Sometimes I see references to "R" resistors. For example:

enter image description here

Obviously the 100 refers to 100 Ohms. What does 100R mean?

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    The R is sometimes used as a decimal point. So 100R is 100 Ohms, 4R7 is 4.7 Ohms, etc. – mkeith Jul 20 '16 at 5:09
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    " Radix point " – Ignacio Vazquez-Abrams Jul 20 '16 at 5:16
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    Traditional ascii fonts do not have an omega symbol, so 'R' is commonly used instead. – pjc50 Jul 20 '16 at 8:28
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    @pjc50 Once I was working with a US student and a Canadian postdoc. The postdoc kept saying "zed" instead of "z", and eventually the student asked "What's a zed?" I told her it was a backwards z, but since we didn't have those on our standard keyboards, we would just type "z" – Scott Seidman Jul 20 '16 at 16:32
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    @bcrist: Just because Unicode has a separate code point for "Ohm" (which is silly) does NOT mean that it is not a capital Omega. – Dave Tweed Jul 21 '16 at 16:16
up vote 42 down vote accepted

The idea is that the multiplier replaces the decimal point. This dates back to pre-CAD schematics which were hand drawn and then photocopied and reduced. A decimal point could easily get lost during the copying process. By writing 4k7 rather than 4.7k the risk of these errors was greatly reduced. R was used for a multiplyer of 1 because omega could easily be mistaken for a 0. So ... 4R7, 47R, 470R, 4k7, 47k, 470k, 4M7, 47M.

The same approach is used with capacitors: 2p2, 22p, 220p, 2n2, 22n, 220n, 2u2, 22u, 220u. In the old days larger values were still marked µF so the next decade was marked 2200u but with large capacitor values common now we're seeing 2m2, 22m, etc. I've never seen an equivalent of the 'R' as in 2C2 for a 2.2 F - yet! 2F2 may be more sensible. The current use of 'R' would then be excused (4R7 instead of 4Ω7) on the basis that Ω isn't readily available on most keyboards.

This system may be more popular in Europe.

Thanks to @JasonC for pointing out that the 'R' notation is covered by British Standard BS 1852.

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    And if you stare at the R for long, it sort of looks like someone got a stretchy omega and pinched its center to join the two legs, and moved it up and to the left a bit. – Kroltan Jul 20 '16 at 11:55
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    @glglgl There are file formats and programs that can only store 8bit characters in strings, and don't support code tables apart from the system code table. Copy & paste into these file formats or programs will produce anything but legible greek characters (except maybe on a greek computer system). – Alexander Jul 20 '16 at 14:06
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    @glglgl you would be surprised how much very old legacy code is running in current (and very expensive) CAD software... – alex.forencich Jul 20 '16 at 14:43
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    @glglgl: Incomplete yes, buggy no. When one considers the cost of support for non-ASCII characters in a schematic entry program, compared to features that are directly related to circuit design, one can see why such a thing may be way down the priority list. – Ben Voigt Jul 20 '16 at 14:44
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    @Alexander: Not only that, but many CAD programs use one or more sets of hard-coded stroked-character shapes, rather than using system fonts, and thus cannot use any characters which are not in their hard-coded set regardless of how many system fonts include it. – supercat Jul 20 '16 at 14:58

It is quite common to see the letter "R" used as a decimal point. As in 47R9 = 47.9 ohms. And likewise, it is common to see the letter "K" or "M". For example 6K81 would be 6,810 ohms and 2M3 would be 2,300,000 ohms.

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    So in this case it just means 100.0 ? – Tyler Durden Jul 20 '16 at 5:15
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    Yes. 100R means 100 ohms. – Richard Crowley Jul 20 '16 at 5:17
  • You may also see 6R8 or 2R2 on an inductor to represent 6.8uH or 2.2uH respectively. – Spehro Pefhany Jul 20 '16 at 18:16
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    It makes sense that they use R as a decimal point as if they actually printed a decimal point on a piece of paper, it could be easily be misread due to its small size or be rubbed out. – Bradman175 Jul 21 '16 at 0:55
  • Alternatively, if it was common to use a decimal point on parts, you might confuse a speck of dust (or a manufacturing flaw) between two digits as a decimal point. – Nick Gammon Jul 23 '16 at 3:52

Adding to the other answers, sometimes you may even see E used in place of R. So a 100 ohm resistor would be 100E and a 9.1 ohm resistor would be 9E1 for example.

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    I've never seen E used as a place holder for a decimal point, "R", "k", "m" and "M" obviously for resistors and "m", "u", "n", "f"and "p"on capacitors and inductors. Where is "E" used in this way? You some times see E used in scientific notation so for example \$1\text{E}3 = 1 \times 10^3 \$ but not on circuit diagrams. – Warren Hill Jul 20 '16 at 6:32
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    Here's a link to a component supplier where E is used extensively. – bitshift Jul 20 '16 at 6:36
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    It may not be very common but if anyone ever comes across a schematic with E instead of R this answer may help them. – bitshift Jul 20 '16 at 7:03
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    @bitshift, I think the people who downvote you have never heard of this, so they downvote because they think you are wrong. I did not downvote you. But I have never heard of this before. I did click on the link you provided, and, indeed, E seems to be used. Learn something new every day. – mkeith Jul 20 '16 at 8:09
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    I have heard of it many times and subsequently upvoted it to not drive away users from this site. – winny Jul 20 '16 at 9:16

Typically, resistor "multipliers" are represented as:

KΩ (thousands of ohms),

MΩ (millions of ohms),

GΩ (thousands of millions of ohms) ...etc.

Since the context usually makes it clear that we're talking about resistor values, it's common short-hand to drop the 'Ω' so that, for example, you can write "39K"* instead of "39KΩ". But, dropping the "Ω" leaves the problem of how to represent a resistor value when the multiplier is 1. So it was decided that "R" would represent a "x1" multiplier. So now you can write "39R" instead of "39Ω".

The multipliers (R, K, M, G... etc) can also be used as shorthand for decimal points.
So, for example, instead of having to write "2.2Ω", you can simply write "2R2". All the multipliers can be used in this way. A final example: "3.3KΩ" can be written as "3K3"

Note that it is common practice to capitalise the "K" multiplier when referring to resistor values. Technically this is incorrect, as "k" is the official '1000' prefix. But it's just a shorthand, limited in its use to resistor values, and the capital K is in common use in this context.

Wikipedia tells,

The notation to state a resistor's value in a circuit diagram varies. The European notation BS 1852 avoids using a decimal separator, and replaces the decimal separator with the SI prefix symbol for the particular value. For example, 8k2 in a circuit diagram indicates a resistor value of 8.2 kΩ. Additional zeros imply tighter tolerance, for example 15M0. When the value can be expressed without the need for an SI prefix, an "R" is used instead of the decimal separator. For example, 1R2 indicates 1.2 Ω, and 18R indicates 18 Ω. The use of a SI prefix symbol or the letter "R" circumvents the problem that decimal separators tend to "disappear" when photocopying a printed circuit diagram.

Also, I've seen, 1. just-like R, also E is being used such as 4E7 etc. 2. the zero for tighter-tolerance sometimes not given, such as 47K, 56K etc.

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