SMD Resistor Coding Explained with Examples

• Understand the different coding schemes for SMD resistors: 3-digit, 4-digit, and EIA-96.
• Learn how to decode values and multipliers, including special notations like “R” and milli-ohm letters.
• Discover practical examples that make reading SMD resistor codes straightforward.

When I first encountered SMD resistors, also known as Surface Mounted Device resistors, I found them quite baffling. Unlike the familiar color bands on traditional resistors, these tiny chip resistors only have numeric or alphanumeric codes printed on their surface. This makes guessing their value at a glance pretty much impossible, especially because their size is so small. But once I understood their coding system, it actually became a simple decoding exercise.

SMD resistors are compact, cost-effective, and crucial in modern electronics. They changed how PCBs are designed by allowing components to be mounted on both sides, which is a massive space saver. However, due to their miniaturized size, SMD resistors don't use the old color band system. Instead, they use specially devised codes to represent resistance values and tolerances.

Three-Digit SMD Resistor Coding

The 3-digit code is the most common and straightforward system you'll come across, especially in general-purpose resistors. It works like this:

• The first two digits represent the significant numbers of the resistance.
• The third digit is the multiplier expressed as a power of ten.

For example:

• Code 102 means 10 × 10² = 1,000 ohms (1 kΩ).
• Code 470 means 47 × 10⁰ = 47 ohms.
• Code 395 means 39 × 10⁵ = 3.9 MΩ (Mega-ohms).

Sometimes, you'll see the letter "R" used where a decimal point is implied, especially for values under 10 ohms:

• 4R7 means 4.7 ohms.
• R54 means 0.54 ohms.

Also, if a short bar is marked underneath the code, that usually indicates a tighter tolerance of ±1%. For example, 122 with an underline equals 1.2 kΩ with ±1% tolerance.

Four-Digit SMD Resistor Coding

The 4-digit code is similar to the 3-digit scheme but offers greater precision. Here’s how it works:

• The first three digits indicate the resistance significant figures.
• The fourth digit is the power-of-ten multiplier.

For example:

• Code 1001 means 100 × 10¹ = 1,000 ohms (1 kΩ).
• Code 5405 means 540 × 10⁵ = 54 MΩ.

Decimal points are again shown with the letter "R":

• 57R6 indicates 57.6 ohms.
• 0R45 means 0.45 ohms.

Low-value resistors used in current sensing sometimes use letters like 'M', 'm', or 'L' to denote values in milli-ohms (mΩ):

• 1M50 means 1.50 milli-ohms.
• 5L00 means 5 milli-ohms.

Current sensing resistors occasionally have long bars over or under their codes to emphasize either the milli-ohm value or to omit certain letters due to space constraints.

The EIA-96 Coding System

For high-precision SMD resistors (1% tolerance), the EIA-96 code uses a different format made up of two digits followed by one letter. It may seem confusing at first but is quite systematic:

• The first two digits correspond to a numerical code from a standard E96 series table.
• The last letter denotes a multiplier (power of 10).

For example:

• 01A equals 100 × 1 = 100 ohms.
• 38C equals 243 × 100 = 24.3 kilo-ohms.
• 92Z equals 887 × 0.001 = 0.887 ohms.

Since the code references external tables, you’ll want to keep an EIA-96 lookup handy when working with these.

Other Notes About SMD Resistors

• Zero-ohm resistors appear as “0” or a single digit zero and act as a jumper or short circuit on the board.
• To estimate the power rating of an SMD resistor, measure its package dimensions—standard sizes correlate with certain wattage ratings, but always check datasheets for accuracy.

Quick Reference: Decoding Examples

Code	Interpretation	Resistance Value
273	27 × 10³	27 kΩ
7992	799 × 10²	79.9 kΩ
4R7	4.7 ohms	4.7 Ω
0R5	0.5 ohms	0.5 Ω
38C	EIA-96: 243 × 100	24.3 kΩ
92Z	EIA-96: 887 × 0.001	0.887 Ω

Mastering these codes can feel a bit like learning a secret language. But once familiar, reading the resistance values off those tiny SMD resistors is quick and reliable. For anyone working with surface mount technology or designing PCBs, understanding this is essential.

If you want to dive deeper, check out resources like [Digikey’s SMD resistor code calculator](https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-smd-resistor-code) or the [PCB Basics guide to SMD resistors](https://www.pcbasic.com/blog/smd_resistors.html).

FAQ

Q: Why don’t SMD resistors use color coding?
A: Their small size makes it impossible to print traditional color bands, so numerical codes are used instead.

Q: What does the letter 'R' mean in SMD resistor codes?
A: It represents the decimal point location in resistance values under 10 ohms.

Q: How do I decode an EIA-96 code?
A: Use a lookup table for the first two digits to find a base value, then multiply by a factor corresponding to the last letter.

Q: Can the SMD resistor code vary by manufacturer?
A: Yes, slight variations can exist, so always verify with the manufacturer’s datasheet when possible.

Q: What are zero-ohm resistors used for?
A: They act as electrical jumpers or short links on a PCB.