How do I read a resistor's colour bands?
Start from the band closest to one end — the tolerance band sits slightly apart at the other. On a 4-band resistor the first two bands are digits, the third is the multiplier (how many zeros to add), and the fourth is tolerance. Brown-black-red-gold is 1, 0, ×100, ±5% — a 1 kΩ resistor. The mnemonic for 0–9 is Black, Brown, Red, Orange, Yellow, Green, Blue, Violet, Grey, White.
What is the difference between 4, 5 and 6-band resistors?
A 3-band resistor has two digits and a multiplier, with tolerance assumed to be ±20%. A 4-band adds an explicit tolerance band. A 5-band uses three significant digits instead of two, giving finer precision — used for 1% and better tolerances. A 6-band adds a temperature coefficient in ppm/K, describing how much the value drifts as the component heats.
Which end do I start reading from?
This trips up beginners constantly. The tolerance band is usually gold or silver and has a wider gap before it, so it goes on the right. If both ends look similar, the tolerance band is the one closest to the lead. Reading a resistor backwards turns brown-black-red into red-black-brown — 200 Ω instead of 1 kΩ, a factor of five out. When in doubt, measure it.
What does tolerance actually mean in practice?
It is the manufacturing spread. A 1 kΩ ±5% resistor is guaranteed only to lie between 950 and 1050 Ω. For pull-ups, current limiting and general use that is fine. For voltage dividers feeding an ADC, timing circuits or precision measurement it is not — those want 1% or better. The calculator shows the actual minimum and maximum so you can judge whether the spread matters for your circuit.
Why are some resistor values so odd, like 4.7 kΩ?
Because standard values follow the E-series, spaced logarithmically so that consecutive values differ by roughly the tolerance band. E12 (10% tolerance) gives 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 and their decades; E24 doubles that density for 5%. The spacing means any target value is within tolerance of a stock part, which is why 4.7 exists and 5.0 does not.
What if the bands are unreadable or the resistor has no bands?
Surface-mount resistors use printed numeric codes instead: 3 digits where the last is the multiplier (472 is 47 × 10² = 4.7 kΩ), or 4 digits for precision parts, or the EIA-96 code of two digits plus a letter. For a colour-banded resistor whose bands have faded or where the body is discoloured by heat, measure it with a multimeter out of circuit — a heat-damaged resistor has often drifted anyway.