# Working out an octagon on buildcalc app



## carpenter uk (Nov 25, 2009)

Im trying to figure out how to work out the side lengths of an octagon with the buildcalc or construction master pro app

Normally i would Halve the width of the square

(Sorry for the metric:whistling)

A square base is 3000mm
1500mm x .828 = 1242mm

= 8 sides @ 1242mm

which is correct

on the buildcalc app 

1500mm [conv] [ radius]
8(no. of sides) [conv] [polygon] =1148.1mm 
which is not correct 

what am i doing wrong?


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## tjbnwi (Feb 24, 2009)

I would just divide the 3000 by 2.414 which equals 1242.751.

I'll send Ben an email about it not being correct on BuildCalc. Get back to you when I get a response.

Tom


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## carpenter uk (Nov 25, 2009)

Thanks, I look forward to his response 
I'm sure it is user error as my math is sum what lacking.


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## tjbnwi (Feb 24, 2009)

carpenter uk said:


> Thanks, I look forward to his response
> I'm sure it is user error as my math is sum what lacking.


Not you. I get the same error. I know my math is correct, as is yours.

Tom


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## tjbnwi (Feb 24, 2009)

We're working on it. 

Tom


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## tjbnwi (Feb 24, 2009)

Happy to say, you're correct, then so is BuildCalc.

It will help if you look at this and read the notes on the screen shots;

https://picasaweb.google.com/tbadernwi/Octagon#

The reason why both are correct is how BuildCalc calculates the octagon. Because it uses a circle to constrain, it measures the diagonal. This causes the side to be short of the circle, the intersecting points touch the circle. The first couple of screen shots show this. The side to side width of the octagon is 2771.6 mm for the 3000 mm octagon.

We as builders work off the parallel sides. Not the tip to tip dimension. To get a 3000 mm side to side, you need a 3247 mm circle. Not easy to know. 

After about 10 emails and the drawings, Ben has decided that in the next MAJOR update to BuildCalc he will make it user selectable between sides and points.

When you look at the 3rd and 4th screen shots in the link, you will see I show the math that proves out ----------side to side width divided by 2.414 will return the length of the sides of an equal octagon. That is all you need to do.

Hope this clears up the issue. For now just divide by 2.414. 

Tom


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## tenon0774 (Feb 7, 2013)

...and I thought I wasn't going to use Trigonometry, today. :whistling

Good clarification on the the Buildcalc, problem...

...even before I knew it was a problem.

:thumbsup:

Thanks.


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## tjbnwi (Feb 24, 2009)

Don't feel to bad about not knowing. I test this app and did not know of the discrepancy. Until last night, I forgot it had the function. Always just divided the width.

Tom


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## festerized (May 19, 2007)

Divide the run by .828, this will give you length of sides


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## tjbnwi (Feb 24, 2009)

festerized said:


> Divide the run by .828, this will give you length of sides


It's MULTIPLY the run by .828. In the 3000 mm octagon In the OP, 3000/2=1500 (run) divided by 0.828 it returns 1811.549, which is not correct. If you multiply 1500 (run) by 0.828 it returns 1242, which is correct. 

Run is half the width, why do the extra math? One more step to have an error. Just divide the total width by 2.414. 3000/2.414=1242.751.

Tom


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## festerized (May 19, 2007)

tjbnwi said:


> It's MULTIPLY the run by .828. In the 3000 mm octagon In the OP, 3000/2=1500 (run) divided by 0.828 it returns 1811.549, which is not correct. If you multiply 1500 (run) by 0.828 it returns 1242, which is correct.
> 
> Run is half the width, why do the extra math? One more step to have an error. Just divide the total width by 2.414. 3000/2.414=1242.751.
> 
> Tom


Oops :whistling


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## CanningCustom (Nov 4, 2007)

festerized said:


> Divide the run by .828, this will give you length of sides


:blink: is it just me or have been absent for a while


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## tenon0774 (Feb 7, 2013)

CanningCustom said:


> :blink: is it just me or have been absent for a while



When's the last time you had a trigonometry lesson? :whistling

Not trying to be flip, but these guys are talking software. The only link I had, to try and understand what the problem was, goes back to mathematics.

(...and there may be a little calc, in there... but I don't do calculus, on Wednesday.)


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## CanningCustom (Nov 4, 2007)

tenon0774 said:


> When's the last time you had a trigonometry lesson? :whistling
> 
> Not trying to be flip, but these guys are talking software. The only link I had, to try and understand what the problem was, goes back to mathematics.
> 
> (...and there may be a little calc, in there... but I don't do calculus, on Wednesday.)


Tenon buddy I wasn't referring to the actually math, was basically making a comment to Festerized who I haven't seen in a while. Take it easy man


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## festerized (May 19, 2007)

CanningCustom said:


> :blink: is it just me or have been absent for a while


Been crazy busy, in the past i would post on CT while doing estimates just to keep my sanity. Im in the middle of a divorce so up and out when i can, no more hiding in my office when there's so much tail out there!:thumbup:


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## CanningCustom (Nov 4, 2007)

Well sorry to hear man, but glad to see your still alive and kicking


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## tjbnwi (Feb 24, 2009)

The ability to calculate polygons by circumscribed (the way it is now) or inscribed circle has been added to BuildCalc in the V5 beta that is currently being tested.

Tom


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## tjbnwi (Feb 24, 2009)

BuildCalc 2.2.0 is now avalible for Apple products.

Tom


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## tjbnwi (Feb 24, 2009)

You may have to update BuildCalc, delete it and reinstall it. Easiest way to tell it is tap-Conv-tap Prefs, if the screen is blank you have to delete and reload. 

The last toggle in Prefs is Inscribed Polygon, toggle it off to get it to figure a circumscribed polygon.

Tom


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## Birch (Jul 20, 2009)

tjbnwi said:


> Happy to say, you're correct, then so is BuildCalc.
> 
> It will help if you look at this and read the notes on the screen shots;
> 
> ...


. . . . way off . . . .


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## tjbnwi (Feb 24, 2009)

Richard,

The issue was how BuildCalc measured the polygon. It used a circumscribed circle, by doing so it measured the polygon tip to tip. 

http://en.wikipedia.org/wiki/Circumscribed_circle

You now have the option to use an inscribed circle, by dong so it now measures side to side. 

http://en.wikipedia.org/wiki/Inscribed_figure

The returned value is to the outside perimeter, so the length is tip to tip long point of the member. The thickness of the member does not affect this number. There are other functions that member thickness is imputable, it is not needed here. 

You should try BuildCal, I purchased the app when it first came out, then became a beta tester. I believe there are a few things you can play with on the website.

Here are a few screen shots of some of the rafter cut calculations;

https://picasaweb.google.com/tbadernwi/HipValley

https://picasaweb.google.com/tbadernwi/MajorMinor

Tom


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## Birch (Jul 20, 2009)

tjbnwi said:


> Richard,
> 
> The issue was how BuildCalc measured the polygon. It used a circumscribed circle, by doing so it measured the polygon tip to tip.
> 
> ...



Tom,

If I get what you posted, the difference is Hip length Vs Common length, w/o thicknesses or cutting process concerns . (Circumscribed Vs Inscribed).

The Picasa pics are a little confusing. The first set is not all about the same roof and the second example defined the Minor Roof as the lesser pitch. I call the pitch containing the ridge the Major Pitch, most frequently the lesser of the two.

I guess we don't all think alike on how a roof is "De-scribed". :no:


Richard


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## tjbnwi (Feb 24, 2009)

Birch said:


> Tom,
> 
> If I get what you posted, the difference is Hip length Vs Common length, w/o thicknesses or cutting process concerns . (Circumscribed Vs Inscribed).
> 
> ...


The roof functions are different than the polygon functions (yes I know a triangle is a polygon). I agree about how things are described/understood. 

If you measure an octagon (or any polygon with 4 sides or more), you can measure from tip to tip or side to side. The tip to tip number will yield shorter sides than the side to side number.

A 5' square measured tip to tip would yield a side length of 3'-6 7/16", we all know a 5' square measured side to side has 5' sides. BuildCalc until this version would not calculate side to side polygons, so if you entered a 5' 4 sided polygon it would return 3'-6 7/16" for the side length. It is obvious when doing a square, not so obvious with more than 4 sides. 

Tom


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## tjbnwi (Feb 24, 2009)

Richard,

Gary does a much better job explaining the roof functions in BuildCalc than I ever could, about ¾ of the way through the article. 

http://www.thisiscarpentry.com/2013/12/13/eave-returns-interpreting-gyhr-details/

Tom


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## Birch (Jul 20, 2009)

Tom,


I read that TIC article a few days ago. They use BuildCalc for the compound miters and rake lengths as expected to process the task. It worked well for Gary and Todd's project.


Also, I notice this thread is already linked on the BC home page when I went there to download the user manual. That is quick thinking! (and reacting) by Ben A.


I noticed quickly in the manual, page 128 . . . equilateral polygons: Example is 6 sided. Does it work for odd sided polygons? (i.e. 5, 7, etc.) (I'm betting yes, circumscribed circle dia., up to 25 sides?)


I also noticed that the User Preferences don't include any framing member thicknesses inputs; so the user will need to know how to formulate correct adjusted inputs. (?)


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## tjbnwi (Feb 24, 2009)

The reason for the change to the poly function in BuildCalc is this thread. I contacted Ben once there was an issue. Took a few correspondences but we got it fixed. I ended up doing Sketch Up drawings to get the point across. You'll find the in my Picasa album.

The poly function works for any equal length polygon. It works on a 25 side, I just tried it, also works on a 3 sided.

To input the framing members for a roof, tap Diag twice, tap advanced, this is where you will find the rafter parameter inputs. I'll check to see if there is another way to input the member parameters. 

Check with Sym, I believe he uses BC now. 

I'm sending you a PM. 

Tom


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## carpenter uk (Nov 25, 2009)

Is there likely to be an update for android to allow for the circumscribed polygon?

I assumed the update had fixed my issue but I had a large polygon to make today and stumbled into the problem again:sad:


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## carpenter uk (Nov 25, 2009)

xfgjdjyj said:


> Thanks, I look forward to his response
> I'm sure it is user error as my math is sum what lacking.



Quoted as I did not want my post to be missed by a silly re-quote



carpenter uk said:


> Is there likely to be an update for android to allow for the circumscribed polygon?
> 
> I assumed the update had fixed my issue but I had a large polygon to make today and stumbled into the problem again:sad:


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## muskoka guy (Nov 16, 2013)

I found this layout calculator works good and is easy to use. http://www.pagetutor.com/octagon_layout/.


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