Resistor Color Code Calculator

How the resistor colour code works

When to use this: you have a resistor in your hand and need to know what it is, or you know the value you need and want to find it in a drawer of loose parts. Either direction works above.

Resistors are too small to print numbers on, so the value is encoded as coloured bands. Each colour maps to a digit, and one band acts as a power-of-ten multiplier. The scheme is defined by IEC 60062 and has been essentially unchanged since the 1920s, which is why a resistor from a 1960s radio reads the same way as one bought today.

On a four-band part the first two bands are significant digits, the third multiplies them, and the fourth states the tolerance. Yellow-violet-red-gold is 4, then 7, then ×100, then ±5% — so 47 × 100 = 4,700 Ω, or 4.7 kΩ, and the real part is somewhere between 4.465 kΩ and 4.935 kΩ.

That last point is the one people skip. The colour code tells you the nominal value. What you actually soldered onto the board is anywhere inside the tolerance band, and your circuit has to work across that whole range.

The colour chart

Gold and silver never appear as digits. That is deliberate, and it is the single most useful fact in the table: if one end of the resistor is gold or silver, that end is the tolerance band, and you read from the other end.

Which way round do I read it?

Three clues, in order of reliability:

• Gold or silver marks the tolerance end. Neither is a valid digit, so a gold band can only be tolerance (or a multiplier, but never the first band).
• The gap. On most parts the tolerance band is printed slightly further from the group than the digit bands are from each other.
• Standard values. If both readings look plausible, work out both. One will be an E-series preferred value and the other almost certainly will not. Resistors are only manufactured in E-series values, so the standard one is correct.

That third check is worth internalising. Brown-black-red-gold read backwards gives gold-red-black-brown, which is not a valid code at all — so there is only one legal reading. Ambiguity is rarer than it feels.

4-band, 5-band and 6-band parts

The extra digit on a 5-band part is not just precision theatre. A 4-band part can only represent two significant figures, so the closest it gets to 4.75 kΩ is 4.7 kΩ. If your divider needs 4.75 kΩ, you need a 5-band part — the code physically cannot express it otherwise.

Worked examples

Reading a 4-band resistor: Orange, Orange, Brown, Gold

Orange = 3, Orange = 3, Brown = ×10, Gold = ±5%
33 × 10 = 330 Ω ±5%
Range: 313.5 Ω to 346.5 Ω

330 Ω is the classic LED series resistor for a 5 V supply — it gives roughly 9 mA through a red LED, which is bright enough for an indicator without wasting current.

Reading a 5-band resistor: Brown, Black, Black, Brown, Brown

Brown = 1, Black = 0, Black = 0, Brown = ×10, Brown = ±1%
100 × 10 = 1 kΩ ±1%
Range: 990 Ω to 1.01 kΩ

Note that the same 1 kΩ value appears as brown-black-red on a 4-band part and brown-black-black-brown on a 5-band. The digit count changes where the multiplier lands — a common source of confusion when you switch between part types.

Tolerance is not decoration

A ±5% resistor in a voltage divider means the output ratio can be off by up to 10% in the worst case, because both resistors can drift in opposite directions. If you need the divider accurate to 1%, ±5% parts cannot deliver it no matter how carefully you pick nominal values.

Where tolerance matters less: current-limiting resistors for LEDs, pull-ups, base resistors for saturated transistor switches. In all of those the exact value is uncritical, and ±5% parts are the right economic choice.

Where it matters a lot: voltage dividers feeding an ADC, gain-setting resistors around op-amps, current-sense shunts, and anything setting a regulator's output voltage. Use ±1% metal film, and consider a matched pair if the ratio matters more than the absolute values.

SMD resistors use numbers, not colours

Surface-mount resistors are printed with digits instead, because there is no room for bands and no reliable way to see them once the board is populated:

The three-digit scheme is used on ±5% parts and the four-digit on ±1%, mirroring the 4-band/5-band split. The RKM code (4R7) also turns up in schematics and BOMs because it survives sloppy printing where a decimal point would be lost.

Why resistors only come in odd-looking values

4.7 kΩ, 3.3 kΩ, 6.8 kΩ — the values look arbitrary but they are not. They come from the IEC 60063 E-series, which spaces values geometrically so that consecutive values differ by roughly the tolerance. E24 (±5%) has 24 values per decade, each about 10% above the last, so any target value is within 5% of a stocked part.

This is why designing around 3,847 Ω is pointless. Pick the nearest E-series value early, then check whether the resulting error matters. Usually it does not; when it does, that is your signal to move to E96 and ±1% parts.