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I was building a round table with a single pole in the middle.

I have the round table top, a base, a pole and a mount to go under the table. The mount has a hole in the middle for the pole and then goes wider to spread out the load (it is around 25cm in diameter and has 8 screw holes to go into the wooden table top).

There are a number of suggestions online on how to find the centre of a circle. However most of these are for smaller circle and suggest using rulers and squares which is not as practical for a larger table (~1.5m diameter).

Is there a best practice for finding the centre of a larger table? How important is it that I am centre of the table?

Because it is a single pole table I am a bit nervous it may come out wobbly if off centre.

  • Find where it balances on a relatively thin dowel rod. Should get you to within 5mm. – Hari Ganti Apr 11 '17 at 22:53
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If you know the radius, use your tape measure from a point on the edge to draw an arc at the radius of the table.

Then choose another point 1/3 of the way round and repeat. Then the other third.

enter image description here

That should leave you a tiny little triangle in the middle if you got your radius right. The centre is the middle of that.

If you are not sure it is totally round, repeat in a few spots around the table and use the "middle" of where they "cross". But not too many or you will get confused.

If you want to be really accurate rather than fiddling with tape and pencil, you can jig up a tool out of a length of wood with a nail in the outer end as a pin, and a hole to fit a pencil Radius distance away from the pin.

  • The diameter is supposedly 1.5m from purchase, but when trying to measure it looks like it is ~1.44m. Does it matter to much if it was off by this much? (or does your suggestion of doing multiple more resolve any amount it is off? – Luke Apr 12 '17 at 1:52
  • @Luke I'd use the measurement, and do three of four if you are not sure. You will end up with a little triangle or "square" in the middle. FInding the middle of that should be easy enough. IF you do three.. do it like a Y. – Trevor_G Apr 12 '17 at 1:55
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    error-factor is high in this method – Carl Witthoft Apr 12 '17 at 17:39
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    @CarlWitthoft how accurate do you need it to be..... For woodwork it's well within a millimeter or so. This is the oldest, compass method, in the book. – Trevor_G Apr 12 '17 at 19:24
  • If the tape measure slips, isn't held on the table edge at exactly the same depth (mostly just a concern if the edge is rounded), the table is not perfectly circular, the pencil slips while you're drawing the arc, or you don't get exactly the same spot on the measuring tape, it'll all lead to this being inaccurate. Only mentioning this because you're complaining about the basically the exact same problems with my solution :) – gregmac Apr 12 '17 at 20:17
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I think this technique using chords with 90 degree lines is probably the easiest to do because it involves standard construction tools and can compensate for inprecise measurements.

You just need a square, a measuring tape and a straight edge. (A large enough carpenter's square is all of these in one tool)

  1. Draw two arbitrary chords (lines across) anywhere on the circle. Note the chords don't have to be the same length or lined up to each other in any way.
  2. Find the exact center of each line
  3. Use a square to project a second line towards the center of the circle at exactly a 90 degree angle to the chord
  4. Where the projected lines overlap is the center

enter image description here enter image description here

  1. You can repeat with additional chords get the center more accurately (which can compensate for any deviation in finding the chord center or getting an exact 90 degree angle)

enter image description here

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    This is absolutely the correct way to go. Everyone else seems to have forgotten their High School Geometry :-) – Carl Witthoft Apr 12 '17 at 17:40
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    @CarlWitthoft I disagree, though technically accurate it leaves you open to measurement errors and not quite finding the middle of the cords and then not quite lining up the 90 degrees correctly. Way too many steps involved for errors to creep in. – Trevor_G Apr 12 '17 at 19:27
  • That is also a good method mathematically. You could also make this a little easier by making a simple enough tool for the job. – Trevor_G Apr 12 '17 at 20:36
  • @Gregmac - Thanks - this is a very good mathematical answer. However Trevors answer below I feel is more practical due to the size of the circle I am measuring. Due to the large radius of the circle any margin of error of a few degrees will be amplified. This would probably be my choice for a smaller table though. – Luke Apr 13 '17 at 0:01
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A slightly simpler, but also slightly less accurate way to do this would be to place a stick with a rounded end (something like a broom handle) in a vice clamp.

Then place the table top on top and proceed to balance it on the end of the stick. Once the table balances perfectly, mark the centre by drawing or placing painters tape around the spot where the end of the stick rests on the table top.

In this way you may not find the exact mathematical centre of the table, however you will be finding its centre of gravity, which in this case of construction may provide you with a less "wobbly" and more centred table as the centre of gravity is what determines if the table is wobbly or not when balanced on merely 1 leg.

  • And when your 120-pound table top falls, sadness ensues. – Carl Witthoft Apr 12 '17 at 17:41
  • @Hammergold - while I like your original idea, this sounds a bit to risky for me – Luke Apr 12 '17 at 23:52
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You could just lay a tape measure across and mark 1/2, then do that again at a different angle. Two measures should be good enough, but if you like you can keep doing this and the marks will generally outline the center.

Chances are good that if you hit that within +-1.5" it's not going to matter. The top doesn't necessarily need to be perfectly balanced on the center, since as soon as you put something on the table it won't be balanced, and the variation in wood density will probably put the center of mass slightly off the geometric center anyhow.

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Tape a piece of string to a spot on the edge of the table. Walk around to the other side with string in hand. Take note where it takes the most string to cross the table; uniform tension, and a thumbnail on string against table edge will tell you this. You are at a diameter. Mark line along string near center of table. Repeat with string taped 30 or 45° away from original spot. Three to four repeats will get you 3-4 lines all of which cross at the center of the table. All lines should cross at table's center. Error may get you a little triangle, square or hexagon. The middle of that is the center of the table.

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