Delta is 50 pounds. BUT, if you've weighed it at home and you're close (within 5-10 pounds), have a plan B for when you get to the airport. Before our recent trip, I weighed our bag at home and it was 47 pounds! Woo hoo, right? We get to the airport and it was 53 pounds!
I had already planned to bring two bags home, so I got the other bag out and rearranged things. No problem then.