It seems about a third of the books I mail the postage estimate is off significantly, often the book is marked media mail but first class is cheaper or vice versa. I just mailed a box set of books where the postage estimate was 3X off! How does PBS estimate weights and posrtage? I'm guessing the database has something like number of pages and that's used along with the format to guess at the weight. How do pritable postage users deal with the discrepancy? 

I always weigh my books and then do the printed postage based on that weight.  I do not by what PBS has listed as I have found it can be wrong or be off enough that it bumps it up to the next level of cost.. Plus I always do it as the wrapped weight not book weight because just the book rate adds in the fact that the book will have extra layers of paper and weigh more.. Usually more then what the actual weight when wrapped is.

William

Invest in a small scale.  That way you can enter the exact weight of YOUR package and pay the correct amount of postage.  The estimate is just that, an estimate.

I always take books to the PO and let them weight them. I was just curious about how PBS derives the estimate. I always have to explain to the PO clerk not to go by what mail method is printed and check for themselves which is cheaper.

So, for each book listing, PBS has a weight field that has a weight for the book. I think when they load book data from Amazon, the field gets filled in then. Sometimes for books that haven't been released yet, when they pull the data from Amazon, the book weight can be missing as it is not yet entered into Amazon's database either.

Also, people can edit the book data and fill in the weight field, as well.

The weight can be just plain wrong, or if the ISBN is reused for different editions of the book, it might be correct for one edition of the book but wrong for the others using that same ISBN.

I don't know how PBS calculates the weight when the book listing doesn't have an actual weight in it ... but my guess would be something like yours, that they maybe base it on the dimensions and/or the number of pages.

If the weight field is empty, the system uses the page count to estimate the weight. That's why, when there is a lot of extra stuff at the end of the story it is recommended to count those in the page # instead of just using the # of pages in the actual story.

I think usually the ones that are way off are because the bad information comes in from another database or a member in a hurry just types a number into the field not realizing that the number is used for something. I have also found some that the ounces was entered and the field is for pounds so a 15oz book was entered as 15lbs.

Scales or having the PO weight are the best way to ensure you get the right number. I have better luck and think I'd average more like 75-80% correct.

This help doc tells you how to correct the weight for the next guy, too:

The *recommended postage* on my wrapper was wrong!

When correcting the database, use the actual book weight, rather than the wrapped weight.