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:

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.

CODE	Value		CODE	Value		CODE	Value		CODE	Value
01	100		25	178		49	316		73	562
02	102		26	182		50	324		74	576
03	105		27	187		51	332		75	590
04	107		28	191		52	340		76	604
05	110		29	196		53	348		77	619
06	113		30	200		54	357		78	634
07	115		31	205		55	365		79	649
08	118		32	210		56	374		80	665
09	121		33	215		57	383		81	681
10	124		34	221		58	392		82	698
11	127		35	226		59	402		83	715
12	130		36	232		60	412		84	732
13	133		37	237		61	422		85	750
14	137		38	243		62	432		86	768
15	140		39	249		63	442		87	787
16	143		40	255		64	453		88	806
17	147		41	261		65	464		89	825
18	150		42	267		66	475		90	845
19	154		43	274		67	487		91	866
20	158		44	280		68	499		92	887
21	162		45	287		69	511		93	909
22	165		46	294		70	523		94	931
23	169		47	301		71	536		95	953
24	174		48	309		72	549		96	976

The letter serves as a multiplier against the values above.

Letter	Multiplier
Z	0.001
Y/R	0.01
X/S	0.1
A	1
B/H	10
C	100
D	1 000
E	10 000
F	100 000

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.