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How to read a resistor by its bands
Small resistors are too tiny for printed numbers, so their value is painted on as coloured bands around the body. Each colour stands for a digit, a multiplier, or a tolerance, and reading them in order gives the resistance. On a four-band resistor, the first two bands are digits, the third band is the multiplier, and the fourth band is the tolerance. On a five-band resistor, the first three bands are digits, the fourth is the multiplier, and the fifth is the tolerance. The tolerance band is usually spaced slightly apart from the others, or printed in gold or silver, which helps you find the correct reading direction.
The digit colours run from black for zero through brown, red, orange, yellow, green, blue, violet, gray and white for nine. The multiplier band uses the same colours as powers of ten, with gold meaning multiply by 0.1 and silver meaning multiply by 0.01. The tolerance band tells you how far the real resistance may wander from the printed value: gold means 5 percent, silver means 10 percent, brown means 1 percent, and the tighter tolerances of precision resistors go down to 0.1 percent and below. This decoder takes your band colours and returns the value, the tolerance and the range the real resistor may fall in.
Worked examples, step by step
Take a four-band resistor with bands brown, black, red and gold. Brown is 1 and black is 0, giving the digits 10. Red as a multiplier means times 100, so the value is 1000 ohms, or 1 kilo-ohm. Gold tolerance means 5 percent, so the real resistor may measure anywhere from 950 ohms to 1050 ohms. That is the classic 1k resistor found in countless beginner kits, and it is worth memorising because it appears everywhere.
Why tolerance matters in real circuits
Tolerance decides whether a circuit works first time or needs adjustment. An LED current-limiting resistor at 5 percent is usually fine, because the LED brightness changes slightly and nobody notices. A voltage divider feeding an analogue measurement, or a timing circuit built around a resistor and capacitor, can drift noticeably at 10 percent, and a 1 percent resistor is the cheap fix. Precision audio, measurement and reference circuits go further, to 0.1 percent or better, and at that point temperature drift matters too, because a resistor changes value as it warms up.
Tolerance also explains why measured resistors rarely read exactly their printed value. A new 1k resistor at 5 percent reading 972 ohms is not faulty; it is inside its specification. Beginners sometimes throw away good resistors because a multimeter reading differs from the band value by a few percent. Check the range the decoder reports before judging a part. If the measured value falls outside the tolerance range, then the resistor is suspect, or it is being measured in circuit, where parallel paths confuse the meter.
Common mistakes when decoding
The most common mistake is reading the bands backwards. The tolerance band is usually gold or silver and sits slightly apart, so start from the opposite end. If both ends look plausible, the reading that gives a standard value from the E-series, such as 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68 or 82 times a power of ten, is almost always the correct one. The second mistake is confusing four-band and five-band parts. Reading a five-band resistor as four-band drops a digit and gives a value ten times too small.
Frequently asked questions
What does the fifth band do? It adds a third significant digit, so five-band resistors carry more precise values. What if my resistor has only three bands? Then it has two digits and a multiplier, with a default tolerance of 20 percent, which is common on very old parts. Why do some resistors have six bands? The sixth band is a temperature coefficient, telling you how much the value drifts with heat. It does not change the resistance value itself. Can I trust the decoder over my meter? Use both: the decoder tells you the intended value and range, and the meter tells you the actual part in your hand.
Safety first: an estimate, not a code approval
Every electrical tool on this page is an educational estimator only. It is never professional advice and never a substitute for a licensed electrician. Electrical codes differ by country, region and building type, and they change over time. A result that looks fine here can still fail a local inspection, and a result that looks tight here can still be acceptable under a rule this simple calculator does not model, such as temperature correction, grouping, or a special location rule. Before you buy cable, cut a wall, or touch a panel, check your local electrical code and talk to a licensed electrician. Breaker, conduit and box-fill results are estimates, not code-compliance guarantees. If you are unsure about any part of a job, stop and ask a professional. Electricity can injure or kill, and no free online calculator changes that fact.
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