Ohms Law Calculator

Ohms Law Calculator

Solve any electrical circuit calculation using Ohm’s Law (V = IR) and the power equation (P = IV). Enter any two known values and the calculator solves for the other two. These relationships govern every electrical circuit from phone chargers to power grids, making this the most fundamental tool in electrical engineering and electronics.¹

Ohms Law Reference

Wire Gauge Reference

The Three Forms of Ohm's Law

Ohm's Law expresses the fundamental relationship between voltage (V), current (I), and resistance (R) in any electrical circuit. The three equivalent forms — V = I × R, I = V / R, and R = V / I — allow you to solve for any unknown value when the other two are known. Voltage is measured in volts (the electrical "pressure" that pushes charge through a circuit), current in amperes or amps (the rate of charge flow), and resistance in ohms (the opposition to current flow). These three quantities are inseparable: change one and at least one of the others must change accordingly. A useful analogy is water flowing through a pipe — voltage is the water pressure, current is the flow rate, and resistance is the pipe's narrowness.

The Power Equation and Its Forms

Power — the rate at which electrical energy is consumed or delivered — connects directly to Ohm's Law through P = I × V (watts = amps × volts). By substituting Ohm's Law into the power equation, you get two additional forms: P = I² × R and P = V² / R. These twelve total relationships (three for each of the four quantities) are sometimes displayed in the "Ohm's Law wheel" or "power wheel" — a circular reference chart that electrical engineers and technicians use daily.

Series and Parallel Circuits

In a series circuit, components are connected end-to-end in a single path. Total resistance is the sum of all individual resistances: R_total = R₁ + R₂ + R₃. The same current flows through every component, but voltage divides across them proportionally to their resistance. If three resistors of 100Ω, 200Ω, and 300Ω are wired in series, total resistance is 600Ω, and on a 12V supply the current is 20 mA (0.02A). The 100Ω resistor drops 2V, the 200Ω drops 4V, and the 300Ω drops 6V — summing to 12V.

In a parallel circuit, components connect across the same two nodes, providing multiple current paths. The reciprocal formula applies: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃. Total resistance is always less than the smallest individual resistance. For two resistors in parallel, the shortcut formula is R_total = (R₁ × R₂) / (R₁ + R₂). Three 300Ω resistors in parallel yield 100Ω total. Each branch carries current independently based on its own resistance, and the total current equals the sum of all branch currents. Voltage across all branches is identical.

Common Circuit Calculations

AC vs DC and Impedance

Ohm's Law applies directly to DC circuits, but AC circuits introduce additional complexity through impedance. Impedance (Z) replaces simple resistance and includes both resistive and reactive components. Reactance comes from capacitors (which resist changes in voltage) and inductors (which resist changes in current). In a purely resistive AC circuit, Ohm's Law works exactly as in DC: V = I × R. But when capacitors or inductors are present, the voltage and current may be out of phase — meaning they peak at different moments in the AC cycle. The impedance formula Z = √(R² + (XL − XC)²) combines resistance with inductive reactance (XL) and capacitive reactance (XC). For household appliances on 120V/60Hz circuits, Ohm's Law with basic resistance is sufficiently accurate for sizing breakers, wiring, and fuses.

Practical Electrical Safety

Understanding Ohm's Law is essential for electrical safety. The human body has a resistance of roughly 1,000–100,000 ohms depending on skin moisture, contact area, and current path. Wet skin drops resistance below 1,000Ω, which is why water and electricity are so dangerous — at 120V with 1,000Ω body resistance, Ohm's Law predicts 120 mA of current. Currents as low as 10 mA cause painful muscle contractions, 30 mA can cause respiratory paralysis, and 75–100 mA across the heart can trigger ventricular fibrillation. This is why GFCI outlets (which trip at 5 mA ground-fault current in under 25 milliseconds) are required in kitchens, bathrooms, garages, outdoors, and anywhere near water. For related electrical tools, see our Wire Gauge Calculator and LED Resistor Calculator.

How to Use This Calculator

1. Enter any two known values — Input voltage and current, voltage and resistance, current and resistance, or any combination including power.
2. Calculate — The calculator instantly solves for all unknown values using the appropriate Ohm's Law and power formulas.
3. Apply the results — Use the calculated values for component selection, circuit design, wire sizing, or troubleshooting.

Tips and Best Practices

See also: Wire Gauge Calculator · LED Resistor · Electricity Bill · Energy Converter

Resistor Color Code Reference

Standard resistors use colored bands to indicate their resistance value. The first two bands represent significant digits, the third is the multiplier (number of zeros), and the fourth indicates tolerance. The color code follows: black=0, brown=1, red=2, orange=3, yellow=4, green=5, blue=6, violet=7, gray=8, white=9. For the multiplier band: black=×1, brown=×10, red=×100, and so on. A resistor with brown-black-red-gold bands reads 10 × 100 = 1,000Ω (1kΩ) with 5% tolerance (gold). Five and six-band resistors provide greater precision with three significant digits and a separate tolerance band. Memorizing the sequence by color mnemonic helps — many technicians learn it through memorable phrases. Standard resistor values follow the E12 series (10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82) multiplied by powers of 10, meaning you cannot buy a 500Ω resistor — the nearest standard values are 470Ω and 560Ω.

Power Dissipation and Component Ratings

Every resistor has a power rating — typically 1/8W, 1/4W, 1/2W, 1W, 2W, 5W, or higher for power resistors. Exceeding the rated power causes the resistor to overheat, potentially catching fire or failing open-circuit. To calculate power dissipation, use P = I²R or P = V²/R. A 100Ω resistor carrying 100mA dissipates P = (0.1)² × 100 = 1W, requiring at least a 2W rated resistor (engineers typically derate to 50–70% of the rating for reliability). This is why Ohm's Law matters practically — you must calculate both the correct resistance value and the power dissipation to select an appropriate component. For high-power applications like motor control, heating elements, and power supplies, power resistors with ceramic or wirewound construction handle 5–100+ watts with proper heatsinking.

Whether you are designing a circuit from scratch, troubleshooting an existing installation, or simply trying to figure out if your space heater will trip the breaker, Ohm's Law is the single most useful equation in all of electrical work. Every calculation in this domain — wire sizing, fuse selection, battery life estimation, motor current draw, LED driver design — ultimately reduces to V = IR and P = IV in some form.

How to Use This Calculator

1. Enter known values — Any two: voltage, current, resistance, or power.
2. Calculator solves — The other two values instantly.
3. Check wire gauge — Based on current for safe wiring.

Tips and Best Practices

See also: Electricity Cost · Scientific · Percentage · Unit Price