Resistor Colour Code Calculator

How to use the resistor colour code calculator

1. Fill in the fields — they start with a worked example.
2. Read the answer, and the arithmetic shown beneath it.
3. Copy the results, or adjust the inputs to compare scenarios.

About this tool

Colour bands encode a resistance because printing numbers on something a few millimetres long is impractical. Each colour is a digit from black at 0 to white at 9, in the order of the spectrum once you are past brown and red — which is why the sequence feels arbitrary at the start and obvious after green.

A four-band resistor gives two significant digits, a multiplier and a tolerance. A five-band one gives three digits for tighter precision, and is the usual form for 1% parts.

The tolerance band matters more than beginners expect. A 100 Ω resistor at 10% tolerance is guaranteed only to be somewhere between 90 and 110 Ω, so the range is shown here alongside the nominal value. In a voltage divider or a timing circuit that spread is the difference between a design that works and one that works sometimes.

Reading direction is the usual source of error: the tolerance band is normally gold or silver and sits slightly apart from the others, at the end you read towards.

Common uses

• Identifying an unlabelled resistor from a parts drawer.
• Checking a resistor is the value you thought before soldering it.
• Learning the colour code.

Frequently asked questions

Which end do I read from?

Start from the end where the bands are grouped closer together. The tolerance band — usually gold or silver — sits slightly apart at the far end.

What is the difference between four and five bands?

Four bands give two significant digits; five give three, for tighter precision. Five-band parts are typically 1% tolerance.

Why does tolerance matter?

A 100 Ω resistor at 10% may be anywhere from 90 to 110 Ω. In dividers and timing circuits that spread changes the behaviour, which is why the range is shown here.