Large breeds can take from 12 to 18 months to reach their adult weight and height, with the most rapid growth occurring between 0-20 weeks. To calculate their adult weight, there are two methods you can use:
Divide their weight at 20 weeks by their current age in weeks and multiply by 52. So for a puppy who weighs 35 pounds at 20 weeks: 35÷20=1.75, 1.75×52=91 pounds.
Simply double their weight at 6 months of age.