Do I use a permutation or array to solve this?

[rfcomm2k]

I am building a spreadsheet to help me quote decks. For a given deck size I know I need 6x6 posts every 8'. I know how to figure that for the perimeter but how do I calculate that for the interior posts? For example, a 8' x 8' free standing deck only needs 4 posts, one at each corner. But a 10' by 10' free standing deck needs one at each corner, plus one in the middle of each side, plus one in the middle (going with symetrical spacing), for a total of 9 posts.

Small evenly shaped decks like this are easy to figure in my head, but when I get into a large or non-square one (30 x 45 for example) a formula would be better.

Any ideas?

[Pauleyb]

Irregular decks would require some programming and some smarts on how to aesthetically 'balance' out the posts between the sections. But for a basic rectangle, I think this works:
=(INT((length)/6)+IF(MOD(length,6)=0,1,2))*(INT((width)/6)+IF(MOD(width,6)=0,1,2))

Note that this can be used for some of the irregular shapes, but you would have to do it in sections and remove any double counted posts along shared sides.

[rfcomm2k]

Thanks Pauley, but yhis is apparently not right. Your formula used with a basic 8' x 8' deck produces 9 posts required, and I know that with 8' spacing only 4 are needed.

[rfcomm2k]

Works perfect, even for a 220' x 148' deck!

[Pauleyb]

Sorry, the 6x6 had me thinking a post every 6'. Use this:
=(INT((length)/8)+IF(MOD(length,8)=0,1,2))*(INT((width)/8)+IF(MOD(width,8)=0,1,2))

That should change the number for the 220'x148' which makes me wonder about how the previous one worked perfectly... Try this one out and see if it does better.