However wood is 3 dimensional so you have to take into account the thickness of wood.
Here's a calculator created by Mark from Amazingly.Services
Here's the link to the Excel / Google spreadsheet to calculate the sizes you need.
https://docs.google.com/spreadsheets/d/1eLQLtMJUWjlplA-zlF7Yxn-ONgaXTMLlYRhB8_qnKq8/edit?usp=sharing
Here's a screenshot of the sheet
And of course if you like a formula...
Where
mt=material thickness
bw=box width
bh=box height
bd=box height
You will need
Qty 2 sides bd X (bh-mt)
Qty 2 front / back pieces (bh-mt) X bw - (mt x 2)
Qty 1 base bw X bd
otherwise enter the dimensions on the spreadsheet and let it work it out for you
https://docs.google.com/spreadsheets/d/1eLQLtMJUWjlplA-zlF7Yxn-ONgaXTMLlYRhB8_qnKq8/edit?usp=sharing
EAOE