Inductors use color bands, printed numbers, or letter codes to indicate their inductance value, and the system is similar to resistor color codes once you know the differences. The base unit is the henry (H), but most inductors you’ll encounter are measured in microhenries (µH), where 1 µH equals one millionth of a henry. Smaller inductors used in high-frequency circuits may be rated in nanohenries (nH), where 1,000 nH equals 1 µH.
Telling Inductors Apart From Resistors
Before you start reading bands, make sure you’re actually looking at an inductor. Axial inductors with color bands look a lot like resistors, but there are a few giveaways. Inductors tend to have a seafoam green, pea green, or cyan body color, while most resistors are tan or blue. Inductors also look physically larger in diameter for their length, almost like oversized resistors. The body often has a slightly lumpy texture because of the wire coil underneath the coating, whereas resistors have a smooth, uniform shape. The ends of an inductor’s body also tend to taper to more pointed tips.
The 4-Band Color Code
The standard 4-band system, defined by the EIA (Electronic Industries Alliance), is the most common marking on through-hole inductors. It works like this: the first two bands are digits, the third band is a multiplier, and the fourth band indicates tolerance. The result is always in microhenries (µH).
Here are the color values:
- Black: 0
- Brown: 1
- Red: 2
- Orange: 3
- Yellow: 4
- Green: 5
- Blue: 6
- Violet: 7
- Grey: 8
- White: 9
The third band (multiplier) tells you what to multiply the two-digit number by. Brown means ×10, red means ×100, orange means ×1,000, and yellow means ×10,000. Black means ×1, so the two-digit number stays as-is. Gold means ×0.1 (divide by 10), and silver means ×0.01 (divide by 100). These last two are how sub-1 µH values are marked.
For example, an inductor with brown, black, red, and gold bands reads as: 1, 0, ×100 = 1,000 µH (or 1 mH), with a tolerance of ±5%. An inductor with green, blue, gold, and silver bands reads as: 5, 6, ×0.1 = 5.6 µH, with ±10% tolerance.
The fourth band is the tolerance indicator. Gold means ±5%, silver means ±10%, and black means ±20%. If there is no fourth band at all, the tolerance defaults to ±20%.
The 5-Band Military Code
Military-specification inductors use a 5-band system that’s slightly different. The easiest way to identify one is the first band: it’s always a wide silver band, roughly double the width of the other four. This band doesn’t carry a value. It simply identifies the component as a military-rated RF inductor.
After that wide silver band, three bands indicate the inductance value in microhenries and the fifth band gives the tolerance. The three value bands can work in two ways. In the straightforward version, the first two bands are digits and the third is a multiplier, just like the 4-band system. But in some cases, a gold band in the second or third position acts as a decimal point, letting you express fractional values. For instance, if the three value bands are brown, gold, green, that reads as 1.5 µH (the gold shifts the 5 to the right of the decimal point).
The tolerance band on military inductors is more precise. Brown means ±1%, red means ±2%, orange means ±3%, yellow means ±4%, gold means ±5%, silver means ±10%, and black means ±20%.
Reading SMD Inductor Markings
Surface-mount inductors are tiny, so they use printed codes instead of color bands. The most common format is a 3-digit code that works just like SMD resistor codes: the first two digits are the value, and the third digit is the number of zeros to add. The result is in microhenries. So a marking of “101” means 10 followed by one zero, giving you 100 µH. A code of “470” means 47 followed by no zeros, or simply 47 µH.
For values below 10 µH, the letter “R” appears in the code to mark the decimal point. The R stands in for the decimal in a µH value. So “4R7” means 4.7 µH, and “R33” means 0.33 µH. For very small inductors rated in nanohenries, the letter “N” serves the same purpose. A marking of “4N7” means 4.7 nH.
Some SMD inductors also include a letter at the end to indicate tolerance. “J” means ±5%, “K” means ±10%, and “M” means ±20%. On very precise nanohenry-range inductors, you may see “D,” which indicates a tight tolerance of ±0.3 nH. Two-digit codes also exist on the smallest components, using the same R and N conventions but with fewer digits.
Converting Between Units
Inductor values span a wide range, so you’ll regularly need to convert between units. The three you’ll see most often are millihenries (mH), microhenries (µH), and nanohenries (nH). The conversions are straightforward:
- 1 mH = 1,000 µH
- 1 µH = 1,000 nH
- 1 mH = 1,000,000 nH
So if you read a 4-band inductor and get 220 µH, that’s the same as 0.22 mH. If an SMD code gives you 4.7 nH, dividing by 1,000 gets you 0.0047 µH. In practice, most color-coded through-hole inductors fall in the 1 µH to 10 mH range, while SMD inductors often sit in the nanohenry to low microhenry range.
Which Direction to Read
On a 4-band inductor, the tolerance band is usually spaced slightly farther from the other three, just like on resistors. Read from the opposite end. If the inductor has a gold or silver band, that’s almost always the tolerance band (the last one you read), since gold and silver don’t appear as the first digit. On military 5-band inductors, orientation is even easier: start from the wide silver band and read left to right.
For SMD inductors, the code is simply printed text. If you can’t read the markings with the naked eye, a magnifying glass or phone camera with zoom will help. On extremely small packages, some manufacturers skip markings entirely, in which case you’ll need a multimeter with an inductance measurement mode or the component’s datasheet to identify the value.