STS2 Draw Probability Calculator
Answer the question every climber asks: “Will I see my key card this turn?” Enter deck size, copies, and cards drawn — get at-least-one odds instantly.
Results
- At least 1 copy
- 36.7%
- Exactly 0
- 63.3%
- Exactly 1
- 33.3%
- At least 2
- 3.3%
- Expected copies in hand
- 0.40
Model: hypergeometric draw without replacement from a 25-card pile, 2 success cards, drawing 5. Reshuffles / random discard piles are not modeled.
How to use this calculator
This tool uses the hypergeometric distribution — the same math that governs drawing cards without replacement from a finite deck. Here's how to get accurate results:
- Deck Size: Enter your current deck size (including cards in hand and draw pile). Do not count the discard pile unless you expect a reshuffle this turn.
- Key Cards in Deck: How many copies of your target card exist in your deck. This includes cards you haven't drawn yet plus any copies already in your hand.
- Cards to Draw: How many cards you'll draw this turn (hand size + card draw effects).
- Desired Copies: Usually set to 1 for "at least one copy," but increase for combo consistency (e.g., 2+ copies for a two-card combo).
Interpreting the results
The calculator shows three key probabilities:
- At Least X Copies: The chance to see your minimum desired copies. This is the most important metric for consistency planning.
- Exactly X Copies: Useful for precision planning, especially when multiple copies have diminishing returns (like redundant defensive cards).
- At Most X Copies: Helps assess overflow risk — valuable when drawing too many copies of a limited-use card (like exhaust-worthy effects).
Decision rule of thumb: If the "at least 1 copy" probability is below 40%, treat your key card as unavailable this turn. Plan alternatives or accept higher variance.
When to use this tool
Draw probability matters most when deck-building decisions hinge on consistency vs power:
- Deck-thinning decisions: Before removing cards, compare draw odds before vs after. A 5-card removal might not significantly improve win-con consistency if the card was already rare.
- Copy count choices: When offered "add 2 copies of X" vs "1 copy of stronger Y," use the calculator to see if the consistency gain outweighs the power loss.
- Relic evaluation: Bag of Preparation and other draw relics effectively increase "cards to draw" mid-combat. Re-calculate odds when considering these relics.
- Combo planning: For 2-card or 3-card combos, set "desired copies" accordingly. A 50% chance to see card A and 50% for card B means only ~25% for both together.
Common mistakes to avoid
Mistake 1: Counting discard pile in deck size
Why it's wrong: The hypergeometric formula assumes drawing from a single pool. Your discard pile doesn't join the draw pile until after a shuffle.
Correct approach: Only count draw pile + cards in hand. If you expect a shuffle this turn (e.g., you'll exhaust your draw pile), run the calculation twice: once pre-shuffle, once post-shuffle.
Mistake 2: Ignoring cards already in hand
Why it's wrong: If you're holding 1 copy of a 2-copy card and calculate based on 2 copies in deck, you'll overestimate your draw odds by counting the card you already have as "drawable."
Correct approach: Subtract cards in hand from both "deck size" and "key cards in deck." You're calculating odds to draw additional copies, not total ownership.
Mistake 3: Overestimating combo consistency
Why it's wrong: Seeing card A (50% chance) and card B (50% chance) doesn't mean 100% combo activation. You need to see both, which multiplies probabilities (50% × 50% = 25%).
Correct approach: For multi-card combos, first calculate each card's individual odds, then multiply them. This conservative estimate prevents over-committing to inconsistent lines.
Quick examples
| Scenario | Deck Size | Copies | Draw | ≥1 Copy Chance |
|---|---|---|---|---|
| Starting deck, 2x key card | 25 | 2 | 5 | ~36.8% |
| Starting deck, 3x key card | 25 | 3 | 5 | ~50.5% |
| Thinned deck, 2x key card | 18 | 2 | 5 | ~48.4% |
| Late game, 1x win-con left | 15 | 1 | 6 | ~40.0% |
| Fat deck, 2x combo piece | 35 | 2 | 5 | ~26.5% |
Key insight: Thinning your deck from 25 to 18 cards provides a massive consistency boost (36.8% → 48.4% for 2-copy cards). Fat decks (35+ cards) severely punish consistency-focused strategies.
Advanced use cases
Once you're comfortable with basic draw odds, explore these advanced applications:
- Relic vs relic comparisons: When choosing between +2 card draw relic and +20% damage relic, calculate how often the extra cards would find your key attacks. If the draw boost increases win-con odds by >10%, it often outweighs raw damage.
- Mulligan decisions: In multi-card starting hands (if STS2 adds this), use the calculator to decide whether to mulligan away low-impact hands based on key card density.
- Exhaust vs retain planning: Some effects exhaust cards after use. Calculate odds based on the effective deck size after exhaustion, not the starting size.
Related tools
Draw probability is one piece of the decision puzzle. Pair it with:
- Necrobinder soul calculator — when planning lethal turns, confirm you'll both see your damage card (draw calc) and have enough resources to cast it (soul calc).
- Regent forge simulator — draw consistency matters for forge-heavy builds. Use both tools to balance immediate power (forge) vs long-term consistency (draw odds).
- First-run guide — new players often overvalue consistency in small decks. The guide explains when to prioritize raw power over percentage points.
Ready to optimize your deck?
Check your current deck's draw odds, then explore our full calculator suite for more Slay the Spire 2 decision tools.