Decoding Resistor Markings
Though they may not display their value outright, most resistors are marked to show what their resistance is. PTH resistors use a color-coding system (which really adds some flair to circuits), and SMD resistors have their own value-marking system.
Decoding the Color Bands
Through-hole, axial resistors usually use the color-band system to display their value. Most of these resistors will have four bands of color circling the resistor, though you will also find five band and six band resistors.
Four Band Resistors
In the standard four band resistors, the first two bands indicate the two most-significant digits of the resistor's value. The third band is a weight value, which multiplies the two significant digits by a power of ten.
The final band indicates the tolerance of the resistor. The tolerance explains how much more or less the actual resistance of the resistor can be compared to what its nominal value is. No resistor is made to perfection, and different manufacturing processes will result in better or worse tolerances. For example, a 1kΩ resistor with 5% tolerance could actually be anywhere between 0.95kΩ and 1.05kΩ.
How do you tell which band is first and last? The last, tolerance band is often clearly separated from the value bands, and usually it'll either be silver or gold.
Five and Six Band Resistors
Five band resistors have a third significant digit band between the first two bands and the multiplier band. Five band resistors also have a wider range of tolerances available.
Six band resistors are basically five band resistors with an additional band at the end that indicates the temperature coefficient. This indicates the expected change in resistor value as the temperature changes in degrees Celsius. Generally these temperature coefficient values are extremely small, in the ppm range.
Decoding Resistor Color Bands
When decoding the resistor color bands, consult a resistor color code table like the one below. For the first two bands, find that color's corresponding digit value. The 4.7kΩ resistor shown here has color bands of yellow and violet to begin - which have digit values of 4 and 7 (47). The third band of the 4.7kΩ is red, which indicates that the 47 should be multiplied by 102 (or 100). 47 times 100 is 4,700!
If you're trying to commit the color band code to memory, a mnemonic device might help. There are a handful of (sometimes unsavory) mnemonics out there to help remember the resistor color code. A good one, which spells out the difference between black and brown is:
Or, if you remember "ROY G. BIV", subtract the indigo (poor indigo, no one remembers indigo), and add black and brown to the front and gray and white to the back of the classic rainbow color-order.
Resistor Color Code Table
Resistor Color Code Calculator
If you'd rather skip the math (we won't judge!), and just use a handy calculator, give one of these a try!
Four Band Resistors
|Band 1||Band 2||Band 3||Band 4|
|Value 1 (MSV)||Value 2||Weight||Tolerance|
1 kΩ ±5%
|Band 1||Band 2||Band 3||Band 4||Band 5|
|Value 1 (MSV)||Value 2||Value 3||Weight||Tolerance|
1 kΩ ±5%
Decoding Surface-Mount Markings
SMD resistors, like those in 0603 or 0805 packages, have their own way of displaying their value. There are a few common marking methods you'll see on these resistors. They'll usually have three to four characters -- numbers or letters -- printed on top of the case.
If the three characters you're seeing are all numbers, you're probably looking at an E24 marked resistor. These markings actually share some similarity with the color-band system used on the PTH resistors. The first two numbers represent the first two most-significant digits of the value, the last number represents a magnitude.
In the above example picture, resistors are marked 104, 105, 205, 751, and 754. The resistor marked with 104 should be 100kΩ (10x104), 105 would be 1MΩ (10x105), and 205 is 2MΩ (20x105). 751 is 750Ω (75x101), and 754 is 750kΩ (75x104).
Another common coding system is E96, and it's the most cryptic of the bunch. E96 resistors will be marked with three characters -- two numbers at the beginning and a letter at the end. The two numbers tell you the first three digits of the value, by corresponding to one of the not-so-obvious values on this lookup table.
The letter at the end represents a multiplier, matching up to something on this table:
|Y or R||0.01||B or H||10||E||10000|
|X or S||0.1||C||100||F||100000|
So a 01C resistor is our good friend, 10kΩ (100x100), 01B is 1kΩ (100x10), and 01D is 100kΩ. Those are easy, other codes may not be. 85A from the picture above is 750Ω (750x1) and 30C is actually 20kΩ.