The formula
Value = (digits side by side) × multiplier
What it means
The first bands are digits written one after another, not added: brown and black are "1" and "0", that is ten, and the next band says what to multiply by. Four bands mean two digits, five mean three — which is what allows tighter values — and the sixth, when present, is the temperature coefficient in parts per million per degree. The last colour band, set apart from the rest, is always the tolerance.
How to work it out by hand
- Hold the resistor with the separated band, or the gold or silver one, on the right
- Read the first bands as consecutive digits: brown and black are "10"
- Multiply by the power of ten the next band gives
- The last band is the tolerance: gold ±5 %, silver ±10 %
What is worth knowing
Knowing which end to start from is half the job: the tolerance band usually sits slightly apart from the others, and if it is gold or silver there is no doubt, because those colours are not valid as a first digit. Reading it backwards gives a completely different but perfectly plausible value, which is the dangerous part. Gold and silver in the multiplier band divide rather than multiply, and that is how resistors below ten ohms are written.