Ohm’s Law & Power Calculator

What it is for

• Choosing a series resistor: an LED or another part needs a certain current – which resistor gives it that?
• Estimating the current of an appliance: the rating plate shows voltage and power, and you want to know how much current flows.
• Checking the load on a part: how much power turns into heat in a resistor, and is its size up to it?
• Making sense of measurements: you have measured voltage and current and want the resistance or power.

The formulas

Two relationships cover everything the tool calculates:

• Ohm’s law: V = I × R. It is built into the unit itself: one ohm is one volt per ampere.
• Electrical power: P = V × I. One watt is one volt times one ampere.

Substituting one into the other gives all six cases:

A note on symbols: this page writes voltage as V, as most English-language textbooks do. International standards and German textbooks use U for voltage, so you will see U = R · I there – it is the same law.

Units and prefixes

The tool works in volts (V), amperes (A), ohms (Ω) and watts (W). Very small and very large values get a prefix that stands for a power of ten. The tool picks a suitable one for the result by itself:

The most common mistake is a forgotten prefix: work 20 mA as 20 A and the resistor comes out a thousand times too small.

Decimal point or comma: both work. “3.15” and “3,15” are read as three point one five; “1,000” with a group of three digits after the comma is read as one thousand.

Example 1: series resistor for an LED

A red 5 mm LED, type Kingbright WP7113SRD-D, has a forward voltage of typically 1.85 V at 20 mA according to its datasheet (2.5 V at most); it can take at most 30 mA continuously. It is to run from 5 V at 20 mA (example values).

1. The resistor gets whatever the LED leaves over: V = 5 V − 1.85 V = 3.15 V
2. R = V ÷ I = 3.15 V ÷ 0.02 A = 157.5 Ω – these are the values the calculator opens with.
3. You cannot buy a 157.5 Ω resistor. The next larger value in the standard E24 series is 160 Ω; with it, I = 3.15 V ÷ 160 Ω ≈ 19.7 mA flows. The Resistor Colour Code tool shows which bands that resistor carries.
4. P = V × I = 3.15 V × 0.0197 A ≈ 62 mW turns into heat in the resistor. How much it can take is in the resistor’s datasheet.

If your LED’s forward voltage is closer to the maximum of 2.5 V, only 2.5 V is left for the resistor and about 15.6 mA flows – the LED is a little dimmer but stays safely below its current limit. That is why it is better to round up to the next larger resistor than down to the next smaller one.

Example 2: current drawn by a fan heater

Say the rating plate reads 230 V and 2,000 W (example values). A fan heater is essentially a resistive load:

1. I = P ÷ V = 2,000 W ÷ 230 V ≈ 8.7 A
2. R = V² ÷ P = (230 V)² ÷ 2,000 W ≈ 26.45 Ω

That resistance applies while the heater is running. If you measure the cold heater with a multimeter, you may get a different value, because the resistance of a metal changes with its temperature.

Checking with a multimeter

Calculating is good, measuring is better. A digital multimeter measures all three basic quantities, but each one differently:

• Voltage is measured in parallel with the part: one probe on each of its two terminals.
• Current is measured in series: the circuit is opened and the meter put in the gap, with the test lead in the current socket. With the lead in the current socket, never hold the probes across a voltage – that is practically a short circuit.
• Resistance is measured only with the circuit switched off, ideally with one leg of the part desoldered – otherwise you measure the surrounding circuit as well.

For measurements on mains voltage the meter must be rated for it (measurement category “CAT” in the manual) – and work on mains wiring belongs in the hands of a qualified electrician.

Limits of this tool

• AC with a power factor: for motors, power supplies and other loads that are not purely resistive, the real power is less than V × I; the power factor (cos φ) enters the calculation there. This tool does not include it.
• Non-linear parts such as LEDs and diodes do not follow V = I × R. That is why example 1 uses the forward voltage from the datasheet and applies Ohm’s law only to the resistor.
• Temperature: the resistance of many conductors and parts changes with temperature. The tool calculates with fixed values.
• Whether a part can handle the calculated power or current is in its datasheet – not in this tool.

Privacy: the calculator runs entirely in your browser. What you enter is not sent to xmoo.app or anyone else.

Sources

• Ohm = volt per ampere, watt = joule per second, volt = watt per ampere: BIPM, The International System of Units (SI Brochure, 9th edition), table of derived units
• Prefixes M, k, m, µ and their factors: BIPM, SI prefixes
• LED Kingbright WP7113SRD-D: forward voltage typ. 1.85 V, max. 2.5 V at 20 mA, continuous current max. 30 mA: datasheet (Kingbright)
• E24 series including the value 160 (× 10 Ω): Vishay, Standard Electronic Decade Value Tables

Page last updated: 31 October 2026