Flatten a cone

SketchUp Pro 20.0.362 on an iMac running Catalina 10.15.4. I have a cone shaped object that I need to fabricate. I need a good way to flatten the image on a sheet so I can cut it out and then wrap it into the desired cone shape.


You could try flattery.

Will that work on a cone? It looks like its made for 3D objects with flat surfaces, not what I have. Any other suggestions?

A cone IS a collection of flat surfaces

I haven’t tried it, but everything in SketchUp is made from flat surfaces.

There’s also Unwrap and Flatten available in the Extension Warehouse.

And for a simple cone it really isn’t difficult to draw an unfolded version with native tools anyway.

This was done with no extensions.

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Here is the cone I’m trying to flatten, and I also need one just like the one you show except smaller.
Cone.skp (411.4 KB)

I thought I had it figured out but when I built the prototype I discovered my calculation theory was all wrong. Shame on me. Now that I actually engaged my brain to use the right formula (arc of circle) I’ll be able to calculate the angle for the given radius to give a desired arc. Then I can just draw that out. But I’m anxious to hear how you did yours.


Makes you dredge up some high school Geometry doesn’t it?

Here’s the manual way I used earlier. I used the same method for yours with the added step of the center hole.

You need the length of the side of the cone from the apex to the edge. 9.9 in your model. You also need the circumference at the bottom edge which you can get from Entity Info. 34.6 in your model.

I drew a circle with a radius of 9.9 and the circle for the hole at 2.7 to match your truncated cone’s lower and upper circles. Then I got the circumference of the larger circle (62.2) and did the following math.

34.6 / 62.2 = 0.556

360 x 0.556 = 200.26

Then I used the Protractor tool to place a guideline at 200.26° from the starting point. (the line at the 3:00 o’clock position on the center circle.) I made a component of this geometry so the next geometry wouldn’t stick to it. Next I used the Arc tool (not 2-Point Arc) to draw arc with 24 segments through 200.26° at the two radii and I connected their ends to get the face which I moved over to the right. For my first example the only difference is there’s no need for the smaller circle.

Note I used 24 sides for my circles and the arcs because that’s what you used. I would match things up with whatever number of sides you used when you made the cone.

Here you can see the segments of the edge of the 3D cone and the flat pattern match as one would hope.

FWIW, since SketchUp recognizes the circles and arcs as true circles and arcs for circumference calculations, this method, though involving some math is likely to be more accurate than pulling a triangle or trapezoid out of the model and copying it. It also means that you can lay the pattern out more easily with basic tools. No need to print a full size pattern or anything.


In theory it should work but the end result is not what I need/want. The “Wedge” shown, when wrapped, does not give the cone in my drawing. Maybe I just misinterpreted what you are showing. Nonetheless, I did arrive at the answer. See my response to DaveR below.

I finally dug up the simple geometry formula for the arc of a circle.
Arc = Radius x Angle (in radians)
I know my desired arc lengths (base of cone and top opening in cone) and I know my radii (9.8" and 2.7") so all I had to do was to calculate the angle. Doing so I arrived at 200º

Then I looked at your approach using a ratio of circumferences and that’s even simpler.

Where was my brain yesterday.

Here’s my result. Now I can get on with building the prototype. For reference, it’s the shade for a dark sky yard light.

Thanks for your help.


Sometimes you just need to come at it from a different viewpoint.

I’ve found the ratio thing to be very useful in a number of cases so it’s always in the back of my head.

The one thing I did that I see you didn’t is draw arcs through the 200°. It’s probably not important in your application. I did it so that I had a vertex at each end of the arc instead of dividing an edge on the circle. That’s more a cosmetic thing. Would be useful if you were going to print a full size pattern or show it to a fabricator.

So of course you know that no pictures: it didn’t happen. :smiley:

Dave… really? …come at it from a different ANGLE!!

…missed opportunity…




I’m choking on my peanut butter and cheese sandwich!


I’m choking at the idea of a peanut butter and cheese sandwich. Never heard of such a thing!

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You should try it. my favorite is peanut butter and Swiss but we were out of Swiss so I had cheddar.

These regional specialities are often an acquired taste. I must have tasted it about 50 years ago but it left no lasting impression. In our parts you can buy it in shops that cater to the nostalgia of local Americans, like the ones in the US that sell rye bread and a certain brand of milk chocolate to local Finns.

try peanut butter, lettuce and banana. mmmmmmmmmmmmm

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