Resistor Colour Code Calculator

What the bands actually say

A resistor is too small to print a number on and too cheap to justify trying. The colour code solves that by turning the value into a short sentence of coloured rings, and once you see it as a sentence rather than as a lookup, it stops needing to be memorised.

Every code says the same three things in the same order: here are the significant figures, then here is the decade to put them in, then here is how far off I am allowed to be. Two digits or three, one multiplier, one tolerance. Nothing else changes between a three-band part and a six-band one — the extra bands only add precision at the front and a temperature figure at the back.

The digits run black, brown, red, orange, yellow, green, blue, violet, grey, white — zero to nine. That is the visible spectrum with black at the dark end and white at the bright end, which is why the sequence is learnable at all: it is not an arbitrary assignment, it is the order light comes in.

Which end do you start from?

This is the part that actually catches people, and no chart helps with it. Read a resistor backwards and 1 kΩ becomes 100 Ω, which will not blow anything up but will quietly halve the brightness of your LED and leave you doubting the rest of the circuit.

Three tells, in the order they are worth checking:

• The gap. The tolerance band sits slightly further from its neighbours than the others do. On a good part it is obvious; on a small one, hold it up to the light and look at the spacing rather than the colours.
• Gold and silver are terminal. Neither carries a digit, so neither can be the first band. If you can see gold or silver at one end, that end is the finish — start from the other.
• Black is never first. A leading zero means nothing, so no manufacturer prints one. Black at one end is a multiplier, which again puts you at the wrong end.

If all three fail you — and on a faded four-band part with a brown tolerance band they can — the answer is a meter, not a harder stare. See below.

Three, four, five and six bands

The band count is a rough proxy for how tightly the part is specified, because it takes a third significant figure to describe anything better than about 2 %.

A four-band part reading ±1 % is worth a second look. Tight tolerances are nearly always printed with five bands, so a four-band brown tolerance usually means you are holding a five-band resistor and have read it from the wrong end. The calculator says so when you produce that combination.

A worked example

Yellow, violet, red, gold — the most common resistor in the world, and the one loaded into the calculator when the page opens.

Yellow = 4      first digit
Violet = 7      second digit
Red    = ×100   multiplier
Gold   = ±5%    tolerance

47 × 100 = 4700 Ω = 4.7 kΩ
Anything from 4465 Ω to 4935 Ω is in spec.

Note how wide that acceptable range is. Nearly half a kilohm of legitimate spread on a part sold as 4.7 kΩ — which is fine in a pull-up and disastrous in a divider setting a reference voltage. The tolerance band is the specification, not the small print.

Why 4.7 kΩ and not 5 kΩ

Beginners ask for round numbers and find they cannot buy them. The reason is worth knowing, because it explains the whole catalogue.

Preferred values — the E-series — divide each decade into equal ratio steps rather than equal arithmetic ones, and the number of steps is chosen so that consecutive values just touch at their tolerance limits. E24, the ±5 % series, has 24 values per decade spaced about 10 % apart, so a ±5 % part at one step reaches halfway to the next. Every possible resistance is covered by something in stock, with no gaps and almost no overlap.

That is why the series and the tolerance always arrive together: ±10 % parts come in E12, ±5 % in E24, ±1 % in E96. Stocking ±10 % parts in E96 would mean ninety-six values whose tolerance bands overlap five deep — paying for precision the specification throws away. The calculator shows the nearest catalogue value for whatever you type, and how far from it you are.

What the code does not tell you

Three things matter as much as the value and none of them are in the bands.

Power rating. Nowhere in the colour code. It is carried by physical size, and you learn it by eye: roughly 3 mm long for an eighth of a watt, 6 mm for a quarter, 9 mm for a half, 11 mm and up for one watt. A 1 kΩ quarter-watt and a 1 kΩ two-watt carry identical bands and differ by a factor of eight in what they will survive. Work out the dissipation with Ohm's law before choosing the body size, not after the first one turns brown.

Tolerance is not accuracy. It is the spread at the factory, at room temperature, before anything has happened to the part. Add the temperature coefficient — 250 ppm/K on ordinary carbon film is 2.5 % over a 100 K rise — then self-heating under load, then drift with age and humidity. A ±1 % resistor running warm in a hot enclosure is not a ±1 % resistor any more, which is exactly why the sixth band exists.

All-black is not a resistor. A single black band on its own is a zero-ohm link: a wire jumper shaped like a component so a pick-and-place machine can fit it. They exist to let one board be built in several configurations, and they read as a dead short because that is what they are.

Reading bands that will not be read

Colour codes were designed for parts on a bench under good light, and that is not always where you meet them.

Brown, red and orange are the problem trio on a small body under warm lighting, and the tolerance ring being gold does not help because gold under tungsten light looks like yellow. Daylight or a cool white LED torch separates them; a phone camera often does better than your eye, because its white balance is not being fooled by the same light.

Never trust a part that has been hot. Overheating darkens the body and shifts the pigments, and it turns brown towards red and red towards black — errors of exactly one decade. A resistor that has cooked has also very likely drifted in value, so the bands are the least of it. Measure it, and then work out why it got hot.

In circuit, a meter reads low. Everything in parallel with the resistor reads with it. Lift one leg before believing a measurement, or you will condemn a good part because the rest of the board is sitting across it.

The full table

Every column below drives the calculator above — same data, so the two can never disagree.

Orange and yellow show no tolerance. Some charts assign them ±0.05 % and ±0.02 %, but those grades are laboratory items you will not meet, and treating an orange ring as a tolerance band is a good way to decode a resistor that is simply the wrong way round.

Surface mount, briefly

SMD resistors are numbered, not coloured, and the numbering is the colour code in disguise. 472 is 47 followed by two zeros: 4.7 kΩ, the same digits-then-multiplier grammar. Four digits means three significant figures — 4701 is 470 followed by one zero, again 4.7 kΩ, this time from a ±1 % part. Where an R appears it marks the decimal point, so 4R7 is 4.7 Ω. The EIA-96 code on the smallest packages abandons the pattern entirely and uses two digits and a letter, which needs a lookup table and defeats everyone.

When to stop reading and start measuring

The colour code identifies a part. It does not verify one. If the circuit misbehaves, measure the resistor rather than re-reading it — the bands tell you what the factory intended, and a meter tells you what you have. That distinction covers faded parts, hot parts, counterfeit parts, and the ordinary case of a resistor sitting quietly at the edge of its tolerance while you stare at the schematic.