Resistance value marking methods
There are many types of Resistor available in the market which can be used in electrical and electronic circuits to control the flow of current. All resistors have some of "resistive" or "resistance" value. Resistors are available in a range of different resistance values from fractions of an ohm to millions of ohms.
Every resistor has a unique value of resistance, watt, and tolerance value. These values are marked on a resistor in different ways. Few methods are explained below.
Color Code Method
The resistance value is generally printed onto the body of the resistor as numbers or letters when the resistors body is big enough to read the print, such as large power resistors. But when the resistor is small such as a 1/4 watt carbon or film type, it's difficult to print. For that we are using color bands an alternate to print. So that is called the color code method. The below figure is shown that how the resistance value is marked in 4 bands and 5 bands color code methods.
The color order can be remembered by memorize the phrase "B.B. ROY of Great Britain has a Very Good Wife".
SMD Resistor Code Methods
Because of the small size of SMD resistors, there is often not space for the color code method to be printed on them. Therefore, new resistor SMD codes were developed. The most commonly seen code methods are the three digit and fourdigit code method and an Electronic Industries Alliance (EIA) system called EIA96.
 Three Digit SMD resistor Code Method

A three digit SMT resistor coding method is the one that is normally used for standard tolerance resistors. As the name indicates this SMD resistor marking system uses three digits. The first two digits in the code indicate the significant figures, and the third digit is a multiplier.
Example:
The SMD resistor with the digits 103 would have a resistance of 10 x 10^{3} ohms, or 10 kΩ. However, beware of resistors marked with digits such as 470. This is not 470 ohms, but it follows the scheme exactly, and it is 47 x 10^{0} or 47 x 1 = 47 Ω.
If the resistance value is in below 10 Ω, the letter R is used to denote the decimal point. For example, a resistor denoted as 4R7, then the value would be 4.7 Ω.
 Four Digit SMD resistor Code Method

The Four digit SMT resistor coding method is the one that is normally used for High tolerance resistors. This SMD resistor marking system uses four digits. The first three digits in the code indicate the significant figures, and the fourth digit is a multiplier.
Example:
The SMD resistor with the digits 1002 would have a resistance of 100 x 10^{2} ohms, or 10 kΩ. However, beware of resistors marked with digits such as 4700. This is not 4700 ohms, but it follows the scheme exactly, and it is 470 x 10^{0} or 470 x 1 = 470 Ω.
If the resistance value is in below 100 Ω, the letter R is used to denote the decimal point. For example, a resistor denoted as 47R0, then the value would be 47 Ω.
 EIA96 or E96 SMD resistor Code Method

A further surface mount resistor code scheme has started to be used, and it is aimed at 1% tolerance SMD resistors, i.e. those using the EIA96 or E96 resistor series. As higher tolerance resistors are used, further figures are needed. However, the small size of SMT resistors makes the figures difficult to read. Accordingly, the new system seeks to address this. Using only three figures, the actual characters can be made larger than those of the four figure system that would otherwise be needed.
The EIA SMD resistor coding scheme uses a three character code: the first 2 numbers indicate the 3 significant digits of the resistor value. The third character is a letter which indicates the multiplier. In this way this SMD resistor marking scheme will not be confused with the 3 figure markings scheme as the letters will differentiate it, although the letter R can be used in both systems. There are only 96 values in the E96 series, only two figures are needed to number each value, and as a result this is a smart way of reducing the number of characters required.
The table shows EIA SMD resistor code and corresponding value:
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 Code Multiplier Z 0.001 Y or R 0.01 X or S 0.1 A 1 B or H 10 C 100 D 1000 E 10000 F 100000 Examples:
01Y = 100 x 0.01 = 1Ω
68X = 499 x 0.1 = 49.9Ω
76X = 604 x 0.1 = 60.4Ω
01A = 100 x 1 = 100Ω
29B = 196 x 10 = 1.96kΩ
01C = 100 x 100 = 10kΩ
Power rating of the SMD resistor
The Approximate Power Rating Value can be calculated by its dimension. A few commonly used package dimensions with the corresponding typical power ratings are presented in the table below.
Package  Size in inches (LxW)  Size in mm (LxW)  Power rating 

0201  0.024" x 0.012"  0.6 mm x 0.3 mm  1/20W 
0402  0.04" x 0.02"  1.0 mm x 0.5 mm  1/16W 
0603  0.063" x 0.031"  1.6 mm x 0.8 mm  1/16W 
0805  0.08" x 0.05"  2.0 mm x 1.25 mm  1/10W 
1206  0.126" x 0.063"  3.2 mm x 1.6 mm  1/8W 
1210  0.126" x 0.10"  3.2 mm x 2.5 mm  1/4W 
1812  0.18" x 0.12"  4.5 mm x 3.2 mm  1/3W 
2010  0.20" x 0.10"  5.0 mm x 2.5 mm  1/2W 
2512  0.25" x 0.12"  6.35 mm x 3.2 mm  1W 
These are estimated power rating only. For accurate power rating value, read the datasheet of SMD resistor.
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