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How to calculate radius of an elliptical curve

Message 1 of 4
308 Views, 3 Replies

How to calculate radius of an elliptical curve



In the drawing mode, I am not able to calculate the radius of an elliptical curve which I split into three curves to be able to calculate two different radii. This curve forms half of an ellipse which measures 3200x1900mm. 


When I go into the drawing mode and select 'radial dimension', nothing appears when I hover my mouse over the curve. 


I have been working out the curve in very DIY ways...tracing over the line and making a circle to calculate the radius of one curve. But I feel like the results are not accurate enough. In one of the screenshots, I was able to get the radius by selecting the curve which I don't get how — I traced the curve I had using the spline tool, but the other curves didn't respond when hovering the mouse over. 


How do I do this??

Message 2 of 4
in reply to: beritYKHLW

The curves in an ellipse are not radius curves so I doubt you will be able to find any radius value.  If you want to model a shape using an ellipse, you will be a lot better off not breaking the elliptical sketch, just dividing it us using other sketch entities.  What are you trying to eventually model?  Can you attach an image?

"If you find my answer solved your question, please select the Accept Solution icon"

John Hackney

Beyond the Drafting Board

Message 3 of 4
in reply to: beritYKHLW

The radius of curvature is smallest at the ends of the major axis (the left and right in your sketch) and largest at the ends of the minor axis (the top in your sketch). For your ellipse the radii are 564.1mm and 2694.7mm respectively. The formula to calculate these values is fairly simple.


It is possible to determine the radius of curvature at other arbitrary points, but the formula is rather more complex.

Message 4 of 4
in reply to: beritYKHLW

By definition, the "radius" of an ellipse is constantly changing, maybe what you are looking for is an oval which is constructed from two big arcs and two small arcs which are tangent where they meet.

See here for an explanation: 


Regards, Peter

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