How many decimal places should a volume measurement in a beaker include?

I have a beaker which goes up in $0.5~\mathrm{mL}$ increments and my teacher told me that my measurement $(15.50~\mathrm{mL})$ was incorrect, and instead the answer was $15.5~\mathrm{mL}$. I don't understand. I know that the liquid is between $15.5~\mathrm{mL}$ and $16~\mathrm{mL}$. My estimated digit is $0$ (it looks like it lies on the line), so my answer is $15.50~\mathrm{mL}$. How is it $15.5~\mathrm{mL}$?




**It may have to do with the fact that it's a beaker, rather than some other kind of glassware.**


First, beakers are poor vessels for volumetric measurements due to their large diameter/height ratios. You have to add a lot of liquid, relatively speaking, in order to increase the height of the fill level by (say) a millimeter, and that means the uncertainty in the fill volume is large.


As well, [beakers are usually not graduated precisely](https://www.dartmouth.edu/~chemlab/techniques/flasks.html). Thus, even if you *very very carefully* fill the beaker to *exactly* the $15.5~\mathrm{mL}$ line with liquid, ***there's a very real chance that the line doesn't actually correspond exactly to a $\mathit{15.5~mL}$ fill volume.***


So, unlike with a piece of glassware specifically made for accurate volumetric measurements, such as the burette mentioned by MaxW in a comment, it is likely not appropriate to assign more significant figures to a beaker measurement than are "available" in the graduation increment.


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