Reading the bands
On a four-band resistor the first two bands are digits, the third is the number of zeros to add, and the fourth is the tolerance. Brown-black-red-gold is 1, 0, two zeros, ±5 percent: 1,000 Ω. Five-band parts, which are usually 1 percent, give three digits before the multiplier, so brown-green-black-brown-brown is 1, 5, 0, one zero, ±1 percent: 1,500 Ω.
The digit colours run in spectrum order once you know where they start: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9. As a multiplier, gold means divide by ten and silver by a hundred, which is how sub-ohm values are marked. As a tolerance band, gold is 5 percent and silver 10 percent.
The two practical failure modes are reading from the wrong end and misjudging a colour under warm light. Reading backwards is common because the tolerance band is only slightly separated from the group and on some bodies the gap is not obvious. If the value that comes out is not close to a standard size, turn the part around and read it the other way. Brown against red and orange against red are the usual colour confusions, and both are worse on old carbon parts that have darkened with heat.
Surface-mount markings
| Marking | Scheme | Value |
|---|---|---|
| 473 | 3-digit: two digits, then zeros | 47 kΩ |
| 101 | 3-digit | 100 Ω |
| 4702 | 4-digit, used on 1% parts | 47 kΩ |
| 4R7 | R marks the decimal point | 4.7 Ω |
| R047 | R notation, sub-ohm | 0.047 Ω |
| 0 or 000 | Zero-ohm link | A jumper, not a resistor |
| 01C, 24B | EIA-96: two digits index the E96 ladder, letter sets the multiplier | Needs the EIA-96 table |
EIA-96 exists because a 1 percent part needs three significant digits and an 0402 body has room for three characters. This page does not carry that table; if you meet one of those markings the meter is faster than looking it up. Note also that many small surface-mount resistors carry no marking at all, and capacitors of that size almost never do.
Why the standard values are the numbers they are
E12 and E24 are the two common ladders. E12 has twelve values per decade — 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 — spaced so that a ±10 percent part covers the gap to its neighbour. E24 has twenty-four, adding 11, 13, 16, 20, 24, 30, 36, 43, 51, 62, 75 and 91, and suits ±5 percent parts. The ladders come from a logarithmic progression, but the published values are not exactly that progression: the true twelfth roots would give 2.6, 3.2 and 4.6 where the standard says 2.7, 3.3 and 4.7. Those three rounding exceptions are historical, and they are the reason this page carries the tables rather than computing the ladder from a formula.
The consequence when designing is that you cannot ask for an arbitrary value. Calculate what you want, take the nearest standard size, and check that the error it introduces is acceptable — which for most bias networks and current limits it is. When it is not, two resistors in series or parallel will reach almost any value, at the cost of a second part and a second tolerance.
The meter is still faster
For a loose part in a drawer, measuring beats decoding. Colour codes matter mainly for parts already installed, where you can see the body and cannot easily lift a leg. Bear in mind that a resistor measured in circuit reads low, because everything else on the board is in parallel with it, so an in-circuit reading below the marked value is not evidence of a failed part.
Once you know the value, the LED resistor calculator and the voltage divider calculator cover what it is likely doing, and the heat sink calculator handles the case where it is running hot enough to have discoloured the board around it.
Questions people ask
How do I know which end of a resistor to start reading from?
The tolerance band is normally set a little further from the others, and it is usually gold or silver, neither of which appears as a first digit. So if one end has a gold or silver band, that end is the last one. Where all the bands are evenly spaced and none is gold or silver, read it both ways and see which result lands on or near a standard value — the wrong direction almost always produces a number that is not in the E12 or E24 ladder. If both directions look plausible, measure it.
What does a resistor with no tolerance band mean?
A three-band part is nominally ±20 percent, an old grade that is rare in new production but common in salvaged equipment and hobby kits. Two digits and a multiplier give you the value; you simply have no tolerance information beyond the assumption. In practice a ±20 percent part measured today may be further out still if it has spent years running warm, since carbon composition resistors drift upward with age and heat.
Why is 4.7 kΩ so common instead of a round 5 kΩ?
Because 47 is a rung on the standard ladder and 50 is not. The ladder is logarithmic, chosen so that consecutive values are separated by roughly the tolerance of the parts, which means a manufacturer can produce one distribution and sort it into adjacent bins without gaps or waste. The consequence for designers is that round decimal numbers mostly do not exist as stock values, and habits form around what does — 4.7 k, 10 k, 47 k, 100 k appear everywhere because they are what has always been on the shelf.
What does the code 104 mean on a capacitor?
One hundred nanofarads, written as 10 followed by four zeros in picofarads: 100,000 pF, which is 100 nF or 0.1 µF. The scheme matches the resistor SMD scheme — first two digits then the number of zeros — and the unit is always picofarads. So 103 is 10 nF, 105 is 1 µF, and 220 is 22 pF rather than 220 pF. A letter after the digits is the tolerance code, most often J for ±5 percent, K for ±10 percent or M for ±20 percent.
Can I measure a resistor without taking it out of the circuit?
You can measure it, but you cannot trust the reading. Everything else on the board connected across that resistor is in parallel with it, so an in-circuit measurement is always equal to or lower than the true value. A reading that matches the marking is good evidence the part is fine; a reading below it tells you nothing on its own. Semiconductor junctions nearby can also conduct at the meter's test voltage and produce odd readings. Lift one end if the value actually matters, and always with the power off and the supply capacitors discharged.