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I found these calculators for sbd interest and steem power interest respectively: https://jscalc.io/calc/mKEqkCwkOOe4IzwV and
https://jscalc.io/calc/KcEkRo6qv2s8CM6S

It seems to vary constantly. A good place to see it is on https://steemstats.com/ just enter your name in the follow then go to steem power tab and it will estimate your interest.

I think it was 10% every year. Also, powering down SP takes 104 weekly payments if I'm not mistaken. You can check the Steemit Whitepaper if you click on the top-right corner menu and select "Steem Whitepaper".

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