Why there is so little printed on the part
Surface-mount passives are too small to carry a full ordering code. Instead they carry a compressed value marking, or nothing at all. The marking is a convenience for handling and rework — the authoritative record of what a part is lives on the reel or bag label, and that is what should be transcribed into your records.
Resistors: the three-digit code
The pattern is two significant digits followed by the number of zeros. So 103 reads as 10 followed by three zeros — 10,000 ohms, or 10 k. Two more examples: 472 is 4.7 k, and 100 is 10 ohms, because the multiplier is zero. A marking of 000 indicates a zero-ohm link or jumper rather than a real resistance.
Resistors: the four-digit code
Precision parts often use four digits, the first three significant followed by the multiplier: 1002 is 100 followed by two zeros — again 10 k — and 4992 is 49.9 k. The extra digit exists to express values that three digits cannot, such as the third significant figure in a 1% part.
The letter R as a decimal point
Where a value falls below 100 ohms, an R replaces the decimal point. 2R2 is 2.2 ohms, R22 is 0.22 ohms, and in a four-digit current-sense part R125 is 0.125 ohms. This is the one place where the code is self-describing rather than a multiplier.
Precision parts and the two-character code
Some 1% parts in small bodies use a different scheme: two characters that index a table rather than being read directly, followed by a letter acting as the multiplier. Because the two digits are a lookup and not a value, they cannot be decoded by arithmetic — the table has to be applied. Where you meet a two-character marking you cannot interpret, treat it as unreadable and confirm the value another way rather than guessing.
Capacitors: mostly unmarked
Ceramic chip capacitors are usually supplied with no marking at all in the small case sizes, and even larger ones frequently carry nothing. Where a code is present it follows the same multiplier logic as resistors but the unit is picofarads:
- 102 — 10 × 10² pF = 1,000 pF = 1 nF
- 103 — 10 nF
- 104 — 100 nF, the familiar 0.1 µF decoupling value
- 105 — 1 µF
Notice that the same three digits mean different things on a resistor and a capacitor. Voltage rating is a separate attribute and is usually not encoded in the same marking, so it cannot be inferred from it.
The polarity mark that catches people out
Polarised capacitors are marked in a way that is genuinely inconsistent between families, and this causes real damage:
- Aluminium electrolytic: the stripe marks the negative terminal.
- Tantalum: the stripe or bar marks the positive terminal — the opposite convention.
A tantalum fitted the way an aluminium electrolytic is fitted is reverse-biased, and tantalum parts fail badly in that condition. When the marking is ambiguous, confirm the terminal from the datasheet rather than by analogy with another capacitor type.
Diodes and other two-terminal parts
Discrete diodes normally carry a band or bar indicating the cathode. On small packages the band may be a corner chamfer or a single dot rather than a full line, and on some parts the marking is a short alphanumeric code rather than a band. Where a code is used, it identifies the device type against the manufacturer's marking document — it is not a value.
When not to trust the marking
- Small case sizes carry nothing. An 0402 resistor may be entirely blank.
- Markings are easily confused under magnification and reflow residue. A 6 and an 8, or a 5 and an S, are genuinely hard to separate on a worn part.
- A plausible marking proves nothing about origin. Re-marking a part's value is trivial, which is one reason marking alone is not a quality check — see our checklist for spotting counterfeit components.
- Mixed parts in one bag defeat the marking entirely. If the packaging has been opened and repacked, the code on the body is the only information left — and it is the least reliable kind.
A workable routine
- Read the reel or bag label first and treat it as the record of what the part is.
- Cross-check a sample of parts against that label.
- Where the marking cannot be decoded with certainty, measure the value or consult the datasheet instead of assuming.
- Keep the label with the parts through kitting, so rework does not depend on reading a 1 mm body.
Confirming a passive
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Send the full ordering code with quantity and destination and we will confirm availability, pricing and lead time. Our guide to reading a part number explains how the suffix sets tolerance and packaging, and you can browse the full component inventory or start from Vishay, YAGEO or KEMET.