Duplex with Single water meter, Splitting The bill

[Tyler Ricci]

Hello Biggerpockets! I recently purchased a duplex with a single water meter. The bill is $360/month for water/sewer. The previous landlord only charged each unit $50 towards the water bill. I need to increase this utility charge but I want to be fair about it. Each unit has 2 bedrooms 1 bath, Unit A has 2 tenants Unit B has 3 tenants. 

I would like to charge each unit by the number of tenants that occupy each unit. So unit A would owe $144/month Unit B would owe $216. 

Would this be fair? 

Does anyone else in this situation charge like this?