Drawing without replacement, done exactly
Every card you see changes what is left in the deck, which is why deck math is the hypergeometric distribution rather than coin-flip multiplication. This calculator computes it exactly: the chance of at least one copy in your opener, the full exactly-0-through-4 spread, the odds by any turn on the play or the draw, and what chasing a card through London mulligans really buys. It works for any "group of cards" question, not just playsets: 17 lands in a 40-card limited deck is the same math wearing different clothes.
The number that quietly matters most is the expected copies decimal. A playset in a 60-card deck averages 0.47 copies per opening hand, and that expectation, not the at-least-one percentage, is the foundation manabase math is built on: it is how you go from "how often do I see a land" to "how many sources of white do I need," which is exactly where our upcoming land calculator picks up.
The formula
Expected copies = n × K ÷ N
N is the deck size, K the copies, n the cards you have seen, and C the binomial coefficient ("choose"). On the play you have seen 7 + (turn − 1) cards, on the draw 7 + turn, because the player on the play skips their first draw step.
Worked example
A playset: 4 copies in a 60-card deck, 7-card hand. At least one copy: 39.95%: almost exactly the coin flip players intuit, but reached honestly. The spread: 33.63% for exactly one, 5.93% for two, 0.38% for three, and one hand in about 14,000 holds all four. Expected copies: 0.47.
By turn 4 you have seen 10 cards on the play (52.77%) or 11 on the draw (56.55%): the draw's extra card is worth 3.78 points here. And if you would ship any hand without it, two London mulligans of 7 fresh cards each raise the chance of finding it to 78.35%, at the price of keeping just 5 cards.
Mulliganing deep, and the free mulligan your pod plays with
Most odds calculators quietly stop at three mulligans, which is a Standard player's assumption wearing a lab coat. Real formats disagree. A Vintage Dredge player chasing Bazaar of Baghdad is not choosing between a fine hand and a great one: without Bazaar the deck does not function, so shipping to five, four, or three is simply the cost of playing the deck, and going six deep to find it is a rational line rather than a disaster. This page therefore lets you mulligan as far as your hand size allows, down to keeping nothing at all. Watch what happens to the percentage as you go: four copies in 60 cards climb from 39.95% at zero mulligans to 78.35% at two, 92.19% at four, and 97.18% at six. Each extra throw buys less than the one before, because you are removing a shrinking fraction of the remaining failure, and each one costs a full card off the top. That crossing point, where the search gets more expensive than the thing you are searching for, is a judgement call the math can inform but not make.
The second switch above is for kitchen tables and casual pods that grant one free mulligan: you throw the first hand back and draw a fresh seven without bottoming anything. Turn it on and the calculator prices it honestly, which means telling you something slightly deflating. A free mulligan does not improve your odds of finding the card at all. The percentage is set purely by how many fresh looks you take, and a free mulligan gives you the same looks for one fewer card paid. It changes what you keep, not what you find: two mulligans with the house rule leave you six cards instead of five. That is a real and substantial gift, since the cards around your key card are what let you cast it. It is just not the gift most players think they are getting.
Three readings players get wrong
The expected count is not a probability. Multiplying 4/60 by 7 gives 0.47, and it is tempting to read that as 47%; the true at-least-one chance is 39.95%, because the lucky multi-copy hands are hiding inside the average. The two numbers answer different questions, and the table above shows both. The play/draw gap is real but small for any single card: about 3 to 4 points by the mid-game; the draw's real gift is card economy, not finding one specific answer. And the London mulligan is a combo player's tool, not a free reroll: each throw genuinely shows 7 fresh cards (the math here assumes you ship every hand without your card), but paying a card per mulligan means the rest of your hand gets worse while your odds of the one card get better. That trade is worth it for a piece your deck cannot function without, and for very little else, which is exactly what the 78% figure in the example is quietly telling you.