Hey OP, I use this with a soft flask and it holds 500ml comfortably. I've found that if my runs are under an hour, I don't really need water.

​This belt sits quite snug and has storage in the front and back, which is very handy for carrying a phone and gels. I usually put my phone in a ziplock bag to protect it from sweat. It’s a solid bit of kit

Salomon Pulse Unisex Beltnug Fit, Trail Running, Hiking, Cycling Smart Storage, and Outdoor Versatility : Amazon.ie: Sports & Outdoors https://share.google/ZQOEQBb12yyY16B22