Honey is ~75% sugar and it's all fermentable. Honey does vary slightly in the content though. The rule of thumb on Gotmead's calculator is 74% sugar content in honey. And so far, the meads I've made with their calculator have been spot-on the actual gravity of the must.
Well I already have a Mead, a Cyser, Two honey added beers with one more comming in a couple days... and I still have about 14lbs still laying around. Soon it will be Honey harvesting time again and I will have even more.
"The key to immortality is first living a life worth remembering."