What percentage of Klondike solitaire games are winnable?

For Klondike on its own, a solver with perfect information, able to see every face-down card, wins about 90.5% of turn-1 deals and about 81.9% of turn-3 deals, give or take a fraction of a percent. Those numbers come from published research, and they describe the most a flawless player could ever achieve, not the average game.

Your own rate will be lower, because the face-down cards stay hidden while you play and a single wrong move early can quietly kill a deal. Turn-3 is the harder setting: drawing three cards at a time reaches fewer stock cards, so fewer deals stay winnable. The rest have no solution at any level of skill.