Bi-directional weigh conversion, Kg to Lbs, and visa versa

I have a client that wants us to enhance their data entry form, which among other data collects a patients weight, so that if the weight is entered as Kg's in one field, a separate field right next to that one will auto-populate with the weight in lbs. They also want the reverse to happen, if lbs is entered then Kg's will auto-populate.

Too me, it sounds like I might need to create a bunch of weight formula fields and have form rules determine which ones show when. Has anyone done this, or can anyone think of an easier way?  Need some guidance.  

Thanks!