# How to find SMD resistor values and size?

I want to do reverse engineering of a PCB, in which some SMD resistors are used. On top of each resistor it has marked with 1R0, 150 , 0 etc. Is that represents the value of the resistor? If so, how to find the size of the resistor.

Say, a 2512 resistor SMD package has a width of 3.2mm and a length of 6.4mm, will it be available with all resistor values like 1kohm , 2kohm etc.

• Go to your favourite on-line component store and check. Commented Jul 6, 2017 at 12:24
• The number on the resistor is its value. Those appear to be 0805 sized parts.
– JRE
Commented Jul 6, 2017 at 12:28
• I feel like this has been asked 1000x times. There's no way this isn't documented.
– Bort
Commented Jul 6, 2017 at 17:35

The size of a SMD resistor affects it's thermal dissipation, and maximum voltage rating (although other factor are also likely to be limiting for most values in a range). Different types will also have different tolerance and temperature performance.

There is no significant interaction between the available resistance values, and the package size.

Yes. Or, to be more precise and exceed the minimum required character count for a cheap answer - Yes.

The number on the resistor is the value. It is gennerally in a form of $ABC$, where this means $AB \cdot 10^C$. For example, 100 would be 10 Ohm, 473 would be 47kOhm.

As others have said, different sizes give you different power and voltage ratings. However, other factors also influence this, such as the construction of the resistor (thick/thin film, wirewound, ...), as well as the PCB and mounting (sometimes you can get higher ratings of power by having thicker copper pads).

• I think you mean A*10^B, or maybe Aa*10^B..... AB*10^Clooks to me like two numbers, multiplied, then multiplied by 10^C.
– Bort
Commented Jul 6, 2017 at 17:41
• I don't know how else to word it, since it's usually three digits, AB being the first two, C the last. I would worry that if I say "AB" it would apear like it's two digits, or at least, there would be uncertanty which one of the two is two digits. Commented Jul 6, 2017 at 18:02
• @Joren (10A+B)*10^C
– Matt
Commented Jul 6, 2017 at 19:26

There are a few different marking systems for SMT resistors and some resistors are not marked at all.

Probably the most common is a system with three or four digits that is similar to resistor color codes but with digits instead of numbers, the last digit is the multiplier and the preceding digits are the significant figures so for example "150" is 15Ω.

For values where the multiplier would be less than (or sometimes equal to) zero, the letter R is used as a decimal point instead of using a multiplier. So "1R0" is 1.0Ω, "R10" would be 0.1Ω. https://electronics.stackexchange.com/a/565417/88614 says that for even smaller values the letter m is sometimes used for milliohms, but I haven't come across this personally.

A single 0 on it's own generally represents a zero ohm link.

There is also a code system called EIA-96 that uses two numbers follwed by a letter to represent values from E96 (which would require four digits to represent with a traditional numeric code). The number is an index into E96 with "01" representing a value of 100 and "96" representing a value of "976" the letters then represent the multiplier, I won't copy the full set of letters here, but they can be found at https://eepower.com/resistor-guide/resistor-standards-and-codes/resistor-smd-code/#

I have vague recollections of another alphanumeric system but with the letter at the start rather than the end but I can't remember the details and google isn't turning it up right now.

The size is sometimes driven by power requirement and sometimes by ease of assembly. In the more common sizes you can usually find all values from E24 and E96 with no problem. In more esoteric sizes there may be a more limited range.

Unfortunately there are two different systems for size codes, an imperial system where the size code approximately represents the size in hundredths of an inch and a metric one where the code represents the size in tenths of a millimeter. Unfortunately unless you already have an idea what size resistor is involved you can't tell just by looking at a code which system it is. https://eepower.com/resistor-guide/resistor-standards-and-codes/resistor-sizes-and-packages/ has the dimensions for some common sizes (though don't assume because a size is not in those lists that it doesn't exist).