Null And Computed Column

how do you change the allow null on a column to be 0,
so instead of having null when empty, it would be 0,
I tried default value or binding to 0 to -1, and to 1
and in 0 or -1, it stays null, in 1 it puts the 1
for example if is quantity, it puts as if I would have 1 quantity,
so it's either null, or whathever number I put visides -1 or 0
how can I make it be 0 as defult?
I'm using it to add and substract with computed column, formula;
and the problem is that if it's the first time it's used it does not add or substract, becsause it does not add null with a number, if it has 0 than it works,
whata I had to do is add it manually, but of course it doesn't suppoesed to be that way
any suggestions will be appriciated: