The 120 ohm resistor color code is Brown, Red, Brown for standard 4-band axial resistors, and Brown, Red, Black, Black for precision 5-band resistors. For surface mount (SMD) components, the marking is 121 (3-digit) or 1200 (4-digit). Because 120Ω is an E24 series value, it will not use the 2-digit EIA-96 code system found on non-standard E96 values.
The Complete 120 Ohm Resistor Reference Table
Use this spec-sheet table to identify a 120Ω resistor across all common physical formats. The multiplier dictates how many zeros to add to the significant digits, or in the case of the 5-band black multiplier, it acts as a times-one (x1) multiplier.
| Format | Band 1 / Digit 1 | Band 2 / Digit 2 | Band 3 / Digit 3 | Multiplier | Tolerance Band | Final Value |
|---|---|---|---|---|---|---|
| 4-Band Axial (5%) | Brown (1) | Red (2) | N/A | Brown (x10) | Gold (±5%) | 120 Ω |
| 5-Band Axial (1%) | Brown (1) | Red (2) | Black (0) | Black (x1) | Brown (±1%) | 120 Ω |
| SMD 3-Digit (5%) | 1 | 2 | N/A | 1 (x10) | N/A | 120 Ω |
| SMD 4-Digit (1%) | 1 | 2 | 0 | 0 (x1) | N/A | 120 Ω |
Standard Variants: IEC 60062, MIL-Spec, and SMD Markings
Resistor color codes are not governed by electrical wiring standards like the NEC or IEC 60446. Instead, they follow component-specific manufacturing standards. Understanding which standard your component falls under prevents misreading the bands.
- IEC 60062 (International): The global standard for electronic component marking. It defines the standard color-to-number mapping (Black=0, Brown=1, etc.) and applies to almost all commercial 4-band and 5-band through-hole resistors you buy from distributors like DigiKey or Mouser.
- MIL-PRF-22684 (Military Spec): Older military-specification resistors often use a 5-band system where the fifth band indicates reliability (failure rate per 1,000 hours) rather than tolerance. If you are salvaging vintage military avionics or radio gear, a 5-band resistor might actually be a 4-band value with a reliability stripe. Always verify with a multimeter.
- SMD EIA-96 vs. Standard Markings: A common point of confusion is the EIA-96 system, which uses a 2-digit code plus a letter multiplier for 1% SMD resistors. However, 120Ω is a standard E24 series value. Therefore, manufacturers do not use EIA-96 codes for it. You will only see the standard 3-digit (121) or 4-digit (1200) numerical markings on 120Ω SMD parts.
Bands and Multipliers People Get Wrong
Even experienced technicians misread 120Ω resistors when rushing. Here are the specific rows and bands that cause errors on the bench:
The 5-Band Multiplier Trap (Black vs. Brown)
In a 4-band resistor, the multiplier for 120Ω is Brown (x10). In a 5-band resistor, the multiplier is Black (x1). Technicians used to 4-band resistors often see Brown-Red-Black and assume the Black is the multiplier (x1), reading it as 12Ω. Always count the bands first. If there are five bands, the third band is a significant digit, not a multiplier.
The Tolerance Gap Spacing
Manufacturers print the tolerance band (Gold, Silver, Brown, Red) with a slightly wider physical gap between it and the multiplier band. On small 1/8W or 1/4W resistors, the bands can bleed together. If the spacing is ambiguous, orient the resistor so the Gold or Silver band is on the far right—these colors are never used for significant digits in the IEC 60062 standard.
SMD '121' vs '120' Confusion
Beginners often look for the number '120' on a 3-digit SMD resistor. A 3-digit SMD marked '120' is actually 12Ω (12 x 10^0). The correct 3-digit marking for 120Ω is 121 (12 x 10^1).
Decision Path: Identifying Your 120Ω Resistor
Use this decision tree to conclusively identify a 120 ohm resistor or verify a suspect component. Follow the path until you reach a concrete verification step.
| Condition / Observation | Action / Next Step | Concrete Result |
|---|---|---|
| Component has 4 colored bands | Check for Brown-Red-Brown-Gold | Confirmed 120Ω (5% tolerance) |
| Component has 5 colored bands | Check for Brown-Red-Black-Black-Brown | Confirmed 120Ω (1% tolerance) |
| Component is SMD with 3 digits | Check for '121' printed on casing | Confirmed 120Ω SMD (5% tolerance) |
| Component is SMD with 4 digits | Check for '1200' printed on casing | Confirmed 120Ω SMD (1% tolerance) |
| Bands are burned, faded, or ambiguous | Proceed to safe multimeter measurement | Verify via DMM (see below) |
Safe Interpretation When Markings Are Faded or Burned
When a resistor has been subjected to thermal stress, the epoxy coating darkens and the color bands turn into indistinguishable brown smears. Carbon composition resistors drift high in value when overheated, while metal film resistors typically fail open. Never trust the visual marking of a heat-damaged component.
To safely and accurately interpret a faded 120Ω resistor, follow this bench procedure:
- De-energize and Discharge: Remove all power from the circuit. Discharge any large capacitors using a bleed resistor to prevent damaging your multimeter or shocking yourself.
- Isolate One Leg: Use a soldering iron to desolder and lift one leg of the resistor out of its PCB pad. This breaks the parallel circuit path, ensuring your multimeter only reads the component itself.
- Zero the Meter: Touch your multimeter probes together. If your meter has a relative (REL) or zero button, press it to null out the probe resistance (which can be 0.2Ω to 0.5Ω on cheap meters—significant when measuring low values, though less critical at 120Ω).
- Measure and Judge: Place the probes on the resistor leads.
- If the reading is between 114Ω and 126Ω, the resistor is within standard 5% tolerance and is likely functional.
- If the reading is between 118.8Ω and 121.2Ω, it is within 1% tolerance.
- If the reading is significantly higher (e.g., 150Ω or OL/Open), the resistor has suffered thermal damage and must be replaced.
For further reading on standard component markings and tolerance calculations, refer to the All About Circuits resistor color code guide or use the DigiKey conversion calculator to verify complex 5-band and 6-band variants on the bench.
