How to caculate the stretched canvas carton size?

When customer place the order, we always give customer an order list, which is very important to show the carton cbm and detail size, also show customer how many containers we should order.

Here I would like to introduce you more so that you could understand:

Take an order for example: there is a stretched canvas size 14x18cm,frame size:1.7x2.5cm, it is 20pcs/ctn, and carton size is 38x16x20cm. How can we got the carton size? 

First, please remember that we need add 2cm for the carton thickness.

The stretched canvas 20pcs/ctn means it is two lines.

So the carton length 38cm is because canvas length 18cmx2 lines+2=38cm.

The carton width 16cm is because canvas width 14cm+2=16cm.

The carton height 20cm is because the canvas thickness (1.7cm frame thickness + 0.1cm canvas thickness)x10pcs+2=20cm.

Is it clear? Hope our information could give you a help.