Ohm’s Law Calculator

How Ohm’s law works

When to use this: reach for Ohm’s law whenever you need to know how much current a circuit will draw, what resistor value drops a given voltage, or how much heat a part will have to get rid of. It is the first calculation in almost every design and the first check when something is running hot.

Ohm’s law says that the current flowing through a resistor is proportional to the voltage across it. Double the voltage and you double the current; double the resistance and you halve it. That single relationship, V = I × R, connects the three quantities you can actually measure with a multimeter, which is why it underpins essentially all practical circuit work.

The useful mental model is water in a pipe. Voltage is the pressure pushing the water, current is the flow rate, and resistance is how narrow the pipe is. Raise the pressure and more water flows. Narrow the pipe and less does. The analogy breaks down for anything involving semiconductors, but for resistors it is exact enough to reason with.

Power follows immediately. Every electron pushed through a resistance gives up energy as heat, and the rate of that energy loss is P = V × I. Substituting Ohm’s law gives two more forms — P = I²R and P = V²/R — which are algebraically identical but let you compute dissipation from whichever pair of values you happen to have. This is the part beginners skip, and it is the part that causes scorched circuit boards.

The four forms you actually use

Given any two of voltage, current, resistance and power, the other two are fully determined. In practice four rearrangements cover almost everything:

The calculator above returns all four regardless of which one you asked for, because the other three are almost always worth a glance. A resistor value that looks perfectly reasonable can still be dissipating half a watt.

Worked design examples

Sizing an LED series resistor

A red LED has a forward voltage of roughly 2 V and wants about 20 mA. Running it from a 5 V supply means the resistor has to absorb the difference:

V_R = 5 V − 2 V = 3 V
R = 3 V / 0.02 A = 150 Ω
P = 3 V × 0.02 A = 60 mW

150 Ω is an E24 standard value, so no compromise is needed. At 60 mW a 0.125 W resistor would technically survive, but a 0.25 W part costs the same and runs cooler. Note that you round the resistor up in current-limiting applications — a higher resistance means less current, which is the safe direction to err.

Checking a current-sense shunt

A 0.1 Ω shunt carrying 2 A produces a measurable voltage for an amplifier to read:

V = 2 A × 0.1 Ω = 200 mV
P = I²R = 2² × 0.1 = 0.4 W

200 mV is a comfortable signal level. The 0.4 W of dissipation is the real design constraint: it demands at least a 1 W shunt, and the resulting self-heating will shift the resistance slightly, which is exactly why precision shunts are specified with a low temperature coefficient.

Common values reference

Values you will meet constantly in low-voltage design:

Choosing a resistor wattage

A resistor’s power rating is the dissipation it can sustain in free air at 70 °C without drifting or failing. Running one at 100% of its rating is legal but unwise: it will reach well over 100 °C, discolour the board, and shift in value over time. Standard practice is a 2× safety factor — size the part for twice the calculated dissipation.

Derating matters too. Above about 70 °C ambient, the permissible dissipation falls linearly to zero at the part’s maximum temperature, typically 155 °C. A 0.25 W resistor inside a warm enclosure at 100 °C is effectively a 0.15 W resistor. If the part sits near a regulator, a motor driver or anything else that runs hot, budget accordingly.

When Ohm’s law does not apply

Ohm’s law describes ohmic components — those whose resistance stays constant as the applied voltage changes. Plenty of things in a circuit are not ohmic:

• Diodes and LEDs. Current rises exponentially with forward voltage. A small voltage increase can multiply the current, which is why LEDs need a current-limiting resistor rather than a carefully chosen supply voltage.
• Transistors. The effective resistance is set by the control terminal, not by the applied voltage.
• Incandescent lamps. The filament resistance rises sharply as it heats, so the cold inrush current is many times the steady-state figure.
• Thermistors and varistors. Designed specifically so that resistance varies with temperature or voltage.

You still use Ohm’s law constantly when working with these parts — just around them rather than through them. Sizing an LED resistor is an Ohm’s law problem; modelling the LED is not.

AC circuits and impedance

In an AC circuit containing capacitors or inductors, resistance generalises to impedance, written Z and measured in ohms. The relationship becomes V = I × Z, but both voltage and current now carry a phase as well as a magnitude, and Z varies with frequency. A capacitor’s impedance falls as frequency rises; an inductor’s does the opposite.

For purely resistive AC circuits — a heating element, an incandescent lamp — the simple form still works using RMS values, and the power calculation is unchanged. Once reactance is involved you also have to distinguish real power (watts) from apparent power (volt-amps), which is where power factor enters.

Practical tips

• Work in the units on the schematic. Type 4k7 rather than 4700; it is faster and matches what is printed on the board.
• Check the power every time. It takes a second and it is the single most common omission in beginner designs.
• Snap to a standard value early. There is no point designing around 3,847 Ω. Pick the nearest E24 value and confirm the circuit still works with the resulting error.
• Remember tolerance. A ±5% resistor at ±5% supply tolerance means your current could be 10% off the calculated figure. Design with margin.