How to draw a parabola and its focal point?

How to draw a parabola and its focal point?

A Bezier curve with a single control point between the ends is a segment of a parabola. You can draw it with Fredo’s Bezier Spline extension from the Sketchucation Plugin store or the Bezier Curve tool in the Extension Warehouse. As for finding the focus, I don’t know of an extension to do it but you could work it out graphically.

How accurately do you need to place the focus?

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have you looked at THIS LINK << click the bold text…


Thanks for answering so quickly. I need to find the focal point to assist in drawing a second parabola off the focal point to be used ina ray-tracing experiment I need to do.thank you.

There’s also an option to draw a parabola in the Curve Maker extension in the Extension Warehouse.

I haven’t found a way to come up with an extremely accurate way to locate the focus, though. It can be approximated but due to the nature of curves in SketchUp–made up of short line segments–it’s not not perfect. The more segments you use to make the curve, the closer you’ll come to the real focus point.

This was done using a parabola drawn with Curve Maker. I expect it’ll be close enough for government work.
Parabola.skp (33.2 KB)

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I made a simple plugin to create an arbitrary parabola and its focal point:

The plugin is here: jimhami42_create_parabola.rb (1.8 KB)


Thank you for the update. how do I open it. I tried to import it into sketchup but it would not open. I tried to open ti on the desktop but to no avail.

Are you referring to the file Jim shared? It’s a plugin file. You need to put it into the Plugins folder which should be under your User name/App Data/Roaming/SketchUp… The App Data folder is normally a hidden folder so you would need to set up Windows to show it.

I went ahead and packaged it as an rbz file. Download it to your Desktop or somewhere you can find it. Then in SketchUp go to Window>Preferences>Extensions. Click Install Extension and find the file. Select it and open it. Let SketchUp put it in the right place.

I did have to edit it so it would show up in the Extensions menu.

jimhami42_create_parabola.rbz (769 Bytes)

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Jim. thanks for the program to calculate a parabola, but something has gone wrong with the calculating. I put in 2500mm x 6mm and the value that was red out was 1485x28125. Unfortunately I could not use it. when I first used it it appeared to work but it was too small so I multiplied the figures to increase the size but could not the result I needed.I left it for a while and the parabola got less and less of a curve.I though I would contact you for some more help.

`Jim I forgot to include that the focal point kept growing.

First of all, this is a very flat parabola! I created a parabola 2500mm x 6mm and got this:

The calculation is based on 4py = x^2 … if x is 1250 and y is 6, then p = (1250 * 1250) / (6 * 4) = 65104.166667 (which is where the c-point is located).

Can you provide a little more information or possibly a screen shot?

BTW, if you’re trying to achieve a specific focal length, I’ve added another version that lets you specify the width and the focal distance: