Online resources say 1:4 sugar to water ratio by volume. This translates to about 1:5 by weight
To be more precise: 0.25 cups sugar is 48 g, 1 cup water is 237 g, so the ratio is 1:4.9375 or 0.2025:1 or 16.8% sugar by weight. For my small feeder, I do 30 g sugar to 150 g water.
Remember to use only refined white sugar. Online resources say that substitutes (e.g. organic sugar that is a bit darker) may be bad for the birds.
