The easiest 21st century way of doing this is to create the table in sketch up. When you draw the angled line from point to point, inquire and get the angle on that line.

I gave up on geometry and trig years ago when I learned AutoCad. Just create points, connect a line and get the info.
Presto! No math needed.

 

I wouldn't calculate the angle. Draw it out full size on a piece of card board and use an adjustable angle
square to copy the angle. Then transfer the angle from the square on to your piece of wood or scrap to test it.
You can then put the piece on you miter saw bed and rotate the saw to the correct angle. You could also just use the angle square right on the bed of your saw. Remember to keep your orientation on the legs in such a way that the cuts are parallel top and bottom

 

I agree with stnich ,you can actually see what the angle is going to look like by making card board templates.I know the math is niece to know but I am not that good with math so I always make full size mock -ups to scale with card board or large graph paper.90% of my projects are my own designs and I also use Scetchup but I have a lot to learn with that program.My cousin works in a hi end furniture shop and they use templates all the time!

 

GAH! I hate sketchup.

Here's how I did it for a coffee table

So…. first question is how wide is the table? 40"?

And then how much of a ledge is there going to be from the edge of the table to the apron?

Then how big are your legs?

Let's say you are building a table 14 feet by 42", and you want a 4" overhang from the apron. widthwise and 6"overhang lengthwise.

That means brings you down to 38" by 156" (13 feet)

Now use pythagoras: 38×38 + 156×156 =c squared

Roughly 160".

This isn't including the amount of space your legs take up, nor is it accomodating for the angled cuts, but it's a decent enough estimate. I then cut boards to that size and then did the final measuring and fitting as I worked.

 

Make a little model. Cardboard put together with tape and hot glue is
fine. You can even use foam-core, available from art supply stores.

If you're having to ask these sorts of questions about the geometry
of your designs, you're better off making models than trying to do
this stuff mathematically.

When I got really into woodworking in my early 20s I bought an
old text on geometry and trig and taught myself a bunch of stuff that
never made any sense when I was in school. It was only the tangibility
of building things that made geometry make sense to me.

 

Can you show us a picture of what "x" style legs you want? I can think of a couple different way to do that

 

36 squared plus 30 squared. Now find the square root of that number. That will give you the length from the outside of the table to the far side on the floor. point to point on the leg. Find the sq root of the sums of the squares. If you have a problem with this then invest in a refrigerator box and a sharp pencil and start drawing.

 

oops. I was thinking it was a bracer. I really should read more carefully and not when so tired.

Grandpa is correct, but you don't say how much space between the legs still. If it's only 40 inches, his 36 is going to leave you with only two inches from the ledge on both sides. Which is adequate. I think if it's 14 feet long, you might also want it slightly wider than the basic rule of thumb to make it more visually appealing. if you're looking for a table top to be 30" high, don't forget to subtract the height you'll gain from the tabletop from your 30".

Also, the length in his example is going to be about 47 inches.

 

this is what i'm guessing you are talking about
this is a picnic table at home depot
fro $379

send me $300 and i'll give you directions
to go measure the legs

saving you $79

 

I sure as hell hope you're wrong patron. I don't like the legs without any sort of crossbrace or apron, and if he's doing a 14ft long version of something like that, one good push and somewhere, some of the legs are going to snap or collapse.

 

I know there are a lot of people who like sketchup. It is something that I want to learn but it seems to have a pretty good learning curve. Maybe someday I'll give it another try but time is always a problem. There's nothing at all wrong with building models or just making a drawing and measuring the angles with a protactor or a combination square. However, for things like this I use a CAD program that is suprisingly affordable, powerful, and has an intuitive work space. It comes with a tutorial that will have you going in a couple of hours. It's called TurboCad

At $129 I think that it is an affordable tool for many woodworkers.

We have a molding plant and have to make our templates for the knife grinder. We have a computerized router that cuts our templates from plastic. The DXF files that run this machine are generated with TurboCad. The accuracy of the drawings are well under 1000th of an inch. We have a machine shop in our plant and all of our engineering drawings are done with this program. It comes with all sorts of templates so that you can use it for architecture, electrical drawings, etc. I have laid out shop floors and offices with it. It's great for chip carvers as well. I have used it to quickly lay out chip carving patterns and then transfer the patterns directly to the wood with my heat transfer tool. It will make all kinds of calculations for you. In short it is a bargain for $129 and I use it at the plant and at home every day. You don't have to be a genius or computer geek to use it. It will save you time, money, and material because you can make some very nice drawings that are completely clear. You can take the drawings to staples and they can print them much larger if you need them.

I've always wanted to have AutoCad. The learning curve for AutoCad is very steep and it is very expensive. I have never found anything that I have needed to do that TurboCad couldn't handle so why bother with AutoCad. At $129 TurboCad is an affordable tool.

I'm not affiliated in anyway with TurboCad and receive nothing for plugging it. I just use it in all kinds of ways and am glad that I have it.

For the current problem: if you know the thickness of the table top, the width you are going to make it, and the height the top surface is above the floor, and how far in from the edge of the top your legs were going to be, you could make a drawing for the top and the x-legs in under a couple of minutes and have all of the dimensions that you needed. It would take you under 30 minutes to learn to do this and much more. So if you have a computer in or near your workshop you can see how much time this program can save you.

 

I always draw it out and then use Trig to figure it out, but you can make a scale drawing and just measure it with a protractor and cut them at that angle.
That picnic talbe show by Paton has a center brace coming dowon from the top to hold the x legs vertical.
If you are doing an X leg on the end like that and you start with the leg inset about 1" from each side and with the top 1.5" thick, you will have a leg height of 28.5 and width of 34". If you have the inside of the leg be directly under the edge of the 34" dim. at the top and the rest of the width of the leg stick past it, it come to a 40 degre angle on the top and bottom of the legs.
figuring the Tangent : 28.5 / 34=.8382 The angle with that Tan is 39.97 degrees

Now if you want the bottom outer edge of the legs to be 34" then you have more calcuating using the width of the leg and I won't get into that.
For a 14 ft long top, I'd consider doing aprons and a different leg configuation and maybe three legs over that span…......................Jim

 

Graph paper…enough said

 

Just go for it with some scraps and it it looks good good it is good.

 

I'm going to take a couple aspirin. My head hurts after tryin' to digest all the math.
There better be some center bracing. Sounds like a landing deck for aircraft. 14'? That's a bunch. I'm sittin' on one end. Please ship me the salad dressing. I'll accept FedEx.
Sounds more like truss rods with turnbuckles are needed.
Bill

 

My goodness…..

Get a big piece of posterboard or cardboard and draw a box 30 inches tall and however wide your leg spread will be.

Let's say you want 2 inches of (the top) overhang on each side. Your box would be (30 inches tall minus the THICKNESS of the top… if the top is 1-1/2" thick, then the box is 30 minus 1-1/2 or 28-1/2) by (table width) 30" minus (total overhang) 4" so 26 inches wide. Now make a trammel 5-1/2 inches wide. (A trammel is just a strip of cardboard or poster board or whatever…. the width of a 2×6). Lay it on your box from corner to corner as though it was a leg.

VOILA! You have your angles and length of leg.

Done

Oh, and at 14 feet you're going to need some serious sag prevention. Like a center beam. A deep one.

 

Jonathan,

You are definitely correct, if I had not figured it out in 10 months then I have bigger problems!

Here is the table.

Here is a picture of the legs.