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You pick up a small resistor in the physics lab and see four coloured rings. The examiner asks for its value. If you know the code, it takes ten seconds; if you don't, there is no way to work it out. This post gives you the code, five worked readings, the tolerance arithmetic and a short practice routine.
Is the colour code still in the Class 12 syllabus?
Not as a textbook topic. The current NCERT Class 12 chapter on current electricity (reprint 2026-27) no longer has the colour-code table. The 2021-22 edition of the same chapter did, as Table 3.2, and that is the version older reference books copy.
The skill still matters in the lab. The CBSE Physics curriculum for 2026-27 lists the activity "To identify a diode, an LED, a resistor and a capacitor from a mixed collection of such items", and names carbon resistors in its apparatus list. The JEE (Main) 2026 syllabus has the same identification task under experimental skills. Check the current year's documents, since both are revised each cycle.
What do the colours stand for?
The 2021-22 NCERT table gives each colour a digit and a multiplier:
Black 0, multiplier 1
Brown 1, multiplier 10
Red 2, multiplier 10²
Orange 3, multiplier 10³
Yellow 4, multiplier 10⁴
Green 5, multiplier 10⁵
Blue 6, multiplier 10⁶
Violet 7, multiplier 10⁷
Grey 8, multiplier 10⁸
White 9, multiplier 10⁹
Gold: multiplier 10⁻¹, tolerance 5%
Silver: multiplier 10⁻², tolerance 10%
No fourth band: tolerance 20%
NCERT's rule: the first two bands give the first two significant figures in ohms, the third is the decimal multiplier, and the last is the tolerance, "the possible variation in percentage about the indicated values". Many students remember the order with a sentence whose initials are B B R O Y G B V G W. Make up your own; one you wrote yourself sticks better.
Why only carbon resistors? NCERT explained that wire-bound resistors, wound from alloys such as manganin, constantan or nichrome, are used for lower values, while carbon resistors are small, cheap, and "hence their values are given using a colour code".
Five worked readings
Orange, blue, yellow, gold (NCERT's own example). Digits 3 and 6, multiplier 10⁴: 36 × 10⁴ Ω = 360 kΩ. Gold means ±5%, and 5% of 360 kΩ is 18 kΩ, so the true value lies between 342 kΩ and 378 kΩ.
Brown, black, red, silver. 10 × 10² = 1000 Ω = 1 kΩ, ±10%, so 900 Ω to 1100 Ω.
Yellow, violet, black, gold. 47 × 1 = 47 Ω, ±5%. 5% of 47 is 2.35, so 44.65 Ω to 49.35 Ω.
Green, blue, gold, gold. The third band is gold, so the multiplier is 10⁻¹: 56 × 0.1 = 5.6 Ω, ±5%, so 5.32 Ω to 5.88 Ω.
Red, red, orange, no fourth band. 22 × 10³ = 22 kΩ, ±20%, so 17.6 kΩ to 26.4 kΩ.
How does tolerance carry into a circuit?
Tolerance tells you the range the maker promises, not an error you made. It carries straight into combinations. Two 1 kΩ, ±10% resistors in series give 2 kΩ, anywhere from 1.8 kΩ to 2.2 kΩ. In parallel they give 500 Ω; the extremes are 900 Ω in parallel with 900 Ω, which is 450 Ω, and 1100 Ω with 1100 Ω, which is 550 Ω. Our post on series, parallel and symmetry shortcuts has the combination rules.
In the lab, check a reading against the range. Suppose (sample number) a multimeter shows 1.04 kΩ for the brown, black, red, silver resistor above. That lies inside 900 Ω to 1100 Ω, so the resistor is fine. A reading of 1.30 kΩ would mean either a misread band or a faulty part.
Which mistakes cost marks?
Reading the third band as a digit. Brown, black, red is 10 × 10² = 1000 Ω, not 102 Ω.
Starting from the wrong end. Find the gold or silver band first and read away from it.
Forgetting that gold as a multiplier means 0.1. It shrinks the value; it does not add a zero.
Mixing up similar colours under poor lab light: red and orange, brown and red, blue and violet. Tilt the resistor towards a window or use your phone torch.
Dropping the unit. Write Ω or kΩ every time; 47 and 47 kΩ are a thousand times apart.
A ten-minute practice routine
Decode. Write five random four-band codes and decode each, with the tolerance range.
Encode. Go the other way: 330 Ω is orange, orange, brown; 4.7 kΩ is 47 × 10², so yellow, violet, red; 100 kΩ is 10 × 10⁴, so brown, black, yellow.
Check in the lab. Read three real resistors, measure them, and note whether each reading lies inside its tolerance range.
Prepare for the viva. Be ready to say why carbon resistors carry a code and what tolerance means. Our guide to the board practical exam and viva covers the rest.
If series and parallel circuits are still hard to picture, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read. Resistance also changes with temperature, which our post on the temperature coefficient of resistance explains.
Before your next lab session, decode the five readings above from memory and check your answers against this page.
