DistCalc.com uses an algorithm which calculates the great-circle distance, also known as geodesic or orthodromic distance.
The surface where this distance is calculated on is the
Using an ellipsoidal model for Earth, DistCalc provides higher accuracy than other (comparable) tools on the internet which mostly use a sphere. But this model requires a more advanced (and more complex) mathematical approach. Although the model is quite good, there is some uncertainty remaining (model errors, numerical effects, accuracy of coordinates). Thus, the calculated distance may be different from the true distance outside your window. That's why I cannot assure the correctness of the result by 100%.
DistCalc.com needs coordinates of your starting point and your destination to do anything for you (except looking nice and clean). If you already have
The algorithm consists of a few mathematical equations. Besides your coordinates (φ,λ) some constants have to be defined (e.g. semi-major axis a and oblateness f of
P.S: Since I am German, please note that my algorithm works with metric sizes. I hope you still use my page now you know this dark secret of DistCalc.com. :-)