Funny thing is, it's not exactly a back of the envelope calc, as technically speaking it depends on your requirements on accuracy (or rather, your tolerance for

Funny thing is, it's not exactly a back of the envelope calc, as technically speaking it depends on your requirements on accuracy (or rather, your tolerance for different flavours of distortion), the projection you wish to draw in, the projection the measurement is done in, and what tradeoffs you are willing to make on circumference/shape/area. Also factors: where you draw the circle, because this matters depending on your choices of projections. ArcGIS or GDAL or whatever can do the math for you, but there's still domain knowledge and careful tradeoffs involved in representing anything spatial and meeting actual IRL need. It's all relative (and a bit of a goddamned nightmare) (src: I used to build spatial tools on ArcGIS)