How to Read a Resistor's Color Bands

Colour codes were put on resistors so a part could still be identified after the printing had worn off. They work perfectly well until you are holding a small brown cylinder under a desk lamp trying to decide whether that stripe is red or orange.

How the bands turn into a number

Every colour has a digit. Black is 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9. The multiplier band uses the same colours as powers of ten, so black multiplies by 1, brown by 10, red by 100, orange by 1,000 and so on. Gold and silver appear only as multipliers of 0.1 and 0.01, or as tolerance.

Tolerance has its own short list: brown is plus or minus 1%, red 2%, gold 5% and silver 10%. A missing fourth band, which you will only meet on very old stock, meant 20%.

Four bands versus five

A five-band resistor gives three digits instead of two, then the multiplier, then tolerance. Brown, green, black, brown, brown reads as digits 1, 5, 0, so 150, multiplied by 10, giving 1,500 ohms at plus or minus 1%. The extra digit is there because precision parts need values the two-digit system cannot express, such as 4,990 ohms.

This is also where the mistakes happen. That example has a brown stripe at each end and no gold anywhere, so it is easy to take it for a four-band part, read brown, green, black as 15 ohms, and land a factor of a hundred away from the 1,500 it really is.

What tolerance actually costs you

Take a 4.7k resistor. At plus or minus 5% the real part is somewhere between 4,465 and 4,935 ohms. At 1% the window narrows to 4,653 to 4,747. For an LED series resistor that spread changes nothing you can see. For a divider feeding an analogue input, or the feedback pair setting a regulator's output voltage, it moves the result by the same percentage, which is usually the moment people start buying 1% parts. If percentage ranges are the part you find fiddly, our guide to calculating percentages covers the arithmetic.

Which end do you start from

The tolerance band is usually gold or silver and sits slightly further from its neighbours than the others do. Start at the opposite end.

When both ends look similar, use the fact that gold and silver cannot be digits. Yellow, violet, red, gold reads as 4,700 ohms. Read from the other end and it starts with gold, which is not a valid first digit, so there is only one legal direction.

If both directions are legal, check which one lands on a value that is actually manufactured. Standard parts follow the E-series, so 4.7k, 10k, 22k and 47k exist while 3.9k exists and 3.8k does not. One direction almost always produces a stocked value and the other produces something nobody sells.

Using the resistor calculator

The tool runs in both directions, and which one you want depends on whether you are holding a part or designing with one.

Bands to value. Set the band count first, since 4, 5 and 6 band parts read differently, then pick each colour from the dropdowns in order. The output gives the nominal resistance plus the minimum and maximum allowed by the tolerance.

Value to bands. Type the resistance you want and the tool returns the stripe sequence, which is useful when you are pulling parts from a drawer of loose stock.

Take those minimum and maximum figures into your circuit sums rather than the nominal value if the design is sensitive; the worst-case current through a load follows straight from Ohm's law with the extreme value substituted in.

Common questions

My resistor has six bands. What is the last one? Temperature coefficient, in parts per million per degree Celsius, not a second tolerance. Brown is 100 ppm, red 50, orange 15. It only matters in precision analogue work where the board gets hot.

How do I tell red from orange under bad light? Hold it against a known part, or stop guessing and measure it. A multimeter on the resistance range settles it in two seconds, as long as the resistor is out of circuit, because anything in parallel with it on the board will read low.

My calculated value does not seem to be sold anywhere. Standard resistors come in fixed steps, so you cannot buy exactly 3,750 ohms in a common series. Pick the nearest stocked value, or put two in series or parallel to land closer.

Does the body colour mean anything? Loosely. Beige or tan bodies are usually carbon film, blue tends to be metal film, and green often means a fusible or flameproof part. It is a hint about the type, never a substitute for reading the bands.