It is often useful to know the value of a resistor already mounted in a circuit. In this case, an ohmmeter cannot be used because the rest of the circuit would affect the measurement, and it is always annoying to unsolder at least one pin, even more if the case is of the SMD type (surface mounted device). Most of the time, a marking allows to know this value.
A colour code is generally used. There is also sometimes a clear marking.
The colour code is well-known, and can be found on many websites. I remind you anyway:
A resistor has several coloured rings, generally four or five, depending on its precision.
First problem: how to know the direction of reading? This is not always obvious, except in the most frequent case where there is a gold or silver ring. It must be on your right. This ring indicates the value tolerance, in percentage. This is not of great interest to us.
Then we have three or four digits left. The first two or three (from the left) can be read as they are. But the last represents the number of zeros to be added after the first digits.
In this first example, one reads successively brown, green, red, i.e. 1 5 2. The value is 15 followed by two zeros, i.e. 1500 ohms, or 1.5 kΩ.
In this second example, we read successively red, violet, black, or 4 7 0. The value is 47 followed by no zero, so 47 Ω. Note that it is not always easy to distinguish between brown and violet.
In this third example, we read successively red, violet, green, orange, or 4 7 5 3. The value is 475 followed by three zeros, i.e. 475,000 Ω or 475 kΩ. Note that the tolerance ring (the rightmost one) is now brown, which doesn’t ease decipher the value.
These resistors (and many other devices) are in the form of tiny rectangular blocks. Marking, where it exists, is in the form of figures. But the principle is exactly the same as the previous one, without the step of colours. It is a priori easier, upon the condition of having a good sight… I therefore resumed the same examples, without further explanations.
The most common standardized values (there are others) are twelve:
1 - 1.2 - 1.5 - 1.8 - 2.2 - 2.7 - 3.3 - 3.9 - 4.7 - 5.6 - 6.8 - 8.2
All multiples by 10, 100, 1000, etc. of these values are also standardized. So, the following values will be 10, 12, ... 82, 100, 120, etc.