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Portico Ω reference

Resistor identification FAQ

Clear answers for reading through-hole color bands, decoding SMD markings, using reverse lookup, and knowing when a datasheet or meter should take over.

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Through-hole color bands

How band order, significant digits, multipliers, tolerance, and temperature coefficient work.

What kinds of resistors can Portico Ω identify?

Portico Ω decodes 4, 5, and 6-band through-hole resistor color codes. It also works in reverse: enter a resistance, tolerance, and—when applicable—temperature coefficient to generate the corresponding bands.

For surface-mount resistors, it recognizes common 3-digit and 4-digit numeric markings, R-decimal notation, zero-ohm jumper codes, EIA-96 codes, RKM notation, and documented L milliohm notation. Manufacturer-specific markings may still need a datasheet.

Which end of a resistor should I read first?

Start at the end opposite the tolerance band. The tolerance band is often gold or silver and is usually separated from the other bands by a wider gap. On many precision resistors it may be brown, red, green, blue, violet, or gray instead.

If spacing is unclear, read the resistor both ways and compare the result with a meter, the circuit context, or the expected preferred-value series. Never rely on band orientation alone when a wrong value could damage equipment.

What is the difference between 4, 5, and 6-band resistors?

A 4-band resistor uses two significant digits, one multiplier, and one tolerance band. A 5-band resistor uses three significant digits, a multiplier, and tolerance, allowing a more precise value to be printed.

A 6-band resistor uses the same first five bands as a 5-band part and adds a sixth band for temperature coefficient of resistance, normally expressed in parts per million per kelvin (ppm/K).

How is a resistance calculated from the bands?

For a 4-band part, combine the first two digits and multiply by the third band's power of ten. Yellow–violet–red is 47 × 100, or 4,700 Ω. The fourth band then supplies the tolerance.

For 5 and 6-band parts, combine the first three digits and multiply by the fourth band's power of ten. The fifth band is tolerance; on a 6-band resistor, the sixth is TCR.

What does resistor tolerance mean?

Tolerance is the permitted difference between the marked nominal value and the manufactured value. A 1 kΩ ±5% resistor may be within 950 Ω to 1,050 Ω when measured under the conditions specified by its manufacturer.

Tolerance does not include every real-world effect. Temperature, aging, self-heating, measurement accuracy, and circuit conditions can add further variation.

What is TCR, and what does the sixth band mean?

TCR is the temperature coefficient of resistance: the approximate change in resistance for each kelvin of temperature change, expressed in ppm/K. A lower absolute TCR generally means the resistor is more stable as temperature changes.

The sixth color band encodes TCR only in the 6-band scheme. It is not another significant digit, and Portico Ω does not infer a generic SMD resistor's TCR from its top marking.

What if a 4-band resistor has no visible tolerance band?

Under the traditional color-code convention, an omitted tolerance band represents ±20%. Age, heat, coatings, contamination, or abrasion can also make a band disappear, so first check whether the part is simply damaged or discolored.

If the markings are uncertain, isolate at least one lead from the circuit before measuring. Parallel paths in a powered-down circuit can still make an in-circuit resistance reading misleading.

Why do the on-screen colors look different from the real resistor?

Color appearance changes with lighting, display calibration, body coating, age, dirt, heat damage, and camera processing. The screen colors in Portico Ω are practical visual approximations, not calibrated physical color standards.

Use band position and context as well as hue. When brown, red, orange, blue, violet, or gray are difficult to distinguish, confirm the result electrically and consult the part documentation.

SMD markings

How the most common printed chip-resistor codes work—and why no single rule covers every family.

How do 3-digit SMD resistor codes work?

In the common 3-digit system, the first two digits are significant figures and the final digit is the number of zeros, or power-of-ten multiplier. For example, 103 means 10 × 10³ = 10 kΩ, while 472 means 47 × 10² = 4.7 kΩ.

This format is commonly associated with wider-tolerance parts, but the marking alone does not guarantee tolerance, power rating, technology, or package size.

How do 4-digit SMD resistor codes work?

The common 4-digit system uses three significant digits followed by a multiplier digit. A marking of 1002 means 100 × 10² = 10 kΩ; 4701 means 470 × 10¹ = 4.7 kΩ.

Four-digit markings are often found on precision parts, but some product families use different conventions. Treat the decoded value as an interpretation until the family is known.

What does the letter R mean in an SMD marking?

R normally replaces the decimal point when the value is expressed in ohms. Examples include 4R7 for 4.7 Ω, R20 for 0.20 Ω, and 1R00 for 1.00 Ω.

R can appear in several manufacturer formats, including fixed-length RKM notation. Portico Ω parses the electrical value, but the datasheet remains authoritative for tolerance and family-specific meaning.

What is an EIA-96 code?

EIA-96 usually consists of two digits from 01 to 96 plus a multiplier letter. The two-digit number selects one of 96 normalized base values; the letter scales it. For example, 01C selects base value 100 and multiplier 100, producing 10 kΩ.

EIA-96 is commonly used for 1% resistors, but it is not universal. Some precision families use plain 4-digit values, and some E192 parts are unmarked.

What do 0, 000, 0000, and R00 mean?

These are common zero-ohm jumper markings. A zero-ohm resistor is used as a configurable link, routing aid, or assembly option; it is not an ideal conductor and still has a small resistance plus current and power limits.

The exact printed code depends on manufacturer, family, and package. Small zero-ohm parts may have no top marking at all.

What does L mean in a very-low-ohm marking?

Some documented current-sense resistor families use L for values below 10 mΩ. In that context, a code such as 5L00 represents 5.00 mΩ rather than 5 Ω.

L notation is family-specific, not a safe universal rule. Confirm the manufacturer and series before using the decoded value in a current-sensing or high-current design.

Why is my SMD resistor unmarked or ambiguous?

Very small packages may not have enough surface area for a readable code. Some precision series, very-low-ohm parts, and particular value ranges are deliberately supplied without markings. Laser marks can also be faint, damaged, or confused with lot and date codes.

Package dimensions do not uniquely identify resistance or power. Use the board documentation, bill of materials, known circuit function, manufacturer series, or an isolated measurement.

Why can the same-looking SMD code have different meanings?

SMD top markings are not governed by one universal decoding rule. Panasonic precision parts may use numeric E96 values instead of EIA-96; KOA documents unmarked E192 values in some families; Vishay current-sense parts include specialized low-ohm rules.

Portico Ω checks the common patterns in layers and warns when family context matters. For repair, safety, precision, or high-power work, match the logo, package, series, and datasheet before fitting a replacement.

Using results safely

Reverse lookup, sharing, measurement, package limits, and the boundary between a calculator and a component specification.

How does value-to-color or value-to-code lookup work?

Enter the resistance and unit, then choose the required band count or SMD format. Portico Ω generates a representation only when the requested format can express the value; EIA-96 lookup reports the closest supported preferred value when there is no exact match.

Generated markings show a valid encoding pattern, not a guarantee that every manufacturer sells or prints that exact combination. Select a real component by series and datasheet.

Can I save or share a Portico Ω result?

Yes. The Share result action stores the selected mode and decoded state in the page URL, so a colleague can reopen the same setup. Copy summary creates a compact text version for notes, messages, or work instructions.

A shared link records the calculator state, not a certified component identification. Include photographs, package measurements, circuit location, and manufacturer information when collaborating on an uncertain part.

Should I trust the code or measure the resistor?

Use the marking as the first identification step and measurement as an independent check. Disconnect power, discharge capacitors, and lift at least one resistor lead or terminal when surrounding components could create parallel paths.

A damaged resistor may have drifted or gone open, so its present measurement may not reveal its original value. In that case, use schematics, identical channels, board references, and manufacturer data together.

Can Portico Ω determine package size or power rating from the code?

No. Resistance markings generally do not encode the package dimensions, working voltage, pulse rating, current limit, power rating, construction, or thermal behavior. Two parts with the same resistance code can have very different limits.

Use package measurements only as a starting hint, then verify the manufacturer's package drawing and derating curves. Leave thermal and surge margin appropriate to the application.

Is Portico Ω suitable for safety-critical repairs or designs?

Portico Ω is an identification and calculation aid, not a substitute for a schematic, approved bill of materials, calibrated measurement, or component datasheet. Do not select a mains, medical, automotive, protection, current-sense, or high-energy component from its visible code alone.

Verify resistance, tolerance, TCR, voltage rating, pulse behavior, power, package, technology, and safety approvals against the exact manufacturer part number.