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Reading a Resistor's Value

Through-hole (PTH) Axial Resistor Colored Bands

Axial resistors, the kind that is most often used when prototyping, have color coded bands that specify the resistance value and tolerance of the resistor. They usually come in 4 or 5 band flavors, with 5 band resistors being more precise.

To read a banded resistor, align it so that the three (or four) closely spaced bands are on the left, and the furthest spaced band is on the right. Then, use the following chart to determine the values from left to right:

4-band resistor color chart

The first 2 (or 3 bands, on a 5-band resistor) specify the resistance value, and the 3rd (or 4th) specify the magnitude multiplier.

Sample Reading

For example, the resistor shown above has the following values:

  • Brown = 1
  • Blue = 6
  • Black = 0
  • Orange = 1k Magnitude Multiplier
  • Gold = 5% Tolerance

Therefore, the resistor has 160Ω * 1k = 160kΩ, with a tolerance of ±5%.

Surface Mount (SMD)

There are 2 primary methods of labeling SMD resistors.

The first uses 3 numbers such as 424, this can be understood as 42 104 = 42 x 10,000 = 420kΩ. If the resistor were labeled as 420 it would be 42 100 = 42 * 1 = 42Ω.

The second method, EIA Standard, uses 2 numbers and a letter such as 66B and this will require the tables below to decode.

The numbers are a lookup code.

CODEValueCODEValueCODEValueCODEValue
01100251784931673562
02102261825032474576
03105271875133275590
04107281915234076604
05110291965334877619
06113302005435778634
07115312055536579649
08118322105637480665
09121332155738381681
10124342215839282698
11127352265940283715
12130362326041284732
13133372376142285750
14137382436243286768
15140392496344287787
16143402556445388806
17147412616546489825
18150422676647590845
19154432746748791866
20158442806849992887
21162452876951193909
22165462947052394931
23169473017153695953
24174483097254996976

The letter serves as a multiplier against the values above.

LetterMultiplier
Z0.001
Y/R0.01
X/S0.1
A1
B/H10
C100
D1 000
E10 000
F100 000

So, that 66B has a value of 475 in the first table and a multiplier of 10 from the second table, which would be 475 * 10 = 4,750Ω.