This resistor calculator decodes the coloured bands on a resistor into its resistance, tolerance, and where applicable its temperature coefficient — and works the other way too, from a value to the band colours. It supports 3, 4, 5, and 6-band resistors, the formats you meet in everyday electronics.
Reading the colour code
Each colour maps to a digit: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9. On a 4-band resistor the first two bands are significant digits, the third is a power-of-ten multiplier, and the fourth is the tolerance (commonly gold ±5% or silver ±10%). A 5-band resistor adds a third significant digit for precision parts, and a 6-band adds a temperature coefficient. The calculator assembles these into the resistance in ohms and back again.
Reading direction is the usual stumbling block: the tolerance band (often gold or silver) goes on the right, and there is normally a slightly wider gap before it. Hold the resistor that way and read left to right. The calculator removes the ambiguity by letting you pick each band's colour and showing the resulting value.
Why the value and tolerance matter
Resistors come in standard values (the E12 and E24 series) rather than any arbitrary number, so a calculated design value is rounded to the nearest standard part. Tolerance tells you how far the real resistance may stray from the marked value, which matters in precise circuits like filters and references where a 5% part may be too loose.
A 4-band resistor reads brown, black, red, gold.
- Brown = 1, black = 0 → first two digits 10.
- Red multiplier = ×100.
- Value = 10 × 100 = 1000 Ω = 1 kΩ; gold = ±5% tolerance.
The resistor is 1 kΩ ±5% (brown-black-red-gold).
