They do, I know one of ours has the oz. on it- it is one with 2 handles, but I forget the brand- all of our others don't. You can always measure in a measuring cup before pouring it in. I bet after one or two times if you look where 4 oz or 6 oz fills on the cup, you'll be able to do it with out premeasuring it!
I marked them with a sharpie for a few weeks. Doesn't she finish the whole sippy at one time? Why are they having trouble telling you the amount? Is she in the age room where they provide the amounts? Once my son moved out of the baby room, they stopped telling me the exact ounces, they would just mark when he had the milk.
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Re: Do they make a sippy with oz measurements marked?
When we first started on milk I'd pour her milk into one of those 16.9oz water bottles and just pour from it all day long.