This is something I struggled with for a while and only recently have been able to do the math for. Purrmaster had asked how and most of this is my response to his question.
If you want the inside of the frame to be **8 inches** in length then, with a board of **2 inch** in width, you would add **the width times 2**. so with a 2 inch wide board the out side of your frame will be **12 inches**.

However, if you want to account for the rabbet that you set the picture into then you have to do a bit more math. If with the same board you cut in a **1/8 rabbit** then the space that the picture will be resting in will be the length of the inside edge plus a **1/4 inch** (*that’s twice the rabbet size*) making the total space for the picture **8 1/4 inches **In the picture below on the right side, that’s a 1/8 indicating the size of the rabbet.

if you wanted the space for the picture **8 inches**, then you would make your frame **1/4 smaller**. so instead of 12 inches you have 11 3/4.

Now my math mathematical to this is the following. (you will have to used this equation for both sides of the picture frame.

(length of picture) + 2[(width of board)-(width of rabbet)]= (outer length of frame)

or

length of picture) + 2(width of board)-2(width of rabbet)= (outer length of frame)

in a prettier version with

picture length=**P**

board width=**B**

rabbet width=**R**

Total=X
**P**+2**B**-2**R**=**X**

or
**P**+2(**B**-**R**)=**X**

and to apply that to the situation i was talking about earlier with

picture length=8

board width=2

rabbet width=1/8

Total=X

8+2(2-1/8)=X

8+2(1 7/8)=X

8+3 3/4=X

11 3/4=X

or

8+2(2)-2(1/8)=X

8+4-1/4=X

11 3/4=X

But honestly, if you have the picture in front of you it’s a hell of a lot easier to go by eye and pencil marks. and it’s always important to take your time and make sure you have the miters going the correct way. I once cut a frame where all the miters where parallel to one another on each board. no fun.

Hope I helped

~Pat

## 4 comments so far

JesseTutt

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854 posts in 2877 days

#1 posted 02-19-2013 02:31 PM

Thanks for the explaniation.

-- Jesse, Saint Louis, Missouri

Don Butler

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1092 posts in 4162 days

#2 posted 02-20-2013 11:44 AM

Not to be picky, and I don’t want to minimize the post, but – - – -

the correct spelling is rabbet (or as some have it, rebate).

sorry.

Don

-- No trees were damaged in posting this message, but thousands of electrons were seriously inconvenienced.

Pat Gornet

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14 posts in 2760 days

#3 posted 02-20-2013 04:21 PM

Really? Ok thank you Don. I need that. I’m a horrible speller.

FaTToaD

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394 posts in 3908 days

#4 posted 02-21-2013 05:25 PM

Great post. I have had to these same calculations before and I always forget by the next project and have to figure it out all over again. Count this one as bookmarked for future reference!

-- David

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