Job Offer as Contract worker?

I work full time at an agency at the moment and have been offered a job as a contract worker at a very big corporate company (that shall remain nameless).

Now they’re offering a contract for a year, 40hrs a week 9-5, at a substantial rate.

What i’m wondering is, to compare the jobs, how much should i account for taxes and benefits? LIke what percent of income made through this contract should I substract and what monetary value would you price full health, vision, dental insurance so that i can factor that in too?

I get full benefits now and make an okay rate at the moment, but full benefits are definately great. I just need to make sure that even though it sounds like a ton of money at the other job; is it really after taxes and benefits are considered.