I'm making a simple syrup with equal parts sugar and water in a glass measuring cup and want to avoid dirtying two measuring cups.
I want to measure one (say standard table sugar,) then add the water. What should the resulting volume of both be?
My experiment with table sugar, pouring 1/2 cup of table sugar into a glass container then pouring in 1/2 cup of water on top without stirring resulted in the water line reaching the 3/4 cup mark after a few seconds of absorption.
So the ratio of the volume of separated sugar and water to the mixture is 3:4.
Well, after this came up in another question and after realizing data on this was hard to find online, I pulled out my graduated cylinder and tried it myself.
As noted in comments, measuring sugar by volume is very inexact. I found that simply by pouring sugar into the graduated cylinder and tapping it, I could start with about 110mL and tap it down to about 90mL. Even without considering other problems like possible clumps in sugar or the fact that different brands of granulated sugar may have different particle size (and thus different densities), this is already a huge source of potential variation.
So, I tried getting a volume of sugar that was about 100mL on average (that is, tapped down to settle slightly, but not completely). Using 100mL of water and adding this volume of sugar gave me a solution of approximately 158mL.
That means the combined volume of the dissolved sugar-water mixture was about 79% of the combined volume of the original sugar plus water. Again, I note about a +/-10% variance in sugar density depending on how it is measured, which means the possible range here should be around 76% to 82%, depending on how "settled" the sugar was when I measured it. This is in close agreement with widebandit's post here that found a ratio of 25 fluid oz. solution to 32 fl. oz. of original ingredients, or about 78%, though just a bit higher than Jeff Axelrod's ratio of 75%.
Maybe someday I'll try this with a few different brands of sugar, but I just thought I'd add one more datapoint that's close to the other answers here, with some information on how much variance to expect.
I've been making hummingbird sugar water for years. Dissolving 2 Cups (16 dry oz) granulated sugar into 2 cups water makes about 25 oz (3-1/8 cups) of sugar-water. The sugar adds about 9/16 of it's dry volume to the liquid solute. Once I have this mix I add more water to make 48oz of sugar-water for the hummers. Sugar content is then a bit less than 19% sugar by volume.