"Squaring" a Room/Basement
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I need a serious sanity check here...I've chalked out, one of my proposed basement bedrooms and was checking to see if I'm correct. Looking at the pdf (and starting in the top right corner) : I took a framing square, leveler and chalk line to create my first wall (top right corner) and continue in an anti clockwise movement to create my bedroom. I used the 3x4x5 "right triangle" rule and the bottom two corners were perfect; however, when I measured the width of the room my measurements were way off (the mosr left was 10' ft x 1/2" and the most right was 10ft). I think that what I did is correct and should build the 2x4 framing on these chalk lines. Can anyone confirm or provide guidance?
thanks 
You're racked a little, one way or the other. 345 measurement is too small for a room that size. Try the next multiple of it at '6810'. Also measure your diagonal corners, should be the same if room is square.
Mike Hawkins:) 
That "framed foundation wall" may not be perfectly straight. Snap a reference line out a few inches at each end and check it in the middle.

firehawkmph...I measured it diagnolly and got 194 5/8 and 194 3/8. And to me this looks pretty close to being good. Would you agree?

If I understand you correcly, you started by laying out the first wall from the existing wall with a framing square. If so, I bet that's your problem, a framing square is too small to square something like that. Even if the existing wall was perfect, and your 2 345 corners were perfect, I wouldn't be at all surprised to see an extra half inch floatin around there. Lay out the first wall from the existing wall using a 345 (6810) triangle.

Thanks...this helped and corrected the problem

It's easy to get cumulative error factored into this method and an adjustment as you did solves the error for the most part but the mentioned longer distances help.

When laying out the foundation for a house the way I always square a house is this. First of all, you know you will have one straight wall or line, and you will know the length. Pop a line for that first wall and make a mark at each end. You now have the first wall, the length and location.
Now from one end of the first line hook your tape on that mark that is the end of the wall. You know the length wall running 90° of the first wall. Now is when you figure the two walls for square. A squared + B squared = C squared, you want to figure the hypotenuse of the triangle. Say the wall is 16 feet 4 inches, convert the feet and inches to all inches this will = 196 inches. Now the wall running 90° to the first wall lets say 13 feet 7 inches which = 163 inches. With a calculator multiply the first number by itself. 196X196 =38,416, not put that in your memory plus of the calculator. Next multiply the second number by itself, 163X163 =26,569, add that number to the memory plus on your calculator. Now hit memory recall button and next hit the square root button. This will give you the number 254.92159 which is the distance from one corner to the other of that room. Convert this back into inches and feet which is by dividing by 12. This = 21.243465 so you have 21 feet .243465. Multiply .243465 by 12 inches (one foot) which will = 2.92158 which is 2 and a fraction of an inch. This fraction of an inch is so close to 15/16 that it is safe to round it up. so the measurement from one corner of the room is 21 feet 2 15/16 inch. This will be the hypotenuse of the triangle the two walls form. You already have the first wall line where it is supposed to go. Now take two tape measures, hook one on the end mark of the first wall. At the other end of that wall hook the other tape on the end of that wall at the mark you made. Now just lay the first tape down anywhere. Pull the second tape out the measurement of the wall running 90° of the first wall. You will want the number on the tape to be 13 feet 7 inches, now take the first tape that is hooked on the other end of the first wall and pull it across the second tape, you will want the number 21 feet 2 15/16 to intersect with the number 13 feet 7 inches. Mark right there, that is the corner of the room and will be square. I hope this isn't too confusing. 
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