In metres. (The unit for SPH on a prescription is the diopter; 1 dpt = 1 m^-1.)
You can also just wander over to a pharmacy's glasses rack and hold some reading glasses in front of your normal ones to test how much SPH you need to add. (Optical power is approximately additive.)
I should have noted: if you don't wear glasses already, or if that the formula gives you a SPH > 0.0 and you don't have astigmatism, then there's no need to order prescription ones. You can just go to the reading glasses rack in any store and buy ones with that SPH.
You can also just wander over to a pharmacy's glasses rack and hold some reading glasses in front of your normal ones to test how much SPH you need to add. (Optical power is approximately additive.)
I should have noted: if you don't wear glasses already, or if that the formula gives you a SPH > 0.0 and you don't have astigmatism, then there's no need to order prescription ones. You can just go to the reading glasses rack in any store and buy ones with that SPH.