interest-only mortgage calculator with early payoff?

I can’t seem to find an interest only mortgage calculator that calculates in dollars (not £), and also lets you calculate what would happen to your loan if you paid extra towards it each month. I’m using a mac, and the one I did find didn’t seem to work on macs. help!
i’ll try imagining it’s $ .
i wish that interest.com one worked, but i guess it’s for windows!

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