Resistor Calculator

A four-band resistor encodes its value the way scientific notation does: a two-digit mantissa followed by a decimal exponent. The first two bands are digits, running black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, gray 8, white 9. The third band is the power of ten to multiply by, with gold and silver extending the scale downward to 0.1 and 0.01 for sub-ohm and single-digit parts. The fourth band, set slightly apart from the rest, gives tolerance. Brown-black-red-gold therefore reads as digits 1 and 0, making 10, times 10 squared, giving 1000 ohms held to plus or minus 5 percent, so any sample between 950 and 1050 ohms is in specification. Tolerance is not sloppiness; it is what makes mass production affordable, and it is tied to the preferred-value series that stock resistors come in. The E24 series packs 24 values into each decade and pairs with 5 percent parts, while E12 packs 12 and pairs with 10 percent, because the steps are spaced so that consecutive tolerance windows cover the number line with minimal gap or overlap. That is why a parts drawer holds 4.7 k and 5.6 k but no 5.0 k in a 5 percent line. Precision parts use a five-band code with three digit bands instead of two, and where a sixth band appears it states the temperature coefficient in parts per million per degree Celsius. Note that the color code says nothing about power rating, which is read from the physical size of the body.

Early radio sets carried resistor values stamped or printed in ink, which was hard to read once a part was soldered into a crowded chassis and worse once heat and dust had aged it. American set makers working through the Radio Manufacturers Association adopted a color-marking scheme in the 1920s and 1930s to solve exactly that. The first widely used version was the body-end-dot system, in which the body color carried the first digit, one end carried the second, and a dot on the body carried the multiplier. As axial-lead components became small and uniform, that gave way to the banded arrangement still in use, with the tolerance band separated by a wider gap so the reading direction is unambiguous. The association became RETMA and then the Electronic Industries Association, and the color code was eventually folded into international standards, published today by the International Electrotechnical Commission as IEC 60062 for marking and IEC 60063 for preferred number series. The preferred-value idea is older than electronics: the French engineer Charles Renard proposed geometric series of sizes in the 1870s to cut the number of balloon cable types the army had to stock, and the same logic of geometric spacing matched to tolerance produced the E-series that resistor values follow. Surface-mount parts have since largely replaced stripes with printed three or four digit codes, but through-hole resistors still ship with bands.