Forty Thieves: the two-deck game that punishes careless play
The solitaire site solitairex.io puts Forty Thieves in its expert tier, alongside four-suit Spider, with win rates of 11% or less, and our test gives that some support. It uses two packs, deals forty cards face up in ten columns of four, and gives you a single trip through the rest of the pack. On the tableau you build down in suit, one card at a time.
We handed 3,000 deals to a computer player that looks a few moves ahead but cannot see the stock and never takes a move back. It won only 5.8% of them, and on average it got 34 of the 104 cards home before running out of moves.
That 5.8% is a floor, not the true odds. Our player is a simple one, and a careful person using Undo should do noticeably better. What it does show is how little room for error the game leaves: one badly timed move with the stock and a game can be over.
At a glance: Forty Thieves in numbers
| What we measured | Result | Deals | How |
|---|---|---|---|
| Games won by our computer player | 5.8% (95% range 5.0–6.7%) | 3,000 | Look-ahead player, does not know the stock order, no Undo |
| Cards on the foundations at the end (of 104) | 34.3 on average | 3,000 | Same player, all games |
| Moves in a won game, stock turns included | about 277 | 174 wins | Same player |
| Deals with an Ace playable from the opening tableau | 57.3% | 3,000 | Counted from the deal |
No published study gives a winnability figure for this version of Forty Thieves, so we cannot say what share of deals is winnable with perfect play. Our number measures how hard it is to win without looking ahead.
The rules on this site
- Two packs, 104 cards.
- Ten tableau columns of four cards, all face up (40 cards).
- A 64-card stock, turned one card at a time onto a waste pile. There is no redeal: one pass only.
- Eight foundations, two per suit, built up from Ace to King.
- On the tableau you build down in the same suit: the 6 of diamonds goes on the 7 of diamonds only.
- Only one card moves at a time.
- Any card may fill an empty column.
Why Forty Thieves is so unforgiving
One pass through the stock
Sixty-four cards, seen once, in order. If the card you need turns up before you have space for it, and you cannot play it anywhere, it goes to the waste and is buried by the next one.
Same-suit building with single moves
A card has only two possible places to go on the tableau (the two higher cards of its suit), and you can only ever move one card at a time. Long runs are hard to relocate.
Progress stalls early
Our player got about a third of the cards home on average (34.3 of 104) before it ran out of useful moves. For our player, getting stuck with about two-thirds of the pack still out was the typical ending.
Strategy the numbers point to
Start with the Aces on show
In 57.3% of deals at least one Ace is already playable from the opening tableau. Play it, and any 2 of the same suit, straight away.
Empty a column before you touch the stock
An empty column is the most valuable thing in Forty Thieves, because any card can go there. With columns of only four cards, clearing one early is realistic, and it gives you somewhere to put stock cards that fit nowhere else.
Keep the waste short
Every card you turn over and cannot play sits on top of the ones below it. Before each turn, ask whether the tableau could take the next few cards. If not, create space first.
Don't bury low cards under long runs
Because runs move one card at a time, a long run sitting on top of a 3 means many moves before that 3 is free. Build runs on high cards where you can.
Use Undo as your look-ahead
Our player's lack of Undo is a big part of why it wins so rarely. You have unlimited Undo here, so try a line, see what the next few stock cards bring, and step back if it goes wrong.
Playing Forty Thieves here
Forty Thieves is free to play in any modern browser on a computer, tablet or phone, with nothing to download or install and no account or email needed. The cards are large and clear with a zoom control, which helps with ten columns on a small screen. Hint, unlimited Undo, Restart and Auto Play are always available, and you can drag cards or tap to move them. Wins, time and moves are tracked for the current game; sound, fullscreen and table colour are up to you.
How we did the research, step by step
The free-solitaire.nz team ran this study in September 2026 to see how hard Forty Thieves is to win on this site, with its exact rules.
- Read the rules from the site's code. Forty Thieves is a branch of the site's Klondike engine: two packs, ten columns of four face up, a 64-card stock turned one at a time with no redeal, same-suit building, single-card moves and any card in a space.
- Rebuilt the game as a program with those rules and checked it only makes legal moves.
- Dealt 3,000 numbered games, seeds 1 to 3,000, so every deal can be replayed exactly.
- Chose a player. A look-ahead player that sees what you see (the tableau and the waste) but not the order of the stock, and never takes a move back. That makes its win rate a floor on what a skilled person can do.
- Ran the games. All 3,000 took about half a minute of wall time.
- Looked for published figures. The Blake and Gent study (2026, Journal of Artificial Intelligence Research) does not include Forty Thieves, and we found no comparable figure elsewhere.
- Worked out the margin of error. The win rate's 95% range is 5.0–6.7%. Repeat the test with fresh deals and the true figure for this player lands there 95 times in 100.
- Stated the limits. Our player is simple and cannot plan around the stock the way a person can. Treat 5.8% as "how hard is it without looking ahead", never as "how many deals can be won".
The technical side
The program is Python 3.11 using only the standard library. Forty Thieves has its own module, fortythieves.py, which reuses the player we built for Diplomat. On each turn it searches up to 300 positions reachable from tableau and waste moves, remembering positions it has already seen, and scores them: points for each card on a foundation, for each empty column and for same-suit links, and penalties for cards sitting on a lower card or piling up in the waste. It turns the next stock card when nothing improves the score. A shared runner.py spreads the deals across parallel worker processes with multiprocessing, each deal is shuffled from its own seed, and the 95% ranges are Wilson score intervals. An AI assistant (Claude, by Anthropic) helped us write and review the code and draft this page, but none of the numbers came from it: every figure comes from running the program, and the team reviewed it before publishing.
By Admin, free-solitaire.nz. Study date: 19 September 2026.
Frequently asked questions
How often can you win Forty Thieves?
Our simple computer player won 5.8% of 3,000 deals. A careful player with Undo should do better, but it is one of the hardest games on this site to win.
Can I go through the stock twice?
No. On this site the stock is turned once, one card at a time.
Can a whole run be picked up in one go?
No. Only one card moves at a time in Forty Thieves.
What can go in an empty column?
Any card.
What is a gentler two-pack game?
Try Solitaire Double, where our computer player won 81.5% of games, or Diplomat.