How accurate is the 16-week weight x2 rule?

For medium breeds measured at or near 16 weeks, it tends to give a reasonable ballpark -- typically within 10-20% of actual adult weight. It's less reliable for small and toy breeds (which tend to be overestimated) and giant breeds (which grow for much longer). Treat the result as a planning estimate, not a guarantee.

Can I use this calculator for a mixed-breed puppy?

Yes, with the caveat that mixed breeds can be harder to estimate since adult size depends on the mix of breeds in the parentage. If you know the parent sizes, comparing the estimate against both parents' weights gives a useful range.

My puppy is 6 months old -- can I still use this calculator?

The formula is validated for puppies 8-20 weeks old; at 6 months (26 weeks), results become less reliable because growth rate has already slowed from the linear pattern the formula assumes. For older puppies, a breed-specific growth chart or a vet assessment gives a better picture.

Does this work for large and giant breeds?

The rule is less reliable for giant breeds, which have longer, more complex growth curves. It can still give a rough ballpark, but the uncertainty range is wider. For giant breeds, your vet's growth tracking over multiple visits is a more reliable guide.

How the Puppy Growth Calculator Works | PetCalculator

The Puppy Growth Calculator uses the well-known "16-week weight × 2" rule of thumb to estimate a puppy's adult weight from a single early weight measurement.