SMD Resistor Marking Codes

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SMD Resistor Marking Overview

Surface-mount device (SMD) resistors use printed codes to identify their nominal resistance. The most common systems are three-digit markings, four-digit markings, decimal-point notation using the letter R, and the EIA-96 code used on many precision resistors.

The marking system depends on the resistor series, package size and manufacturer. Very small resistors are frequently unmarked.

A printed resistance code does not necessarily identify tolerance, power rating, temperature coefficient or maximum working voltage. Those properties must be established from the component specification.

Three-Digit Resistor Codes

The three-digit system uses the first two digits as significant figures and the third digit as a decimal multiplier.

Resistance in ohms:

R = (10 × D1 + D2) × 10^D3

Where D1, D2 and D3 are the three printed digits.

Three-Digit Examples

A resistor marked 100 is 10 Ω, not 100 Ω.

The final digit indicates the power of ten. It is not a third significant digit.

Three-Digit Multiplier Reference

Three-digit markings are commonly associated with E24-series resistors, often having ±5% tolerance.

The code itself does not guarantee that tolerance.

Four-Digit Resistor Codes

The four-digit system provides three significant figures followed by a multiplier.

Resistance in ohms:

R = (100 × D1 + 10 × D2 + D3) × 10^D4

Four-digit markings are frequently used on precision resistors, including many ±1% components.

Four-Digit Examples

Distinguishing Three and Four Digits

A four-digit marking must not be decoded by applying the three-digit rule to its first three characters.

R Notation and Low-Value Resistors

The letter R represents the decimal point in resistance markings expressed in ohms.

This notation is particularly useful for values below 10 Ω and for precision values that cannot be represented conveniently using an ordinary numerical multiplier.

R-Notation Examples

The R is a decimal separator, not a multiplier.

For instance:

4R7 = 4.7 Ω

It does not mean 47 Ω or 4,700 Ω.

Four-Character Precision Markings

The number of decimal places in the marking does not, by itself, establish guaranteed resistor accuracy.

Milliohm Resistor Markings

Low-resistance SMD components are frequently used for current measurement, power monitoring and current limiting.

Their values may be expressed in milliohms.

1 mΩ = 0.001 Ω

Common Low-Resistance Markings

For very low resistance, the resistance of PCB tracks, solder joints and test probes can become significant.

Current-sense resistors often use four-terminal Kelvin connections to reduce measurement error.

A code such as R005 does not specify the component’s current rating or resistance tolerance.

EIA-96 Resistor Marking System

The EIA-96 marking system is commonly used for precision resistors, particularly E96-series components with nominal ±1% tolerance.

The marking consists of:

• A two-digit number from 01 to 96.
• A letter representing the multiplier.

The two-digit number is a lookup index, not the resistance value.

EIA-96 Calculation

Resistance = E96 base value × Letter multiplier

Examples

The code 01C is 10 kΩ, not 1 Ω or 100 Ω.

EIA-96 Letter Multipliers

Some published EIA-96 marking conventions use S as an alternative to Y and R as an alternative to X.

Manufacturer-specific use of multiplier letters should be checked before interpreting an ambiguous component.

The letter R can therefore have two different meanings depending on the marking system:

• In ordinary R notation, it represents a decimal point.
• In an EIA-96 variant, it can represent a ×0.1 multiplier.

Complete EIA-96 Base-Value Table

The following table provides the 96 base values used by the EIA-96 marking system.

Decoding Example: 68X

Code 68 corresponds to base value 499.

Letter X represents ×0.1.

499 × 0.1 = 49.9 Ω

Decoding Example: 20E

Code 20 corresponds to base value 158.

Letter E represents ×10,000.

158 × 10,000 = 1,580,000 Ω

Result: 1.58 MΩ

Decoding Example: 47C

Code 47 corresponds to base value 301.

Letter C represents ×100.

301 × 100 = 30,100 Ω

Result: 30.1 kΩ

EIA-96 Versus Four-Digit Markings

Both systems can represent precision resistance values, but their encoding methods differ.

EIA-96 codes are compact and can fit on some packages that cannot accommodate four numerical characters.

The manufacturer determines which marking system is used.

Zero-Ohm SMD Resistors

Zero-ohm resistors are commonly used as PCB links rather than as conventional resistance elements.

Common Markings

Typical applications include configuration jumpers, alternate assembly options and connections that may be removed during testing.

A zero-ohm resistor is not an ideal short circuit.

It has a finite resistance, current rating and maximum permissible operating conditions.

For high-current designs, use the manufacturer’s actual current-rating specification.

Unmarked SMD Resistors

Many small surface-mount resistors have no printed identification.

This is common in packages such as 0402, 0201 and 01005, although marking practices vary.

Common SMD Package Sizes

Dimensions are nominal and may vary slightly between manufacturers.

The package size does not determine the resistor value.

Identifying an Unmarked Resistor

Useful methods include checking the PCB bill of materials, circuit schematic, assembly drawing or component reel label.

If documentation is unavailable, measure the resistance using appropriate test equipment.

An in-circuit reading may be affected by parallel circuit paths.

SMD Marking Orientation

Unlike axial resistors with coloured tolerance bands, SMD resistors generally have no standard visual indicator establishing the reading direction.

A code may appear rotated through 90° or 180°.

Common Ambiguities

For ambiguous characters, inspect the component with magnification.

Check the likely value against the circuit function and confirm with measurement.

A physically plausible code is not necessarily the correct interpretation.

Resistor Tolerance and Markings

Three-digit, four-digit and EIA-96 markings are often associated with particular resistor series.

These are common conventions rather than guaranteed specifications.

A three-digit resistor may have a tighter tolerance than ±5%, and four-digit markings do not guarantee ±1%.

Tolerance Calculation

For nominal resistance R and tolerance t expressed as a percentage:

RMIN = R × (1 − t/100)

RMAX = R × (1 + t/100)

For 4.99 kΩ ±1%:

RMIN = 4990 × 0.99 = 4940.1 Ω

RMAX = 4990 × 1.01 = 5039.9 Ω

The permitted initial resistance range is approximately 4.940–5.040 kΩ.

Tolerance does not include every possible change due to temperature, ageing, moisture or voltage stress.

Power Rating and Package Size

The resistance code does not identify the rated power dissipation.

Package size provides only an approximate indication because ratings differ by resistor construction and manufacturer.

Common Indicative Ratings

These are typical values, not universal ratings.

High-power and specialised resistor series may differ substantially.

Power Dissipation

The principal relationships are:

P = I²R

P = V²/R

Where P is power in watts, I is current in amperes, V is voltage in volts and R is resistance in ohms.

For 1 kΩ with 5 V across it:

P = 5² / 1000 = 0.025 W

The actual resistor must have sufficient power rating after accounting for PCB layout, ambient temperature and manufacturer derating requirements.

Working Voltage

A resistor must remain within both its power rating and its maximum working-voltage specification.

The voltage corresponding to its power rating is:

V = √(P × R)

For 100 kΩ at 0.1 W:

V = √(0.1 × 100000)

V = 100 V

This is the voltage producing 0.1 W dissipation.

The resistor’s specified maximum working voltage may be lower.

For high-voltage circuits, also consider voltage transients, pulse ratings, creepage, clearance and appropriate safety requirements.

Measuring SMD Resistors

A multimeter can confirm a resistor value when the marking is ambiguous.

Measurement Procedure

1. Disconnect circuit power.
2. Discharge relevant capacitors.
3. Select the resistance range.
4. Contact both resistor terminals using suitable probes.
5. Allow the measurement to stabilise.
6. Compare the result with the decoded marking.
7. Remove one connection from the circuit if parallel paths affect the reading.

In-Circuit Measurement Errors

A resistor connected in parallel with another conductive path can appear lower in resistance than its nominal value.

For two resistors:

REQ = (R1 × R2) / (R1 + R2)

Two 10 kΩ resistors in parallel produce 5 kΩ.

Measuring 5 kΩ across one of these resistors while it remains installed does not establish that it is defective.

Low-Resistance Measurement

Ordinary two-wire multimeter measurements can be inaccurate for milliohm resistors.

For a 10 mΩ component, even small probe and contact resistances can introduce substantial error.

Use four-wire Kelvin measurement where the required accuracy justifies it.

Special and Manufacturer-Specific Markings

Not every SMD marking follows the common resistor codes.

A component marked with three characters is not necessarily a three-digit resistor.

Small semiconductor devices, inductors and other components also use compact printed identifiers.

Component type should be established before applying a resistor code.

Common Marking Errors

Quick Decoding Procedure

1. Confirm that the component is a resistor.
2. Read the printed code using magnification if necessary.
3. Count the characters.
4. Identify any R decimal notation.
5. For three digits, use two significant figures and one multiplier.
6. For four digits, use three significant figures and one multiplier.
7. For two digits followed by a letter, check the EIA-96 code.
8. For zero markings, consider a zero-ohm jumper.
9. Compare the decoded value with the expected circuit function.
10. Verify by measurement if the result is uncertain.
11. Check the resistor’s tolerance, power rating and construction before replacement.

SMD Resistor Marking Checklist