SMD Resistor Guide: Sizes, Codes & Identification

Every PCB I design these days uses SMD resistors almost exclusively. After fifteen years of transitioning from through-hole to surface mount technology, I can confidently say that understanding SMD resistor sizes, codes, and identification is essential knowledge for anyone working in electronics. Whether you’re debugging a prototype, selecting components for production, or repairing a failed board, knowing how to read those tiny markings can save hours of frustration.

This comprehensive guide covers everything you need to know about surface mount resistors: package sizes from tiny 01005 to power-handling 2512, all three marking systems (3-digit, 4-digit, and EIA-96), and practical identification techniques I use daily at my workbench.

What Is an SMD Resistor?

An SMD resistor, also called a surface mount resistor or chip resistor, is a passive electronic component designed for surface mount technology (SMT) assembly. Unlike traditional through-hole resistors with axial wire leads, SMD resistors are compact rectangular components that solder directly onto PCB surface pads.

Surface mount resistors perform the same function as their through-hole counterparts: limiting current flow, dividing voltages, and setting bias points in circuits. The difference lies in their form factor, which enables automated assembly and dramatically higher component density.

Feature	SMD Resistor	Through-Hole Resistor
Mounting	Surface pads	Drilled holes
Marking	Numeric codes	Color bands
Size	0.4mm to 6.4mm	3mm to 10mm+
Assembly	Pick-and-place machine	Manual or wave solder
Board density	Very high	Limited
Typical tolerance	1% or 5%	5% or 10%

The construction of a typical SMD resistor includes a ceramic substrate (usually alumina), a resistive element (thick film or thin film), terminations on each end for soldering, and a protective overglaze coating where markings are printed.

SMD Resistor Package Sizes Explained

Surface mount resistor sizes follow JEDEC standards, identified by four-digit codes that represent physical dimensions. The code format indicates length and width in hundredths of an inch (imperial) or tenths of a millimeter (metric).

For example, the imperial code 0805 means 0.08 inches long by 0.05 inches wide. The metric equivalent is 2012, meaning 2.0mm by 1.2mm. Most engineers use imperial codes even while working with metric dimensions in PCB software, which can cause initial confusion.

Complete SMD Resistor Size Chart

Imperial Code	Metric Code	Length (mm)	Width (mm)	Height (mm)	Typical Power Rating
01005	0402M	0.4	0.2	0.13	1/32W (0.03W)
0201	0603M	0.6	0.3	0.23	1/20W (0.05W)
0402	1005M	1.0	0.5	0.35	1/16W (0.063W)
0603	1608M	1.6	0.8	0.45	1/10W (0.1W)
0805	2012M	2.0	1.25	0.5	1/8W (0.125W)
1206	3216M	3.2	1.6	0.55	1/4W (0.25W)
1210	3225M	3.2	2.5	0.55	1/2W (0.5W)
1812	4532M	4.5	3.2	0.55	1/2W (0.5W)
2010	5025M	5.0	2.5	0.55	3/4W (0.75W)
2512	6332M	6.3	3.2	0.55	1W

Choosing the Right SMD Resistor Size

Package selection depends on several factors beyond just fitting components on the board:

Power dissipation drives size selection more than anything else. Calculate your worst-case power using P = I²R or P = V²/R, then select a package rated for at least 50% more than your calculated value. Running SMD resistors at their maximum rating causes reliability problems due to heat.

Assembly capability matters significantly. Packages smaller than 0402 require specialized pick-and-place equipment and are difficult or impossible to hand solder. If you’re prototyping by hand, stick with 0603 or larger.

Inspection and rework become challenging below 0603. Those 0402 and 0201 packages are nearly invisible without magnification, making visual inspection and manual rework extremely difficult.

Cost considerations favor common sizes. The 0402, 0603, and 0805 packages hit the sweet spot of availability, price, and handleability for most applications.

For most of my designs, I default to 0603 for general-purpose resistors and move to 0805 or 1206 when power handling matters. I only go smaller when board space absolutely demands it.

How to Read SMD Resistor Codes

Unlike through-hole resistors with color bands, surface mount resistors use alphanumeric codes printed on their top surface. Three distinct marking systems exist, and knowing which one you’re looking at is half the battle.

The 3-Digit SMD Resistor Code System

The 3-digit code is the most common marking system for standard tolerance (5%) SMD resistors. The format is simple: the first two digits are significant figures, and the third digit is the multiplier (power of ten).

Formula: Resistance = (First two digits) × 10^(Third digit)

Code	Calculation	Resistance
100	10 × 10⁰	10Ω
101	10 × 10¹	100Ω
102	10 × 10²	1kΩ
103	10 × 10³	10kΩ
104	10 × 10⁴	100kΩ
220	22 × 10⁰	22Ω
471	47 × 10¹	470Ω
682	68 × 10²	6.8kΩ
105	10 × 10⁵	1MΩ

Important: The code 220 means 22Ω, not 220Ω. The trailing zero is the multiplier (10⁰ = 1), not part of the value. This trips up many beginners.

Values Below 10 Ohms: The “R” Notation

For resistances below 10Ω, the letter “R” indicates the decimal point position:

Code	Resistance
R10	0.10Ω
R22	0.22Ω
R47	0.47Ω
1R0	1.0Ω
2R2	2.2Ω
4R7	4.7Ω
5R6	5.6Ω

Current sensing resistors often use this notation, sometimes with “M” or “L” for milliohm values (1M5 = 1.5mΩ).

The 4-Digit SMD Resistor Code System

Precision SMD resistors (1% tolerance) use a 4-digit code for greater accuracy. The first three digits are significant figures, and the fourth is the multiplier.

Formula: Resistance = (First three digits) × 10^(Fourth digit)

Code	Calculation	Resistance
1000	100 × 10⁰	100Ω
1001	100 × 10¹	1kΩ
1002	100 × 10²	10kΩ
4700	470 × 10⁰	470Ω
4701	470 × 10¹	4.7kΩ
4702	470 × 10²	47kΩ
1503	150 × 10³	150kΩ
2001	200 × 10¹	2kΩ

The 4-digit system provides three significant figures instead of two, matching the precision of 1% tolerance parts.

The EIA-96 Code System for Precision SMD Resistors

The EIA-96 system was developed for 1% tolerance resistors on smaller packages where even 4-digit codes wouldn’t fit. This system uses two digits plus a letter, requiring a lookup table.

The two-digit number (01-96) corresponds to a specific value from the E96 series. The letter indicates the multiplier.

EIA-96 Multiplier Letters:

Letter	Multiplier	Letter	Multiplier
Z	0.001	A	1
Y	0.01	B	10
X	0.1	C	100
		D	1,000
		E	10,000
		F	100,000

Common EIA-96 Code Values (Partial Table):

Code	Base Value	Code	Base Value	Code	Base Value
01	100	25	178	49	316
02	102	26	182	50	324
03	105	27	187	51	332
04	107	28	191	52	340
05	110	29	196	53	348
06	113	30	200	54	357
07	115	31	205	55	365
08	118	32	210	56	374

EIA-96 Examples:

Marking	Calculation	Resistance
01A	100 × 1	100Ω
01B	100 × 10	1kΩ
01C	100 × 100	10kΩ
29B	196 × 10	1.96kΩ
53C	348 × 100	34.8kΩ
68X	499 × 0.1	49.9Ω

The letter in EIA-96 prevents confusion with 3-digit codes. If you see a letter at the end, you’re dealing with EIA-96.

SMD Resistor Tolerance and Temperature Coefficient

The marking code only tells you resistance value, not tolerance or temperature coefficient. These specifications depend on the resistor series and must be verified from datasheets or purchasing records.

Understanding Tolerance Markings

Some larger SMD resistors include a tolerance letter after the value code:

Letter	Tolerance
F	±1%
G	±2%
J	±5%
K	±10%
M	±20%

General rules of thumb:

• 3-digit codes typically indicate 5% tolerance
• 4-digit codes typically indicate 1% tolerance
• EIA-96 codes indicate 1% tolerance

These are guidelines, not guarantees. Always verify from the component datasheet or bill of materials.

Temperature Coefficient Specifications

Temperature coefficient (TCR) describes how resistance changes with temperature, measured in parts per million per degree Celsius (ppm/°C).

Resistor Type	Typical TCR	Application
Thick film (general)	±100 to ±200 ppm/°C	General purpose
Thick film (precision)	±50 to ±100 ppm/°C	Better stability
Thin film	±5 to ±25 ppm/°C	Precision circuits
MELF (thin film)	±25 to ±50 ppm/°C	High stability

For a 10kΩ resistor with ±100 ppm/°C TCR, a 50°C temperature change causes up to 50Ω (0.5%) resistance shift. In precision analog circuits, this matters significantly.

Thick Film vs Thin Film SMD Resistors

Surface mount resistors come in two main construction types, each with distinct characteristics.

Thick Film SMD Resistors

Thick film technology uses a paste of metal oxide and glass frit screened onto a ceramic substrate, then fired at high temperature. The resistive layer is typically 10-50 micrometers thick.

Characteristics:

• Lower cost
• Wider resistance range (0Ω to 10MΩ+)
• Higher TCR (±100 to ±200 ppm/°C typical)
• Tolerances from 1% to 5%
• Good for general purpose applications

Thick film dominates the market for commodity applications. The vast majority of SMD resistors you’ll encounter are thick film.

Thin Film SMD Resistors

Thin film technology deposits a metal alloy (often nichrome) onto a ceramic substrate through sputtering or evaporation. The resistive layer is under 1 micrometer thick.

Characteristics:

• Higher cost (3-10× thick film)
• Tighter tolerance (0.1% available)
• Lower TCR (±5 to ±25 ppm/°C)
• Better long-term stability
• Lower noise

Thin film resistors are essential for precision measurement, instrumentation, and RF applications where stability and accuracy justify the cost premium.

Zero-Ohm Resistors (Jumpers)

You’ll frequently encounter SMD resistors marked “0” or “000” on PCBs. These are zero-ohm jumpers, essentially wire bridges in resistor form.

Zero-ohm resistors exist because pick-and-place machines handle all SMD components the same way. Using a zero-ohm jumper instead of a wire link keeps assembly automated and consistent.

Common uses:

• Routing signals between layers
• Configuration options (populate or don’t populate)
• Test points
• Fuse positions (in some designs)

Zero-ohm jumpers aren’t truly zero resistance; they typically measure 10-50 milliohms due to termination and contact resistance.

Practical SMD Resistor Identification Tips

After years of debugging boards, I’ve developed reliable methods for identifying unknown SMD resistors.

Visual Identification Workflow

1. Note the package size by measuring or comparing to known components
2. Examine the marking under magnification (10× minimum for 0603 and smaller)
3. Determine the code system (3-digit, 4-digit, or EIA-96)
4. Calculate or look up the value
5. Verify with a multimeter when possible

When Markings Are Unreadable

Markings fade, burn off during rework, or get obscured by conformal coating. When visual identification fails:

Use a multimeter: Remove the resistor from circuit or power down and disconnect one end. Measure resistance directly. This is the most reliable method.

Check the schematic: Cross-reference the component designator with your documentation.

Compare to known good boards: If you have a working board, measure the corresponding resistor.

Use magnification: USB microscopes work surprisingly well for reading tiny markings that appear illegible to the naked eye.

Common Identification Mistakes

Confusing 3-digit and 4-digit codes: A marking of “1001” could be 4-digit (1kΩ) or two separate components. Package size provides context.

Misreading orientation: SMD resistors are symmetrical and can be placed in either orientation. The marking should read normally left-to-right, but placement can vary.

Assuming tolerance from code type: While 3-digit usually means 5% and 4-digit means 1%, exceptions exist. Always verify critical tolerances from documentation.

Ignoring the “R” notation: Codes like “4R7” are 4.7Ω, not 47Ω or 4.7kΩ. The R marks the decimal point.

Useful Resources and Tools

These resources simplify SMD resistor identification and selection:

Online Calculators:

• DigiKey SMD Resistor Code Calculator (digikey.com)
• Hobby-Hour SMD Resistor Calculator (hobby-hour.com)
• Utmel SMD Resistor Code Calculator (utmel.com)
• Kiloohm EIA-96 Calculator (kiloohm.info)

Reference Tables:

• E96 Series Value Chart (eepower.com)
• EIA-96 Code Lookup Tables (learnabout-electronics.org)
• JEDEC Package Size Standards

Manufacturer Resources:

• Vishay Resistor Application Notes
• Yageo Product Selector Guide
• Panasonic Resistor Datasheets
• KOA Speer Technical Library

Component Distributors:

• DigiKey Parametric Search (digikey.com)
• Mouser Component Search (mouser.com)
• LCSC Electronics (lcsc.com)

Frequently Asked Questions About SMD Resistors

How do I know if an SMD resistor is 3-digit or 4-digit code?

Count the characters. Three characters without a letter at the end means 3-digit code. Four numerical characters means 4-digit code. Three characters with a letter at the end (like 01C) indicates EIA-96. Package size provides context too; very small packages (0402 and below) often use EIA-96 because there’s no room for 4-digit codes. When in doubt, measure with a multimeter.

Can I replace an 0805 SMD resistor with 0603 or 1206?

You can substitute different package sizes if the footprint allows and power ratings are adequate. An 0603 fits on most 0805 pads with careful soldering, though it’s not ideal for production. Going larger (0805 to 1206) rarely works without footprint modification. The resistance value and tolerance must match regardless of package size.

Why are some SMD resistors unmarked?

Very small packages (0201 and 01005) lack space for any marking. Some manufacturers skip markings on commodity parts to reduce cost. Unmarked resistors must be identified through measurement, documentation, or reel labels. This is why maintaining accurate BOMs and component organization is critical when working with small SMD parts.

What does a resistor marked “000” or “0” mean?

These are zero-ohm jumpers, used as wire bridges in surface mount assembly. They conduct current with minimal resistance (typically 10-50mΩ) and serve for routing, configuration options, or maintaining automated assembly processes. Zero-ohm jumpers are not actually zero resistance but are close enough for most signal routing purposes.

How accurate are SMD resistor code calculators?

Online calculators are highly accurate for standard marking systems. However, they can’t account for non-standard markings used by some manufacturers, damaged or partial markings, or unusual coding schemes on specialty resistors. Always verify critical components with a multimeter, especially when debugging a circuit that isn’t working as expected.

Wrapping Up

SMD resistors have become the standard for modern electronics manufacturing, and understanding their sizing conventions and marking codes is fundamental knowledge for any PCB engineer or electronics hobbyist. The three marking systems (3-digit, 4-digit, and EIA-96) each serve specific purposes, from general-purpose 5% parts to precision 1% components.

The key points to remember: package codes indicate physical size and power handling, marking codes indicate resistance value, and tolerance comes from the resistor series rather than the marking itself. When visual identification fails, your multimeter is the ultimate arbiter of truth.

Whether you’re designing new boards, debugging prototypes, or repairing existing equipment, this knowledge of surface mount resistor identification will serve you well throughout your electronics career. Keep a good magnifier handy, bookmark a reliable code calculator, and don’t hesitate to measure when markings are unclear.