Resistor Colour Code Calculator
Decode resistor colour bands into resistance values.
What is the Resistor Colour Code Calculator?
A resistor colour code calculator decodes the coloured bands printed on a resistor into its resistance value and tolerance, using the standard electronics colour-code system. It works by assigning each colour a digit value (black=0 through white=9), reading the first two bands as significant digits, the third band as a multiplier (a power of ten to scale those digits by), and the fourth band as the tolerance percentage — the resistance is then (first digit × 10 + second digit) × multiplier. To decode a 4-band resistor by hand, look up each band colour's value on a standard resistor colour code chart, combine the first two digits, multiply by the third band's multiplier value, and read the tolerance from the fourth band's colour (gold = ±5%, silver = ±10%).
How it works
Resistors are marked with coloured bands instead of printed numbers because it's a compact, easily read-at-any-angle system. On a standard 4-band resistor, the first two bands represent significant digits, the third band is a multiplier (how many zeros to add, or a fractional multiplier for very low values), and the fourth band indicates the manufacturing tolerance — how much the actual resistance might vary from the stated value.
UK context
This colour band system is an internationally standardised convention (part of the IEC 60062 standard), which is exactly why it's consistent across manufacturers and has remained stable for decades — unlike figures such as tax rates, this is a fixed engineering standard, not something that changes over time.
Tips
- Reading direction matters — the tolerance band (often gold or silver, and usually spaced slightly apart from the others) marks the end of the resistor, so read from the opposite end.
- 5-band and 6-band resistors follow a similar principle but with an extra significant-digit band for higher precision — this calculator covers the common 4-band format.
Frequently asked questions
What do the resistor colour bands mean?
On a standard 4-band resistor: the first two bands are significant digits, the third is a multiplier, and the fourth indicates tolerance (how much the actual value may vary from the stated resistance).
Which end do I start reading from?
Start from the end furthest from the tolerance band — the tolerance band (commonly gold or silver) is usually spaced slightly apart from the others and marks the end of the sequence.
A quick note
Based on the internationally standardised (IEC 60062) resistor colour code convention, which is fixed and doesn't change over time — unlike some of the other reference figures on this site.