The 100 ohm resistor colour code is Brown, Black, Brown for standard 4-band axial resistors, and Brown, Black, Black, Black for 5-band precision resistors. If you are looking at a 6-band resistor, add a temperature coefficient band (usually Brown for 100ppm/°C). This guide provides the exact IEC 60062 reference tables, common misreads, and how to verify faded components on the bench without guessing.
The IEC 60062 Resistor Color Code Reference Table
Before decoding the specific 100 ohm value, you need the master key. The international standard IEC 60062 dictates the color-to-digit mapping for axial leaded resistors. Read the bands from left to right, keeping the tolerance band (usually Gold or Silver) on the far right.
| Color | Digit (Bands 1-3) | Multiplier (Band 4/5) | Tolerance (Band 4/5/6) | Temp Coeff (Band 6) |
|---|---|---|---|---|
| Black | 0 | ×1 (10⁰) | — | 250 ppm/°C |
| Brown | 1 | ×10 (10¹) | ±1% (F) | 100 ppm/°C |
| Red | 2 | ×100 (10²) | ±2% (G) | 50 ppm/°C |
| Orange | 3 | ×1k (10³) | — | 15 ppm/°C |
| Yellow | 4 | ×10k (10⁴) | — | 25 ppm/°C |
| Green | 5 | ×100k (10⁵) | ±0.5% (D) | — |
| Blue | 6 | ×1M (10⁶) | ±0.25% (C) | 10 ppm/°C |
| Violet | 7 | ×10M (10⁷) | ±0.1% (B) | 5 ppm/°C |
| Grey | 8 | — | ±0.05% (A) | — |
| White | 9 | — | — | — |
| Gold | — | ×0.1 (10⁻¹) | ±5% (J) | — |
| Silver | — | ×0.01 (10⁻²) | ±10% (K) | — |
100 Ohm Specific Breakdown & Standard Variants
While IEC 60062 is the global baseline, the physical format of the resistor changes how the 100 ohm value is encoded. Below is the exact breakdown for a 100Ω resistor across different band counts and surface mount packages.
| Resistor Type | Band 1 | Band 2 | Band 3 | Band 4 | Band 5 / 6 |
|---|---|---|---|---|---|
| 4-Band (5% Tolerance) | Brown (1) | Black (0) | Brown (×10) | Gold (±5%) | — |
| 5-Band (1% Tolerance) | Brown (1) | Black (0) | Black (0) | Black (×1) | Brown (±1%) |
| 6-Band (High Precision) | Brown (1) | Black (0) | Black (0) | Black (×1) | Brown (±1%) / Brown (100ppm) |
| SMD (3-Digit EIA) | 101 (10 × 10¹ = 100) | — | |||
Regional and Standard Variants
When working on legacy or specialized equipment, you will encounter deviations from the standard IEC color bands:
- US MIL-SPEC (MIL-PRF-22684): Military-spec resistors often use 5 bands exclusively, even for 5% tolerance parts. Furthermore, the body color of the resistor indicates the reliability level (e.g., a yellow body indicates a specific failure rate per 1,000 hours). The color bands still follow IEC 60062 math, but the body color is a data point you must not ignore.
- Old UK BS1852: If you are restoring vintage British audio gear from the 1960s or 70s, you might encounter the BS1852 standard. While the digit colors largely match IEC, the body color sometimes represented the first digit, the tip color the second, and a dot the multiplier. Always cross-reference with a schematic when working on pre-1980s UK equipment.
- SMD EIA-96 Code: For 1% tolerance surface mount resistors, the 3-digit '101' code is replaced by the EIA-96 alphanumeric system. A 100 ohm SMD resistor in this standard is marked 01A (01 = 100, A = ×1).
Rows and Bands People Get Wrong on the Bench
Even experienced technicians misread resistors under poor lighting or when dealing with cheap manufacturing tolerances. Here are the specific rows from the reference table that cause the most bench errors, and how to avoid them.
Row 1: Black in the Multiplier Column
The most common mistake happens when reading a 5-band 100 ohm resistor (Brown-Black-Black-Black-Brown). Technicians see the fourth band is Black and assume it means 'zero' or an open circuit. In the digit columns, Black is 0. But in the multiplier column, Black means 10⁰, which is ×1. The value is 100 × 1 = 100 ohms, not 0 ohms.
Row 2 vs Row 3: Red vs. Orange
Carbon composition resistors baked by decades of heat often experience pigment shifting. A Red band (2) can easily fade or bake into a shade that looks exactly like Orange (3). If you are reading what you think is a 100 ohm resistor but the second band looks slightly orange, you might actually be holding a 130 ohm or 300 ohm resistor. Fix: Use a bright, cool-white LED bench light (5000K+) to separate the red/orange wavelengths, and verify with a multimeter.
Rows 11 & 12: Gold and Silver Tolerance Confusion
Gold (±5%) and Silver (±10%) only appear in the tolerance column (the final band). However, on 4-band resistors with a Gold multiplier (×0.1), people sometimes misalign the reading direction. If you read a 100 ohm resistor backward (Gold-Black-Brown-Brown), you'll calculate a nonsensical value. Rule of thumb: The tolerance band is usually spaced slightly further apart from the other three bands. If the spacing is identical, start reading from the end that does not with Gold or Silver.
Safe Interpretation When Markings are Faded or Missing
When a resistor has been subjected to thermal runaway, the epoxy body turns a uniform dark brown, completely obscuring the color bands. Here is the systematic procedure to safely identify and verify a 100 ohm resistor when visual decoding is impossible.
Step 1: Isolate the Component (Out-of-Circuit Measurement)
Never measure a resistor while it is still soldered into the board. A 100 ohm resistor in-circuit will be in parallel with other traces, IC pins, and capacitors. A parallel path of 100 ohms will make your 100 ohm resistor read as 50 ohms on your multimeter. Desolder at least one leg of the resistor to lift it off the PCB pad before measuring.
Step 2: Compensate for Test Lead Resistance
At 100 ohms, the resistance of your multimeter probes actually matters. Standard cheap test leads have a resistance of 0.2Ω to 0.5Ω. If your meter reads 100.4Ω, the resistor might actually be exactly 100.0Ω. Action: Short your probe tips together before measuring. Note the baseline resistance (e.g., 0.3Ω) and subtract it from your final reading. For absolute precision on 1% metal film resistors, use a bench multimeter with a 4-wire Kelvin measurement setup to eliminate lead resistance entirely.
Step 3: Check for Thermal Drift and Damage
If your meter reads significantly higher than 100 ohms (e.g., 115 ohms), the resistor has likely suffered permanent thermal drift. Carbon composition resistors are notorious for drifting upward in value when exposed to sustained heat. Metal film resistors (like the Vishay MRS25 series) are far more stable but will still drift if pushed past their 0.6W power rating. If a 100 ohm resistor measures out of its stated tolerance band (e.g., >105Ω for a 5% part), discard it. Do not reuse heat-stressed resistors in precision or high-current paths.
For further study on resistor construction and failure modes, refer to the All About Circuits resistor guide, or use the DigiKey Color Code Calculator to double-check your bench math on complex 6-band components.
