How it works
Through-hole resistors are marked with colored bands instead of printed numbers. On a 4-band resistor the first two bands are digits, the third is the multiplier and the fourth is the tolerance. A 5-band resistor adds a third digit for precision parts.
Read from the end with the bands bunched closer together. The tolerance band (often gold or silver) is usually set apart from the others. Gold and silver never appear as the first band, which helps you orient the part.
Example: yellow, violet, red, gold reads 4, 7, ×100, ±5%, which is 4,700 Ω (4.7 kΩ).
Formula
R = (digits as a number) × multiplier, ± tolerance %
Questions people ask
Which end do I read first?
Start from the end where the bands are closest to the lead. The tolerance band is usually separated by a wider gap. If in doubt, measure with a multimeter.
What does a sixth band mean?
A 6-band resistor adds a temperature coefficient band after the tolerance band, in ppm/°C (brown 100, red 50, orange 15, yellow 25). The value is read the same way as a 5-band part.
Why are resistor values like 4.7k and 2.2k?
Values follow the E-series (E12, E24, E96), which space values logarithmically so that the tolerance ranges of neighbouring values just touch or overlap.