LessonReal resistors: values, codes and tolerance · 15 min
Reading resistor values: the colour code and SMD codes
A resistor is too small to print "4.7 kΩ" on. Through-hole resistors carry coloured bands; surface-mount (SMD) resistors a tiny number code. Reading both quickly is a basic skill - for building, and for repairing boards where the parts list is long gone.
Writing values: the letter is the decimal point
On drawings and parts lists, values are often written with the prefix letter in place of the decimal point: 4k7 = 4.7 kΩ, 2R2 = 2.2 Ω (R marks the point for plain ohms), 1M5 = 1.5 MΩ, R10 = 0.10 Ω. A decimal point can vanish on a smudged print; a letter cannot.
The colour code
| Colour | Digit | Multiplier | Tolerance |
|---|---|---|---|
| Black | 0 | × 1 | |
| Brown | 1 | × 10 | ±1 % |
| Red | 2 | × 100 | ±2 % |
| Orange | 3 | × 1 k | |
| Yellow | 4 | × 10 k | |
| Green | 5 | × 100 k | ±0.5 % |
| Blue | 6 | × 1 M | ±0.25 % |
| Violet | 7 | × 10 M | ±0.1 % |
| Grey | 8 | ±0.05 % | |
| White | 9 | ||
| Gold | × 0.1 | ±5 % | |
| Silver | × 0.01 | ±10 % |
- Turn the resistor so the tolerance band (gold, silver - often set a little apart) is on the right. Start from the band nearest the other end.
- 4 bands (5 % parts): digit, digit, multiplier, tolerance.
- 5 bands (1 % parts): digit, digit, digit, multiplier, tolerance.
Brown, black, orange, gold.
- Brown = 1, black = 0: the digits are 10.
- Orange = × 1000: 10 × 1000 = 10 000 Ω.
- Gold = ±5 %.
Answer: 10 kΩ ±5 % - the most common resistor of all.
Yellow, violet, black, brown, brown.
- Digits: 4, 7, 0 → 470.
- Brown multiplier = × 10: 4700 Ω.
- Brown tolerance = ±1 %.
Answer: 4.70 kΩ ±1 % (a metal-film part, usually with a blue body).
| Bands | Value |
|---|---|
| red, red, brown, gold | 220 Ω ±5 % |
| yellow, violet, red, gold | 4.7 kΩ ±5 % |
| brown, black, yellow, gold | 100 kΩ ±5 % |
| green, blue, gold, gold | 5.6 Ω ±5 % |
| brown, black, black, red, brown | 10.0 kΩ ±1 % |
SMD codes
- 3 digits (5 % parts): two digits and a multiplier power of ten. 103 = 10 × 10³ = 10 kΩ; 472 = 4.7 kΩ; 220 = 22 × 10⁰ = 22 Ω (not 220!).
- 4 digits (1 % parts): three digits and the multiplier. 1002 = 100 × 10² = 10.0 kΩ; 4701 = 4.70 kΩ.
- R for the point: 4R7 = 4.7 Ω, R100 = 0.1 Ω (current-sense resistors).
- EIA-96 (tiny 1 % parts): two digits that index a table, and a letter multiplier - 01C = 100 × 100 = 10 kΩ. Look these up.
Practice 1Red, violet, yellow, gold is:
- 27 kΩ
- 270 kΩ
- 2.7 MΩ
- 274 Ω
Show the answer
Answer: B. 270 kΩ
2, 7 and × 10 000: 270 000 Ω = 270 kΩ ±5 %.
Practice 2What is an SMD resistor marked 222?
Show the answer
Answer: 2200 Ω
22 × 10² = 2200 Ω = 2.2 kΩ.
Practice 3What is an SMD resistor marked 1001?
Show the answer
Answer: 1000 Ω
100 × 10¹ = 1000 Ω = 1.00 kΩ (1 %).
Practice 4Which bands mark a 68 kΩ ±5 % resistor?
- blue, grey, red, gold
- blue, grey, orange, gold
- grey, blue, orange, gold
- blue, grey, yellow, silver
Show the answer
Answer: B. blue, grey, orange, gold
6 = blue, 8 = grey, × 1000 = orange, ±5 % = gold.
Challenge 5Brown, green, red, gold. What is the HIGHEST resistance this part may really have?
Hint
Read the value, then add the tolerance.
Show the answer
Answer: 1575 Ω
1, 5, × 100 = 1500 Ω ±5 %: anything from 1425 Ω to 1575 Ω.