i don't know of such an expression, but you could use the approximate circumference of elipses geometry formula which is C = 2 * pi * sqrt((a^2+b^2)/2);

So the expression should be something like this

`a = thisComp.layer("Ellipse").content("Ellipse 1").content("Ellipse Path 1").size[0]/2;`

b = thisComp.layer("Ellipse").content("Ellipse 1").content("Ellipse Path 1").size[1]/2;

2 * Math.PI * Math.sqrt((a*a+b*b)/2);

The expression was added to a slider on a null layer in the same composition as the ellipse layer named Ellipse.

Andrei

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