This solver uses only logic, not estimation. In picture there are colored number 7, that can't found by solver because you need assumption to find it. (Like Y chain.....it is advanced sudoku solver technic.) So solver will solve the soduku with number 7, but it can't solve without it. But if unsolved, you can save current and pick some number like picture unsolved, then run solver. Enjoy! PS: I'm freshman. So I don't know this is right forum to upload this. If I should upload another forum. Please send me some message. Thank you!