The actor, who played half-man, half-giant Hagrid in the wizard franchise, tipped the scales at 280 pounds (127 kilograms) but he has already shed 63 pounds (29 kilograms) in the last six months after giving up his favourite junk foods.
He is hoping to drop another 56 pounds (25 kilograms) before going under the knife to fix an old sports injury, which has left him with arthritis in his right knee.
Revealing the key to his diet success, Coltrane explains, ""Avoiding all the things I like. Make a list of the things you like to eat and avoid them all. It's all come off in six months, but four stone's (56 pounds) not much to lose when you're my weight.""
Coltrane, who hurt his knee playing rugby at school in Scotland, will head to the U.S. for surgery to replace his cartilage with teflon once he reaches his goal weight.
He says, ""It's very painful... The long-term plan is to have it done in America but not until I've lost another four stone.""