How best to get box dimensions

When reaching the API, I need to submit width, height, length, along with weight to get accurate rates. How best do I go about adding up all the product dimensions to get realistic results?

For example, the customer buys 4 products measuring 2 x 4 x 4. Do I multiply these numbers by the quantity to get a box size of 8 x 16 x 16?

It doesn't seem rational.. wouldn't it instead be something closer to 8 x 4 x 4 box?