PHP change size box when the quantity change

Hi my name is Leo and i'm a e-commerce website toa brazilian store. I create the cart and put some products in database but now i need to calculate the freight using their mail system putting the height, width, lenght and weight of the box for the products, until there i've created. But i have a problem their products are customized products and i need to change the size of every height, width, lenght and weight when the customer change the quantity, i'm doing it using php but i can't do this final part to change when derminated quantity is change. Exemple:

Product 1: height - 8cm width - 20cm length - 20cm weight - 0.200g

this for a quantity of 100 but when the quantity is greater then 100 addition the sizes twice, but my problem is i can´t tell to my database what id i want to this happen.

Can someone put some exemple code or help me please.

Thanks Leo!