How to Decode the Numbers on SMD Resistors

A chip resistor has a flat top only a millimeter or two long, which leaves no room for color bands. Most sizes carry a short printed code instead, and the smallest sizes carry nothing. Three marking systems are in common use, and you can tell them apart from the characters on the part before you start any arithmetic.

How to tell which system you are looking at

Count the characters and look for letters:

• Three digits, such as 103 or 472: the standard 3-digit code, usually on ±5% parts.
• Four digits, such as 1002 or 4992: the 4-digit code, usually on ±1% parts in 0805 and larger packages.
• Two digits followed by a letter, such as 01C or 68X: the EIA-96 code, used on ±1% parts too small for four characters.
• An R between digits, such as 4R7 or R47: the R marks the decimal point, and the value is in ohms.
• A single 0, or 000: a zero-ohm jumper.

The physical look helps too. Thick film chip resistors usually have a black top over a white or grey ceramic body, with silver end terminations. Ceramic capacitors of the same size are normally a uniform tan, grey or brown color with no marking at all, so an unmarked beige chip is more likely a capacitor than a resistor.

Three-digit codes

The first two digits are the significant figures, and the third digit is the number of zeros to add. The result is in ohms.

• 103 = 10 followed by three zeros = 10,000 Ω = 10 kΩ
• 472 = 47 followed by two zeros = 4,700 Ω = 4.7 kΩ
• 221 = 22 followed by one zero = 220 Ω
• 220 = 22 followed by no zeros = 22 Ω
• 100 = 10 followed by no zeros = 10 Ω

The last two catch a lot of people. A part printed 100 is a 10 Ω resistor, not 100 Ω; a 100 Ω part is marked 101. The logic is identical to the multiplier band of a color code, so a 103 chip has exactly the same value as a through-hole resistor banded brown, black, orange.

Values below 10 Ω cannot be written with a zero count, so an R stands in for the decimal point. 4R7 is 4.7 Ω, 1R0 is 1 Ω and R47 is 0.47 Ω. Schematics use the same idea with k and M: 4k7 means 4.7 kΩ and 1M5 means 1.5 MΩ. Putting a letter where the decimal point goes means a smudged or badly photocopied drawing cannot lose the point and turn 4.7 kΩ into 47 kΩ.

Four-digit codes

Precision values need three significant figures, so the 4-digit code uses the first three digits as the value and the fourth as the number of zeros.

• 1002 = 100 followed by two zeros = 10,000 Ω = 10 kΩ
• 4992 = 499 followed by two zeros = 49,900 Ω = 49.9 kΩ
• 4701 = 470 followed by one zero = 4,700 Ω = 4.7 kΩ
• 1000 = 100 followed by no zeros = 100 Ω
• 49R9 = 49.9 Ω, and R100 = 0.1 Ω

The trap is the same as before, moved one digit along: 1000 is 100 Ω, not 1 kΩ. A 1 kΩ precision part is marked 1001. The 49.9 kΩ part above would carry five color bands if it were a through-hole resistor:

EIA-96 codes

An 0603 resistor measures 1.6 × 0.8 mm, and four legible characters barely fit on its top. The EIA-96 system squeezes a ±1% value into three characters. The two digits are not the value itself; they are an index from 01 to 96 into the E96 series of standard values. The letter is a multiplier.

To decode, look up the index, then apply the letter:

• 01C: index 01 is 100 and C is ×100, so 100 × 100 = 10 kΩ.
• 39D: index 39 is 249 and D is ×1,000, so 249 kΩ.
• 68X: index 68 is 499 and X is ×0.1, so 49.9 Ω.
• 96B: index 96 is 976 and B is ×10, so 9.76 kΩ.

The index cannot be worked out in your head, because it counts steps along a geometric series rather than spelling out digits. Keep the EIA-96 code table open, or type the code into the SMD resistor code calculator. One more caution: R is a valid EIA-96 multiplier letter. A code such as 68R read as EIA-96 means 499 × 0.01 = 4.99 Ω, while the R-as-decimal-point reading would suggest 68 Ω. A 68 Ω ±5% chip would normally be marked 680, so when a code could be read two ways, measure the part.

Jumpers, underlined codes and unmarked parts

Zero-ohm jumpers

A chip marked 0, 00 or 000 is a zero-ohm resistor: a jumper in a resistor package. Designers use them to hop one trace over another on a single-layer board, to select options at assembly time, or as a placeholder where a real resistor might go later. Their resistance is not exactly zero; datasheets typically guarantee a maximum of a few tens of milliohms. They are rated by maximum current rather than by power, and that figure depends on the package, so look it up before routing a motor supply through an 0402 jumper.

Underlined codes

Some codes are printed with a bar underneath, and manufacturers do not all use it the same way. One common meaning is a precision part, ±1% or better, whose value happens to fit the 3-digit format: an underlined 102 would be 1 kΩ ±1% rather than ±5%. Other makers use the bar only to show which way up to read the code, since 106 (10 MΩ) turns into 901 when the part is rotated. Treat the underline as a hint and confirm with the datasheet or a meter.

Unmarked parts: 0402 and smaller

An 0402 resistor is 1.0 × 0.5 mm and an 0201 is 0.6 × 0.3 mm. There is nothing practical to print on, so these are almost always unmarked, and many 0603 parts from volume suppliers are unmarked too. The value lives on the reel label and in the board's bill of materials. A few habits prevent mix-ups:

• Keep loose parts in their cut tape and label each strip the moment you cut it.
• Never tip two unmarked values into the same compartment. Once they are mixed, every part has to be measured.
• Measure with SMD tweezers, or press the meter probes onto the end caps with the chip resting on a piece of card.
• On a board you are repairing, read the value from the schematic or measure the matching part on a known-good board. An in-circuit reading includes everything else connected to those pads.

Package sizes and typical power ratings

SMD package names describe length and width. The common names are imperial, in hundredths of an inch, but metric names also exist and overlap confusingly: an imperial 0603 is metric 1608, while a metric 0603 is the much smaller imperial 0201. Check which system a parts list uses before you order.

These are common figures for general-purpose thick film parts, but ratings vary by manufacturer and product line. High-power ranges can rate an 0603 at 1/5 W or more, while some precision thin film parts are rated lower than thick film in the same size. The rating also assumes the recommended pad layout, because a chip resistor sheds most of its heat through its solder joints into the copper; thin traces mean a hotter part. For hand soldering, 0805 is comfortable, 0603 is manageable with fine tweezers, and 0402 really needs magnification. The resistor power rating guide shows how to work out the dissipation, and the Ohm's law calculator handles the arithmetic.

A quick decoding checklist

1. All zeros: zero-ohm jumper.
2. An R between digits: the R is the decimal point, value in ohms.
3. Three digits: two significant figures plus a count of zeros.
4. Four digits: three significant figures plus a count of zeros.
5. Two digits and a final letter: EIA-96, so look up the index and apply the letter.
6. No marking: measure out of circuit or check the parts list.

Whatever the code says, sanity-check the result against the list of standard resistor values. A 4-digit code that decodes to 49.9 kΩ lands on a real E96 value; one that decodes to 49.5 kΩ does not, which usually means a 9 and a 5 were confused on a worn print. For a sense of where a decoded value is typically used, the 10 kΩ and 4.7 Ω value pages list common applications along with every marking for those values.

Decode an SMD resistor code →