# measurement ?

Discussion in 'Off-Topic' started by Mathematics!, Mar 1, 2010.

1. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
I have had a problem with installing a prehung door...

First I measured the rauf opening to be height 80 1/2"
width 33 1/2"

Then I measured a prehung door at home depot that was exactly 80 1/2"
But it said on it rauf opening for height is for 82 1/2" so where is this extra
2" comeing from mathematically if you measure the prehung door and the opening for it and they are both the same then why is it not fitting by a hair? Why do you need the extra 2" for the height on specifications of the prehung door?

Second I measure all the house doors in my place (just the door) and all of them measure 80" height wise ... when I went to home depot to just by a replacement door I noticed the height is always 80" standard...

But I measured my prehung doors size (just the door ) and it was 79" so
I guess I will have to special order every time I need to replace just the door for it??? All the prehung door doors are about 79" but the standard doors are 80"

WTF so everytime you install a prehung door and want to change just the door not the frame out them you would have to special order a 79" door....
Seems BS to me or you would have to shave an inch off ....???
Either way all these doors give that the door itself is 80" but when I measure it I get 79"... wondering if these measurements on the door that they say 80" because their measuring to where it hits the jamb door and spaces between door and jamb...

Either way I am totally confused and mathematically it is wrong anyway you slice it?

Another question why do they call 2 by 4 wood , it is not 2" b 4ft or anything like that so I am lost with what freaking measuring system their using....????????????????????????????

Thanks for any help because most of what I am doing depend on precise measurements and if their not labeling things correctly then no matter what I measure I will be off................

2. ### blueroomelectronics AAC Fanatic!

Jul 22, 2007
1,758
98
Maybe a Home Depot or home DIY forum might get you the answer you need.

Jul 7, 2009
1,585
141
Can't help with the doors. The doors around my house (about 40 years old) vary from 79" to 81".

The screwball American sizes of things developed over the years independently in each industry. Once entrenched, it is enormously difficult to get people to think differently. For example, 1/2 inch diameter pipe was once a half inch in inside diameter. As material strength improved, the wall thickness could be reduced, yet people wanted to keep the outside diameter the same. Look at all the different gauge dimensions for sheet, wire, plate, etc. And it's comparatively simple with linear measurements -- if you go into science/engineering, the problem is worse because of compound units. Ultimately, the real problem is all the units/conventions just increase the chances of errors. And it costs real money too (wasn't that one Mars mission lost because one software group used SI and the other used fps? And I think it was to the tune of \$100 million or so (too lazy to look it up)).

2x4 lumber was so named because that is the size it is when it is rough cut. The finished size is about half an inch under the rough size. You quickly learn that a 2x4 is actually about 1.5x3.5 and get on with life. But, I agree, it is a bit aggravating for the beginner.

Oh, there's also cheap manufacturers. In the old days (when my kids say dad was running with the allosaurs), if you bought a piece of 3/4" plywood, it was -- surprise -- 3/4" thick. Nowdays the skinflint manufacturers have figured out how to shave off thickness to save money -- I haven't seen a real 3/4" thick chunk of plywood in years. So greed enters into the equation also...

4. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
I am just wondering because it seems like a common error.

And for the 2 by 4 wood that is the rough lumber before planning the 1/2 " off of it. (orignal the wiki said 2 by 4 where exactly made 2" by 4" for the width and depth then some moron's kept changing that.

It just seems <snip> to sell something with a name that implies a different measurement

I know now that 2 by 4 finished lumber is 1 1/2" by 3 1/2" some day maybe it will be different and they wonder way we have building falling over some times....

As for the door I am still totally stumbed if you are selling a prehung door wouldn't you want to have the door compatible so you could buy a replacement door ..... I would think 80" should be really 80" to make all the doors compatible.

I will try to poke around on another forum that may have more info on the subject

I have been a scientist/mathematician for many years except now I am getting into building stuff and seeing how the materials aren't mathematically correct is alittle bit wierd. Since if people want to be precise they should rely on precise name meanings for things ..... just to make it easy on the mind if anything else

later

thanks

gotcha

Last edited by a moderator: Mar 1, 2010
5. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Just wait until you find out that your door frames aren't really rectangular, either - rather more like a parallelogram made out of barrel staves

6. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
yeah , but that is what the shims and a level are for
And I got lucky and everything happens to be almost level.
But I know what you are talking about

Just so happens when I was doing the measurements that is what was screwing me up........

Last edited: Mar 1, 2010
7. ### loosewire AAC Fanatic!

Apr 25, 2008
1,584
435
That why they Invented a proper word,tradesman that Is good In
his craft,what ever It Is.