Two formulas are derived for the capacitance of a flat circular annulus. The first has an accuracy of 1 part in 1000 or better when the ratio of the external to the internal radius lies between 1.1 and ∞ and is exact at ∞. The second,C=34.96R/ln(32R/c) &mgr;&mgr;f, has an accuracy of 1 part in 1000 when the ratio lies between 1.000 and 1.1 and is exact at 1.000. In this formulacis the difference of the radii andRtheir mean value in meters. An approximate expression for the charge density is given in the first case.