Through Hole (PTH)

4-band resistor color chart

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 = 42x10,000 = 420k Ohms. If the resistor were labeled as 420 it would be 42 * 100 = 42 * 1 = 42 Ohms.

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 = 4,750 Ohms.