Here's a simple maths problem. Take a circle of radius 10. This radius could be any unit of measurement; millimetres,inches,light years,baby steps. To find the area of this circle, you apply the formula:

πr²

Therefore the area of our circle is 3.14159 x (10²) = 314.159

Seems simple enough?

Not according to Revit

Draw a filled region using a circle of radius 10.

Firstly, do this using metric units (mm & mm²) and you get:

Now do this in imperial units (decimal in & in²) and you get:

The metric figure is 99.98% accurate, the imperial a little closer. Big deal you may say, and in most cases you'd be correct, certainly in a small example like this. But what if I take this circle and make it bigger, then I extrude it to make some 3D geometry, then I copy this around many times, then I want to know the volume of these elements? Small errors start to compound into more significant overall errors

pi is a constant. Revit knows this is a circle. So why the discrepancy?

## 6 comments:

Looks like it only affects filled region area. Rooms and spaces seem fine. Fwiw, I personally don't rely on or use filled regions for area values that really matter.

Hi Steve, thanks for your thoughts. It's actually affecting volume calculations, which is how I first came across this. I used filled regions here as an example to demonstrate the point. I first thought it might be because circles are essentially faceted, but I've checked that and it's not the case. If you take a cylinder and do an Area*Length calculation and compare it to Revit's volume calculation, the figures are different that in some cases, even rounding doesn't bring out the same figure

I personally use Filled Regions in calculations for sloped roof calculations in MEP design due to the fact that the Roof slopes are not recognized Rooms/Spaces. Thank you for the information.

Here is my take on it:

What Revit Wants: A really small piece of Revit pi

If you took that circle, made it bigger, and copied it around a bunch of times, it's still going to be 99.98% accurate. Sure, you're going to be off by a larger absolute number, but its still a 0.02% error, and it will always be a 0.02% error, no matter how large your sample size.

On a hundred dollar item, that's two cents. Now say that error happens on a $100,000 house. That's about 20 bucks. I thinks It's safe to say, you'll find a more than a few 20 dollar errors.

So is that another glitch of Revit? Or another Revit LT System Requirements flaw?

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