The tool is just an XLS document accessible via Google docs, or if you download it you can play with it in Excel or any other spreadsheet program. There’s not a functional HTML version, though hopefully someone with some basic programming savvy can port it into a format that anyone could just click on via a webpage.
Check out the Diablo 3 Seasonal Character Paragon Level Calculator. There are two easy ways to access it:
Note that the exp per paragon level in normal and seasonal is identical: You need exactly the same amount of exp to reach Paragon 100 in each, Paragon 200 in each, etc. This tool just adds those totals and gives you the combined amount. So when calculating, Paragon 300 in normal + Paragon 150 in Seasons is exactly the same as P150 in normal + P300 in Seasons.
A few samples, for quick rule of thumb calculations.
This makes Paragon 100 seem pretty sad, huh? Adding a Paragon 100 Seasonal account will take your main from P400-407, or P500-504, or P600-601. Above that Paragon 100 doesn’t even move the bar a single click. And check that last one! If you’ve got Paragon 1000, you need to add Paragon 245 just to raise your total Paragon a single level, to 1001. Good luck, Gabbynator!
Of course your main account with Paragon 500 or more can also earn EXP much faster than a new Seasons account with no gear, no paragon points, etc. Everyone knows that going in, and it’s not like Blizzard said Seasons were triple exp or anything, but we might as well prepare now for the inevitable cries of “where did mah Paragons go!!1!?” when Season One ends, players merge their accounts, and see their main account Paragon tick up like… 3 levels.