Where should it sit?
Start around belly-button level, then move it slightly higher or lower until the load feels stable for your body shape and what you wear. Some women and men carrying accessories prefer it lower towards the hips, while others feel more secure higher up. There is no single position that works for every runner.
The aim is to keep hard items flat, prevent the fabric folding and avoid the carrier creeping upwards. It should feel close rather than restrictive and still allow a full breath. If it rotates or rides up, check the size first, then redistribute heavier items towards the back or side.
How to measure
Measure where you plan to wear it and use the STRYQ size guide rather than trouser size. Small fits 26–30 inches, Medium 31–35, Large 36–39, X-Large 40–43 and XX-Large 43–46.
If you are near a size boundary, think about the layers you normally train in and whether you prefer a firmer or more relaxed fit. Women and men using different bags or layers with the same waist measurement can still prefer a different position because body shape changes how the load sits.
Test the fit with the phone, keys and accessories you genuinely carry for regular training sessions. Walk, jog and change pace before relying on a new setup for a long session. That quick check is more useful than judging the fit while the storage is completely empty.