Resistor Color Code Calculator & Decoder

Four stripes of paint on a component a few millimetres long have to tell you its resistance and how far it may be off. Click the colours you see on each band and this Resistor Color Code Calculator reads them back as a value, a tolerance and the range the part is guaranteed to fall in.

It works in reverse as well. Type a value in the lookup box and press Find Bands to set the colours. Values can be written plainly (4.7k, 220) or in the RKM style printed on schematics, where the unit letter replaces the decimal point: 4k7, 2R2, R22.

Reading Example: Yellow, Violet, Red, Gold

The calculator opens on a 4-band resistor painted yellow, violet, red and gold:

• Yellow is 4 and violet is 7, giving the digits 47
• Red as the multiplier means ×100, so the value is 4.7 kΩ
• Gold tolerance is ±5%, so the part measures between 4.465 kΩ and 4.935 kΩ
• The RKM code is 4K7, and 4.7 kΩ is flagged as an E12 preferred value

Load Sample continues with a 5-band 10 kΩ ±1%, a 6-band 3.32 kΩ with a 50 ppm/K temperature band, and a gold multiplier that makes red-red-gold read 2.2 Ω.

Tricky Inputs: 4k7, R22 and Two-Digit Rounding

A 4-band code holds only 2 significant digits. Switch the 3.32 kΩ sample from 6 bands to 4 and the value is re-encoded as 3.3 kΩ, with a note saying what changed. Going the other way keeps the value and adds a third digit. The smallest markable value is 0.01 Ω (a silver multiplier) and the largest is 999 × 10⁹ Ω with 5 or 6 bands.

A lowercase m means milliohms, as in SI, so use a capital M for megohms. Text that is not a resistance gets a specific reply, for example “"abc" is not a resistance. Use digits with an optional k, M or G (4.7k, 1M) or RKM style (4k7, 2R2).” When a value is not in the E12, E24 or E96 series, the result names the nearest E24 value instead.