If you've never trained before, you should consider using the Extra Small (XS) (10 lb / 4.5 kg) or the Small (S) (14 lb / 6 kg). Our Beginner Kit contains a pair of both the XS and S and might be the perfect solution for you.
If you've been training a few times a week but do primarily bodyweight movements, then you should consider using the Small (S) (10 lb / 4.5 kg). More fit people may be able to use the Medium (M) (18.5 lb / 8 kg) if they have a history of weight training.
If you train a few times a week and primarily use dumbbells and kettlebells, then we would recommend the Medium (M) (18.5 lb / 8 kg). Those individuals looking to challenge themselves strength wise, may be able to use the Large (L) (22.5 lb / 10 kg). Our Intermediate Kit contains a pair of S, M, and L and would be a great fit for you.
For those that train every day, have a history of weight training, and are very fit, we would recommend the Large (L) (22.5 lb / 10 kg) or Extra Large (XL) (27 lb / 12 kg). Our Advanced Kit contains a pair of L and XL YBells which is the perfect combination for you.