user45009: 
I frequently see 9g mentioned in servos. I searched the web but couldn't find any worthwhile answers. At first I thought it meant 9grams but the weight of the servo isn't 9 grams, so have to be something else.

user45797: From a model airplane perspective...
The "9g" is supposed to mean 9 grams in weight. Each vendor has their own way to define their weight.

with wires
without wires
with or without control horns
"about the same as a competitor's 9 gram servo"

Most are not accurate.
It is not a standard for size, and has nothing to do with hole size or screw spacing

user19300: According to Flite Test web site, the 9g reference is the weight without the wire lead. They measured one model at 8 grams rounded down. I suspect the weight also ignores the horn as well as the wire.
The article referenced on the web site calls the weight reference as a standard, which means that it more likely allows for heavier or lighter servos as long as the size of the servo matches. 
If you purchase a servo for a model known to use a 9g servo, you know the hole you have to build for it to fit. If you purchase a 5g servo, it will be a different hole and you may have a 6 or 7 gram weight, but the servo fits as it should.

45009: <p><a href="https://i.stack.imgur.com/050DJ.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/050DJm.png" alt="Servos"></a></p>

<p>I frequently see 9g mentioned in servos. I searched the web but couldn't find any worthwhile answers. At first I thought it meant 9grams but the weight of the servo isn't 9 grams, so have to be something else.</p>
 45797: <p>From a model airplane perspective...</p>

<p>The "9g" is supposed to mean 9 grams in weight. Each vendor has their own way to define their weight.</p>

<ul>
<li>with wires</li>
<li>without wires</li>
<li>with or without control horns</li>
<li>"about the same as a competitor's 9 gram servo"</li>
</ul>

<p>Most are not accurate.</p>

<p>It is not a standard for size, and has nothing to do with hole size or screw spacing</p>
 19300: <p>According to <a href="https://www.flitetest.com/articles/servos-101" rel="noreferrer">Flite Test web site</a>, the 9g reference is the weight without the wire lead. They measured one model at 8 grams rounded down. I suspect the weight also ignores the horn as well as the wire.</p>

<p>The article referenced on the web site calls the weight reference as a standard, which means that it more likely allows for heavier or lighter servos as long as the size of the servo matches. </p>

<p>If you purchase a servo for a model known to use a 9g servo, you know the hole you have to build for it to fit. If you purchase a 5g servo, it will be a different hole and you may have a 6 or 7 gram weight, but the servo fits as it should.</p>
