voltage equals current multiplied by resistance. Knowing any two of these three quantities lets you calculate the third.
›What units are used in Ohm's Law?
Voltage is measured in volts (V), current in amperes (A), and resistance in ohms (Ω). These units are defined so that 1 V = 1 A × 1 Ω. In practice, you often use milliamperes (1 mA = 0.001 A) for small currents, kilohms (1 kΩ = 1,000 Ω) for larger resistances, and megaohms (1 MΩ = 1,000,000 Ω) for very high resistances like those in input-protection circuits.
›How do I calculate voltage using Ohm's Law?
Multiply the current (in amperes) by the resistance (in ohms): V = I × R. For example, if 2 A flows through a 50 Ω resistor, the voltage drop across it is 2 × 50 = 100 V. If your current is in milliamperes, convert first: 200 mA = 0.2 A, so V = 0.2 × 50 = 10 V.
›How do I find current when I know voltage and resistance?
Rearrange Ohm's Law to I = V / R. If you have a 9 V battery connected to a 470 Ω resistor, the current is 9 / 470 ≈ 0.0191 A, or about 19.1 mA. This is the current that flows through the resistor and sets the power it dissipates.
›What is the relationship between power and Ohm's Law?
Power (P) measures how fast energy is consumed, in watts. It connects to Ohm's Law through three equivalent formulas: P = V × I, P = I² × R, and P = V² / R. All three give the same answer; choose the one that matches the quantities you already know. For a resistor carrying 0.1 A at 10 V: P = 10 × 0.1 = 1 W.
›Does Ohm's Law work for AC circuits?
Ohm's Law applies to pure resistors in AC circuits just as in DC circuits. However, AC circuits also contain capacitors and inductors, which introduce reactance (frequency-dependent opposition to current). The AC generalization uses impedance (Z) instead of simple resistance: V = I × Z, where Z is a complex number. For audio, radio, and power electronics work, you need complex impedance calculations beyond basic Ohm's Law.
›Why does resistance cause heating?
When current flows through a resistor, electrons collide with the atoms of the material, transferring kinetic energy as heat. The power dissipated as heat equals P = I² × R — this is called Joule heating. Higher current or higher resistance means more heat. This is why high-current wiring uses thick, low-resistance conductors, and why resistors have power ratings to prevent them from overheating.
›What happens if I exceed a resistor's power rating?
Exceeding the power rating causes the resistor to overheat. At moderate overload it may drift in value or become unreliable. At severe overload the resistor can smoke, crack, or catch fire. Always calculate P = I² × R and choose a resistor rated for at least 1.5 to 2 times the expected power. Common ratings are 0.1 W, 0.25 W, 0.5 W, 1 W, and 2 W for through-hole components.
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