Resistor basics
Resistors are fundamental electronic components that limit electrical current flow. Rather than printing tiny numbers on miniature parts, manufacturers use a standardized color-coding system with colored bands wrapped around the component body. A 4-band resistor tells you two things: the resistance value (in ohms) and how much it might vary from that value (tolerance).
How it works
Each band on a 4-band resistor serves a specific purpose. The first two bands represent individual digits (0–9). The third band is a multiplier that scales those digits by powers of 10, or by fractions like 0.1 or 0.01. The fourth band indicates the tolerance—how closely the manufacturer guarantees the actual resistance matches the stated value.
To decode: read the first band as the first digit, the second band as the second digit, combine them into a two-digit number, then multiply by the value of the third band. The fourth band tells you the acceptable error range.
The formula
Resistance (Ω) = (First digit × 10 + Second digit) × Multiplier; Tolerance = ±X%
Worked example
Suppose you have a resistor with bands: Brown, Black, Orange, Gold.
Step 1: Identify each band value
- Band 1 (Brown) = 1
- Band 2 (Black) = 0
- Band 3 (Orange) = ×1,000
- Band 4 (Gold) = ±5%
Step 2: Combine the first two digits
- 1 and 0 → 10
Step 3: Multiply by the multiplier
- 10 × 1,000 = 10,000 Ω
Step 4: Express with tolerance
- Result: 10 kΩ (kilohms) ±5%
- Actual value will fall between 9,500 Ω and 10,500 Ω
Another example: Red, Violet, Red, Brown
- Red = 2, Violet = 7, Red multiplier = ×100, Brown tolerance = ±1%
- Resistance: (2 × 10 + 7) × 100 = 27 × 100 = 2,700 Ω or 2.7 kΩ ±1%
Common mistakes
The most frequent error is reading bands in the wrong direction. Always start from the end nearest the tolerance band—this end is typically closer to the edge of the component. If you're unsure which end is which, look for a slight gap or observe that the tolerance band is usually gold or silver (less common in the first three bands).
Another pitfall is misidentifying the multiplier band. Gold and Silver are multipliers (×0.1 and ×0.01), not digit bands. If your first band is gold or silver, you've likely flipped the resistor around.
Finally, don't ignore tolerance in circuit design. A ±10% resistor in a precision analog circuit can cause significant errors. Check your circuit's requirements and select appropriate tolerance grades—Brown (±1%) and Red (±2%) are more precise, while Gold (±5%) and Silver (±10%) are looser.
This calculator provides an estimate based on standard IEC 60062 color codes. Always verify component values with a multimeter before critical applications.