As others have said, most unis (that I know of) don't let you graduate with failed modules in your final year, which would include the dissertation.
There's usually a handbook/guidance of some kind provided by the uni that explains the intricacies of how you're grade will be calculated, whether you can fail any credits (and if so how many), whether all your credits are counted towards your grade, how each module is weighted, etc. Often it will also include a formula or calculator for you to input your marks and figure out your own grade as it stands.
That would be your best place to consult as it will be specific to your uni/course.
If it's confusing or you can't find the info you want, email your tutor for clarification.
If you're worrying about your dissertation, I would also suggest emailing and/or meeting up with your supervisor asap to check you're on the right track and to get some pointers - that's what they're there for after all.