Understanding IWD (Improvement When Drawn)
What is IWD?
IWD (Improvement When Drawn) is a statistical metric developed by 17lands.com that measures how much a card improves your win rate when you draw it during a game.
Mathematically, IWD is calculated as: IWD = Win Rate (when drawn) - Win Rate (when not drawn)
For example, a card with +5% IWD means that games where you drew that card had a 5 percentage point higher win rate compared to games where you didn't draw it (but it was in your deck).
Important: IWD measures correlation, not causation. A high IWD doesn't necessarily mean the card caused you to win - it could mean you were already winning when you drew it.
IWD Limitations & Biases
1. Aggro Bias
Fast, aggressive decks often win or lose before drawing many cards matters. This gives aggro cards artificially low IWD values.
Example: A 2-mana 2/2 creature might have low IWD not because it's bad, but because aggro decks win quickly (before drawing it matters) or lose quickly (making it irrelevant). The games where it gets drawn often end before its impact can be measured.
2. Situational Cards (Sideboard Effect)
Niche cards that are only good in specific matchups can have inflated IWD because they're primarily played when they're likely to be effective.
Example: Artifact or enchantment destruction cards might show high IWD because players sideboard them in against artifact/enchantment-heavy decks where they're strong. They're rarely in losing matchups, inflating their correlation with winning.
3. Archetype Paradox
Medium-quality cards in dominant archetypes can have low IWD because the deck wins regardless of whether you draw specific cards.
Example: If Blue-White fliers is the best archetype in a format, a mediocre flier might have low IWD not because it's weak, but because the deck is so strong that drawing any particular card doesn't significantly change the outcome.
4. Context & Sample Size
IWD is based on aggregated data from thousands of games. The metagame, format speed, and player skill level all affect the numbers.
Considerations:
- Rare cards have smaller sample sizes than commons
- Early format data can be less reliable as the meta evolves
- Flashback drafts may have different dynamics than new releases
- 17lands data comes from enfranchised players, not casual players
GIH WR vs IWD
This app also displays GIH WR (Games In Hand Win Rate), which is the win rate when a card is in your opening hand.
IWD
Measures the improvement from drawing a card during the game. Better for evaluating late-game impact and card quality in a vacuum.
GIH WR
Measures the absolute win rate with the card in your opening hand. Better for evaluating overall deck strength and early-game impact.
Comparing both metrics gives you a more complete picture of a card's value.
Learn More
All data in IIHGuessr comes from 17lands.com, an incredible resource for Limited Magic data and analysis.
For more details on their methodology, visit the 17lands metrics page.