Hi and Welcome.
I have not done this stuff for a while, but here is a translation of some old Swedish notes I found in my collection.
There are different types of boxes, closed, ported, bandpass etc.
Each of these are calculated in different ways.
For a sealed box, here is a formula which can be used.
But first you need this data:
The Qtc value of the music you listen to. Techo has a Qtc of 1-1.2, organ music 0.5-0.55, pop 0,75-0,9 and classical 0,707.
Qts is a Q value you need from the speaker spec sheet
VAS is a value you need from the speaker spec sheet
Vphys is the volume of the speaker, a value you need from the speaker spec sheet
a = (Qtc / Qts)2 – 1
(that is Qtc / Qts to the power of 2)
Gross box volume = VAS /a
Net box volume = Gross box volume + Vphys
If the speaker is filled with dampening materials, this also needs to be factored in.
If you have the bottom of the box filled with dampening materials (up to about 3 cm under the speaker), divide Net box volume by 1,2.
Now you have the volume you need to calculate the length of the sides.
There are two “golden ratios”,
1:0,8 and 1:1,25
1:1,62 and 1:0,62.
The base length L, (i.e. the 1 in the ratios above) is found calculated by:
L = (Net box size / 1000) 1/3
(that is to the power of 1/3)
Regarding ported boxes (the most common type) it is important to place the port in the right place in order not to cancel out the sound waves, this needs to be calculated based on the wave length. The port must be more than half of the wave length from the speaker if it is on the same side as the speaker. The formula is C =f x v, where C is speed of sound in air (344 m/s).
Assuming 50 Hz (f) then the port needs to be approx 3,5m from the speaker.
Therefore it is common to try to place the port on the opposite side from the speaker.
There are both books of formulas and software available to calculate boxes, but it is then important that you have all the in-data you need for the specific speaker you are using.
I know where to buy a good Swedish book on-line, but I guess that will not help you.
If you have no previous experience, the best is perhaps if you can find a ready made box drawing to follow.