Surface mount device (SMD) resistors rely on three primary coding systems to indicate their resistance value: the 3-digit system (standard for 5% tolerance), the 4-digit system (standard for 1% tolerance on larger packages), and the EIA-96 alphanumeric system (used for 1% tolerance on 0603 and smaller packages where physical space prevents printing four numbers). If you are staring at a tiny component marked 473, it is 47,000 ohms (47 kΩ). If it reads 4702, it is 47,000 ohms. If it reads 68B, it is 4,990 ohms (4.99 kΩ).
Decoding the Markings: 3-Digit, 4-Digit, and EIA-96 Tables
How to read these tables: The tables below map the printed characters to their final resistance value. For the standard numeric systems (governed by IEC 60062), the first two or three digits represent the significant figures, and the final digit is the multiplier (number of zeros to add). For the EIA-96 system, the first two digits are a lookup code for the significant figures, and the final letter is the multiplier. Bookmark the quick-jump rows below for the most commonly queried bench values.
10 kΩ = 103 (3-digit) / 1002 (4-digit) / 01B (EIA-96)
4.7 kΩ = 472 (3-digit) / 4701 (4-digit) / 68A (EIA-96)
100 Ω = 101 (3-digit) / 1000 (4-digit) / 01H (EIA-96)
Table 1: IEC 60062 Multiplier and 'R' Notation (3-Digit & 4-Digit)
| Last Digit / Letter | Multiplier | 3-Digit Example | 4-Digit Example | Final Value |
|---|---|---|---|---|
| 0 (or R for decimal) | × 1 (or decimal point) | 100 / 4R7 | 1000 / 4R70 | 10 Ω / 4.7 Ω |
| 1 | × 10 | 101 | 1001 | 100 Ω |
| 2 | × 100 | 102 | 1002 | 1 kΩ |
| 3 | × 1,000 | 103 | 1003 | 10 kΩ |
| 4 | × 10,000 | 104 | 1004 | 100 kΩ |
| 5 | × 100,000 | 105 | 1005 | 1 MΩ |
| 6 | × 1,000,000 | 106 | 1006 | 10 MΩ |
Table 2: EIA-96 Base Values and Multiplier Letters
The EIA-96 standard assigns a two-digit code (01 through 96) to the 96 values of the 1% E96 series. Below is a dense lookup for the most frequently used base codes, followed by the multiplier letter chart. Source standard: EIA-96 / IEC 60062.
| Code | Base Value | Code | Base Value | Code | Base Value | Code | Base Value |
|---|---|---|---|---|---|---|---|
| 01 | 100 | 12 | 130 | 25 | 178 | 47 | 301 |
| 02 | 102 | 15 | 140 | 29 | 196 | 51 | 332 |
| 04 | 107 | 17 | 147 | 33 | 215 | 60 | 412 |
| 07 | 115 | 20 | 158 | 38 | 243 | 68 | 499 |
| 09 | 121 | 22 | 165 | 43 | 274 | 76 | 604 |
| 11 | 127 | 24 | 174 | 45 | 287 | 85 | 750 |
EIA-96 Multiplier Letters: A=10⁰ (×1), B=10¹ (×10), C=10² (×100), D=10³ (×1k), E=10⁴ (×10k), F=10⁵ (×100k). Example: 68B = Base 499 × 10 = 4,990 Ω (4.99 kΩ).
Package Derating and What the Code Cannot Tell You
A common point of confusion on the bench is assuming the printed code dictates the component's physical limits. The code chart only provides the nominal resistance at room temperature. It does not tell you which coding system applies to your specific installation, nor does it account for thermal or power limits.
Which Coding System Applies to Your Installation?
The coding system is strictly dictated by the physical package size and the manufacturing tolerance. If you are hand-soldering or reworking a board, you must identify the package first. Manufacturers like Yageo and Vishay follow this general rule:
- 0805, 1206, 1210, and larger: Typically use the 3-digit code for 5% tolerance and the 4-digit code for 1% tolerance. There is ample space on the ceramic substrate for four characters.
- 0603: The transitional size. 5% uses 3-digit. 1% historically used 3-digit, but modern high-precision 0603 parts almost exclusively use the EIA-96 alphanumeric system to avoid ambiguity.
- 0402, 0201, and 01005: These packages are physically too small for laser-etched alphanumeric codes to be legible or manufacturable at scale. They are entirely unmarked. You must rely on the schematic, BOM, or measure them individually with a multimeter before placement.
How Derating Modifies the Base Value
While the code chart gives you the base resistance value, the manufacturer's thermal derating curve modifies the base power rating of the package based on ambient temperature. A 10 kΩ resistor coded 103 might be rated for 1/8W (125 mW) in an 0805 package. However, that 125 mW base value is only valid up to an ambient temperature of 70°C.
According to standard thick-film derating curves, the allowable power dissipation drops linearly from 100% at 70°C to 0% at 155°C. If your SMD resistor is operating in an enclosure where the ambient PCB temperature reaches 105°C, the derating curve modifies the base power value down to roughly 60 mW. If your circuit pushes 100 mW through that resistor at 105°C, it will overheat, drift in value, and eventually crack the ceramic substrate, regardless of what the resistance code says.
What the Table Cannot Tell You
Never specify a replacement SMD resistor based solely on the code chart. The printed characters omit critical parameters:
| Hidden Parameter | Why It Matters on the Bench | Typical Range for Thick Film |
|---|---|---|
| Tolerance | Determines if the part is 1%, 5%, or worse. A 10k 5% part could be 9.5k or 10.5k. | ±1% to ±5% |
| Temperature Coefficient (TCR) | Dictates how much the resistance drifts as the part self-heats. Critical for precision ADC dividers. | ±100 to ±200 ppm/°C |
| Max Working Voltage | A 10 MΩ 0402 resistor will arc over and fail if subjected to 200V, even if power dissipation is low. | 25V (0402) to 200V (1206) |
| Composition | Thick film is cheap and standard; thin film is required for low-noise audio or precision RF paths. | Thick Film / Thin Film |
Bench Verification: Measuring SMD Codes with a Multimeter
When reworking a board or verifying a reel of SMD components, you cannot blindly trust the printed code. Laser etching errors happen, and reels can be mislabeled at the factory. Here is how to verify SMD resistors accurately at the bench.
The In-Circuit Measurement Trap
Never trust a multimeter reading taken while the SMD resistor is still soldered to the PCB. The multimeter injects a small test current and measures the voltage drop. If the resistor is in-circuit, that current will flow through parallel paths (traces, IC pins, bypass capacitors), yielding a falsely low resistance reading. If your 103 (10 kΩ) resistor measures 4.2 kΩ in-circuit, it is likely fine; the surrounding circuitry is just pulling the reading down. To verify the code, you must desolder at least one pad to lift the component out of the circuit, or use a specialized in-circuit ESR meter that guards against parallel impedance (though this is rarely effective for standard resistance values).
Measuring Low-Value SMDs (Under 10 Ω)
If you are verifying current-sense resistors (often marked with 'R' notation, like R010 for 0.010 Ω), a standard 2-wire multimeter will fail. The resistance of your test leads and the contact resistance of your probes will easily add 0.2 Ω to 0.5 Ω to the reading, completely masking the actual value of the SMD part.
To measure sub-10 Ω SMD resistors accurately, you must use a 4-wire Kelvin measurement. If your bench multimeter lacks a 4-wire mode, you can improvise: use one pair of probes to source a known constant current (e.g., 100 mA from a bench power supply) through the resistor, and use a second pair of probes connected to a high-impedance millivolt meter to measure the voltage drop directly across the resistor's ceramic body. Apply Ohm's Law (R = V / I) to find the true resistance, completely eliminating lead resistance from the equation.
For comprehensive specifications on power ratings and thermal limits for specific SMD series, always defer to the manufacturer's datasheet, such as the Bourns Surface Mount Resistor guides, as physical dimensions and pad layouts can vary slightly between vendors.
