Almost all online calculators are somehow accurate, viabtc has a nice little tool
https://www.viabtc.com/tools/calculator?symbol=BTC which I prefer to use, the good thing about it is the fact that you can change all variables such as fees, difficulty, and price (most online calculators don't let you do that).
It's important to be able to change those variables, as I type, the difficulty is adjusting
in about 14 hours, with at least 13% to the upside, the current difficulty on all those online calculators is 31360548173144 but in 14 hours it will be 35645328467032 which is roughly 13% less profit.