The main difference between a three-band and a four-band resistor is the explicit tolerance band. Both formats use two significant digits and a multiplier, but a three-band part normally leaves tolerance unstated. In common resistor-color practice that missing tolerance is treated as +/- 20 percent. Select the band count in the Resistor Color Code Calculator before comparing examples.
What stays the same
Both formats use the first two bands as significant digits and the third band as the multiplier. The nominal formula is (10 x digit 1 + digit 2) x multiplier. Brown, black, red therefore means 10 x 100 = 1,000 ohms before tolerance is considered.
What the fourth band adds
A four-band code adds an explicit tolerance band after the multiplier. Brown, black, red, gold means 1 kohm +/- 5%, with a nominal range of 950 to 1,050 ohms. Silver in the fourth position commonly means +/- 10%; brown or red can indicate tighter tolerances where the part series supports them.
Three-band tolerance is a convention
A three-band resistor has no separate tolerance band. Common decoding practice treats the missing tolerance as +/- 20%, so brown, black, red is commonly described as 1 kohm +/- 20%, or 800 to 1,200 ohms. This is a decoding convention, not a universal promise about every historical or manufacturer-specific part.
Gold and silver can change roles
Position matters. Brown, black, gold is a valid three-band example of 10 x 0.1 = 1 ohm. Red, red, silver is 22 x 0.01 = 0.22 ohm. The same gold color in the fourth position is normally a tolerance mark, not a multiplier. Gold and silver cannot be significant digits.
Comparison table
| Format | Band order | Significant digits | Tolerance |
|---|---|---|---|
| 3-band | Digit 1 / Digit 2 / Multiplier | 2 | Not printed; commonly +/- 20% |
| 4-band | Digit 1 / Digit 2 / Multiplier / Tolerance | 2 | Band 4 |
Examples to compare
| Marking | Format | Calculation | Result |
|---|---|---|---|
| Brown, Black, Red | 3-band | 10 x 100 | 1 kohm, commonly +/- 20% |
| Brown, Black, Red, Gold | 4-band | 10 x 100 | 1 kohm +/- 5% |
| Brown, Black, Gold | 3-band | 10 x 0.1 | 1 ohm, commonly +/- 20% |
| Brown, Black, Gold, Gold | 4-band | 10 x 0.1 | 1 ohm +/- 5% |
| Red, Red, Silver | 3-band | 22 x 0.01 | 0.22 ohm, commonly +/- 20% |
| Red, Red, Silver, Silver | 4-band | 22 x 0.01 | 0.22 ohm +/- 10% |
| Orange, Orange, Black | 3-band | 33 x 1 | 33 ohm, commonly +/- 20% |
| Orange, Orange, Black, Brown | 4-band | 33 x 1 | 33 ohm +/- 1% |
| Yellow, Violet, Red | 3-band | 47 x 100 | 4.7 kohm, commonly +/- 20% |
| Yellow, Violet, Red, Gold | 4-band | 47 x 100 | 4.7 kohm +/- 5% |
| Green, Blue, Black | 3-band | 56 x 1 | 56 ohm, commonly +/- 20% |
| Green, Blue, Black, Gold | 4-band | 56 x 1 | 56 ohm +/- 5% |
When band count is uncertain
Do not infer three versus four bands from body size. Power rating and construction affect size more than the code format. Count the visible marks, look for the wider gap before tolerance, and check whether paint has been lost. If a mark is hidden or both directions look plausible, keep both candidates and verify with a schematic, a parts list, or an isolated meter reading.
How the calculator handles the two formats
Choose three bands when the component visibly has two digits and one multiplier. The calculator treats the missing tolerance as 20 percent. Choose four bands when a tolerance band is visible and select its color. It updates as the fields change; it does not identify band count from a photograph or decide whether an unusual marking follows the common convention.
Frequently asked questions
Is every three-band resistor +/- 20%?
No. +/- 20% is a common decoding convention for a missing tolerance band. The actual part specification takes priority.
Can gold or silver be the third band?
Yes. In the multiplier position, gold is x0.1 and silver is x0.01. In the final position of a four-band code, those colors normally indicate tolerance.
Does a four-band code always identify a replacement part?
No. It does not identify power, voltage, pulse, construction, safety approval, or the correct manufacturer series.
Source and limits
This guide follows IEC 60062-style resistor color marking practice and the color/value conventions used in common through-hole manufacturer tables. Color decoding identifies the nominal marking; it does not prove that a particular part is genuine, undamaged, correctly installed, or available in the stated power rating.
- Use the Resistor Color Code Calculator as a reading aid, then compare the result with the component body, schematic, bill of materials, and datasheet.
- Gold, silver, faded paint, unusual spacing, and manufacturer-specific marks can create more than one plausible reading.
- Do not use a color-only result as a safety, installation, procurement, customs, or compliance decision.
Related guides
4-Band Resistor Color Code gives a focused worked example; 5-Band vs 6-Band covers three-digit formats; and Tolerance Bands explains the uncertainty range.