This calculator uses the data gathered at this page, more specifically; the assumptions that there are 25,000 Star Destroyers in the Imperial Navy, and that we assume an ISD masses 2.78E10 tonnes, and that they make up 14% of all mass in the Imperial Navy -- these assumptions are not fixed in stone; you may edit them as you feel necessary.

You may also use it to estimate the composition of the pre-Clone Wars' Republic Enforcement Corps -- simply set the number of Star Destroyers in the Imperial Navy to 1000; and ISD mass as a percentage of Imperial Fleet Mass to 100%; and you'll have a starting point of 1000 ISD masses to use to adjust the ship distribution for your much smaller Republic Navy.

The calculator will handle scientific notation for you automatically; just enter it as 1E6 and that's all you need to do.