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Prep

Choose a deck for an event from your own matches, with honest ranges.

Getting the field ready…

How the numbers are made, and where they come from

How the numbers are made

Facts and labeled estimates, never a pick

Prep summarizes the matches you give it under assumptions it states, and leaves the choice of deck to you. Every rate is wins over wins and losses; draws are left out. Nothing leaves your browser unless you copy a team link yourself.

Each matchup

Your matches against an archetype are counted from the decks you link (with the filters for dates, where you played and best of one or three), plus matches you type by hand and teammates' matches you switch on. Opponents are matched to archetypes by name, ignoring case, spaces and punctuation, or by a pointer you set. Each matchup starts from a prior and moves with every match:

  • 50%, weighted like k matches (k = 10, 20 or 40; 20 is the Beta(10, 10) Frank Karsten uses in his Metagame Mentor columns on magic.gg).
  • 50%, weighted by the format: the weight is fitted to how far this format's recorded MTGO matchups spread in the window (the same Beta-Binomial model Meta Lens uses for the field, over 241 weights from 1 to 1,000 matches), and every estimate averages over the weights by how well each fits, this matchup's own matches included.
  • MTGO's record plus yours: the format-fitted prior, with MTGO's recorded matches between the archetype your deck plays like and each column added to your own. The mirror never uses MTGO's record.

posterior = sum over weights a of w(a) x Beta(a + wins, a + losses), with w(a) proportional to prior(a) x B(a + wins, a + losses) / B(a, a)

The estimate is the posterior mean; the range runs from its 2.5% to its 97.5% point, found by bisection on the mixture's distribution function.

Against the field

The shares start from MTGO lists in the window that are legal today (a list holding a card banned since, or gone from the format, by Scryfall's legality as Meta Lens reads it, is left out, as Meta Lens leaves it out by default, so the two fields agree; the page says how many and the day of the legality read), counted as every list, Challenge lists, or each Challenge deck weighted by its match wins minus losses with a floor of zero, Karsten's winners' metagame); the archetypes past the number you show are pooled as everything else. Any share can be typed over; shares that don't add to 100% are scaled. Formats with no MTGO lists in Meta Lens's data (Premodern, Historic, Penny Dreadful, Canadian Highlander, 7 Point Highlander, Old School) take a field you type: archetype names and shares, the rest of 100% as everything else; their matchups start from the weight you choose, since there are no MTGO matches to fit.

expected match win rate = sum over columns of share x matchup estimate

The range comes from 4,000 draws. In each, every matchup is drawn from its posterior and the shares from a Dirichlet distribution centered on yours with a weight of 100 (some drift), 30 (a lot) or the number of lists counted (as the counts say). "Highest in" is the share of draws in which that deck's rate is the top one. The draws are seeded, so the same inputs give the same numbers.

What would change the order: for the two decks with the highest expected rates, a and b, and each column j with share x, the share s at which they'd be level when column j moves and the others scale to fit:

s = R / (R - d), where d = a's estimate - b's estimate in column j and R = (overall difference - x d) / (1 - x)

What to test next

For each matchup and n more matches (5, 10 or 20), every possible number of wins W is weighed by its predictive chance (beta-binomial, averaged over the prior's weights), and gives a new estimate for that matchup and a new expected rate for that deck, with the shares held at yours.

chance the top deck changes = sum over W of P(W) x [the deck with the highest expected rate after W is a different deck]

expected gain = sum over W of P(W) x (highest expected rate after W) - highest expected rate now

The expected gain is the value of the information in match win rate. "Matches to plus or minus 10 points" uses the normal approximation: the matches until the 95% range is about 10 points either side of the estimate.

If the field moves

From MTGO's recorded matches among the archetypes listed (each pulled toward 50% with the format-fitted prior), when there are 50 or more: one replicator step and the Nash mix, solved as a linear program as on Meta Lens. Your decks' rates against each use their own estimates. These are two simple models; real fields don't settle at the Nash mix, as Frank Karsten points out.

replicator step: x'(i) = x(i) x f(i) / (sum over j of x(j) f(j)), where f(i) is archetype i's expected result against the field

Best of three

Exact, by walking every path through the match: game one on the play or the draw (a die roll, or set), then the seat each later game follows from the Magic Tournament Rules section 2.2 (the loser of a game chooses whether to play first in the next; you can set what each player chooses). Games two and three use the after-sideboarding rates. Drawn games aren't modeled. From logged matches, each game's result and your seat give four counts; each rate is pulled toward 50% like 0, 10 or 20 games, and the range comes from 4,000 draws of the four rates.

the common shortcut: 3p^2 - 2p^3, with p the game-one average

The shortcut assumes every game has the same chance; sideboarding and the seat break that, which is why the two can differ by several points.

Rounds and the cut

From the Magic Tournament Rules, Appendix E (effective February 27, 2026): 5 to 8 players, three rounds of single elimination; 9 to 16, five Swiss rounds and a top 4 (or four and a top 8 when the playoff drafts again); 17 to 32, five and a top 8; 33 to 64, six; 65 to 128, seven; 129 to 226, eight; 227 to 409, nine; 410 and up, ten. A player with a one-round bye counts as 2 players, a two-round bye as 4 and a three-round bye as 8.

Your chances

A simulation of the whole event, a few thousand times: you win each match with the rate you give, every other match is even, a share of matches end in an unintentional draw, and games inside a match follow from the match rate (the game rate g with 3g^2 - 2g^3 equal to it). Pairings go by match points, in random order within a point group, avoiding a rematch when one can be avoided; an odd player out gets a bye (a 2-0 win) if they haven't had one. With intentional draws on, two players in the last round draw (reported 0-0-3, MTR 2.5) when both would make the cut however every other match goes: at most the cut size minus two other players can still reach the lower of their two totals. Standings follow MTR 3.1 and Appendix C: match points, opponents' match-win percentage, game-win percentage, opponents' game-win percentage, each with a floor of 0.33, then chance. The playoff seeds 1 against 8, 4 against 5, 2 against 7 and 3 against 6. The record table runs the same event with every player even.

The last round

From standings you type: every result at the tables that can still move someone across the line is tried (a win, a loss or a draw at each; at most twelve tables, so at most 531,441 outcomes), with tables where neither player can reach the line set aside. For each outcome, a player is in when their points are above the line, or on it with no more players there than places; on tiebreakers when more players share the line than there are places; out below it. The chances weigh each table as a win or a loss evenly and a draw as often as you say. Tiebreak percentages you type are shown but not used: they change in the last round.

Your tiebreakers

MTR Appendix C, as written: 3 match points for a win and 1 for a draw; game points the same way per game; your match-win and game-win percentages are points over the most you could have, each with a floor of 0.33; a bye is a 2-0 win and is left out of your opponents' averages; opponents' game-win uses the percentages you type, each floored at 0.33.

Prizes and the day

Prize splits: what MTR section 5.2 allows, in our words, and the arithmetic of playing it out (the chance of each finish with the same match win rate every round) against an even split. The day: the recommended round times of MTR Appendix B (50 minutes a round, 90 for quarterfinals and semifinals, no limit for the final, at least 40 for any match), and, when the playoff drafts again, about 42 minutes to draft and build before the quarterfinals: Appendix B's pick timers for three 14-card Play Boosters, reviews of 60, 90 and 120 seconds, and 25 minutes to register and build.

The Season tab

RCQ rounds, Regional Championships and Pro Tours as Wizards and Star City Games published them, kept by hand in one file the page reads (data/season.json, facts as of October 21, 2026), each with its source and the day it was read. The site never lists events and reads nothing from the event locator, which has no public feed; it links to the official finders instead.

  • RCQ round 1: April 4, 2026 to August 2, 2026, Modern or Limited at stores.
  • RCQ round 2: August 15, 2026 to November 29, 2026, Standard or Limited at stores.
  • RCQ round 3: December 11, 2026 to March 7, 2027, Standard or Sealed Deck at stores.
  • Regional Championships, fall 2026 (Modern): September 11, 2026 to November 1, 2026, 11 events by region.
  • Regional Championships, winter 2027 (Standard): January 8, 2027 to February 14, 2027, 11 events by region.
  • Pro Tour Nauctis, February 26, 2027 to February 28, 2027.
  • Pro Tour at MagicCon: Tokyo, May 14, 2027 to May 16, 2027.
  • Pro Tour at MagicCon: Las Vegas, August 27, 2027 to August 29, 2027.
  • Magic Limited Championship, June 18, 2027 to June 20, 2027.
  • Magic World Championship 33, December 3, 2027 to December 5, 2027.

"Running now", "starts in" and "days left" count whole days between your device's date and the dates above; the round running today is marked, and on a round's last day it says today is the last day. Which Regional Championships an RCQ round feeds is shown only where a source says so. The qualification facts are Star City Games' for the United States unless marked otherwise; other regions' organizers publish their own. "Your season" counts the Game Diary's events of kind RCQ dated inside the round running now, read only.

days left = the round's last day - today; starts in = the first day - today

The season

The Season tab is hand-kept facts with a source and a date on each (data/season.json): the RCQ rounds, the Regional Championships with which Standard the winter ones use where a source says (St. Louis, January 22 to 24, 2027, falls before the rotation by every date given; for San Diego, February 5 to 7, the sources read don't say), the 2027 Pro Tours, World Championship 33 and the Limited Championship with their formats, Arena Championships 13 to 16 and the Qualifier Weekends, the three Eternal Weekends with each organizer's words on proxies, Standard's rotation, and each format's next banned or points list date. A date worked out from a committee's stated schedule says it is estimated. "Coming up" lists the next of them from today.

The pocket card

The Event tab prints it and shows it on the page; both read one list. RCQs run at Competitive rules enforcement, so that is the level it describes. It holds, in our words with the section to look up: what to do before round one and in each round (MTR 2.2 to 2.7, 3.9 to 3.12, and 7.1 and 7.3 for Limited decks), how to talk during a game (MTR 4.1 player communication: status, free, derived and private information; 4.2 tournament shortcuts; 4.3 out-of-order sequencing; 4.4 loops; 4.5 triggered abilities; 4.8 reversing decisions), appeals and a playoff draft (MTR 2.9, 2.11, 2.12), and the infractions that come up most with their usual penalty (IPG 2.1 to 2.6, 3.1 to 3.9 including 3.6 Limited procedure violation and 3.7 communication policy violation, 4.3 and 4.4). The search box shows the points that contain every word you type, ignoring capitals, across the lists and the penalty table, and says how many of them match. The printout fits one page of Letter or A4.

Team codes

A team link carries your decks' names, the archetypes they play like, the field's archetype names and your wins and losses in each cell, compressed (deflate) and written into the link after the #, which browsers never send to a server. Nothing else: no card lists, dates, notes or opponents' names. A teammate's deck joins yours when the names or archetypes match.

Run an event

One device holds the event; nothing goes to a server. Standings follow Magic Tournament Rules 3.1 and Appendix C: 3 match points for a win and 1 for a draw; then opponents' match-win percentage, game-win percentage and opponents' game-win percentage, each percentage at least 0.33; a bye is a 2-0 win (3 match points, 6 game points) left out of the opponents' percentages; players tied on all four share a place.

OMW% = average over opponents of max(0.33, their match points / (3 x their rounds played))

GW% = max(0.33, your game points / (3 x games played)); OGW% = average over opponents of their GW%

Swiss pairings: MTR 10.4 says Swiss events use the Swiss pairing algorithm without spelling it out, so the page names the common practice it follows: round one at random; after that, players with the same match points meet at random, the odd one paired down to the next group; no rematch while another pairing works; the bye to the lowest ranked player who hasn't had one. The rounds start at Appendix E's recommendation for the player count. A round robin seats everyone by the circle method, so each pair meets once; gauntlet pods pair each player of one pod with each of the other. A dropped player (MTR 2.10) isn't paired again. The timer runs 50 minutes by default (Appendix B) and, at time, says the end-of-round rule (MTR 2.4: the active player finishes the turn, then five more turns in total). The standings go out as text or in a link that carries only what it shows.

An event holds up to 128 players and 64 rounds, so a round robin takes up to 64 players and each pod up to 64; the setup form says so before the event starts. Every event is kept until you delete it, and a deleted one waits 30 days in My Stuff's Recently deleted. Prep open in two tabs: each save carries which tab wrote it, a save that finds another tab's newer copy merges with it (a change on either side is kept, events and their rounds matched by id and number), and an open tab shows another tab's change at once.

The event ledger

The ledger's events are the Game Diary's own, so one added in either place is in both. An event's record comes from its rounds when it has them (a linked match read from its deck), else from the record typed in the ledger. Fees and prizes stay in Prep's saved state, in one currency you pick, read the way people write amounts (1,250 and 1 000 are a thousand two hundred fifty and a thousand; 12,50 is twelve and a half; one that could be read two ways, like 1.250, is refused with a question); totals add them per format or per deck, and prizes are counted at the value you type. The CSV holds every row.

The season in your calendar

Calendar files are written to the iCalendar standard (RFC 5545): one all-day entry per date, with the source and its date in the description, and an id that stays the same, so importing again updates instead of doubling. Find an event links out to Wizards' event locator and the Companion app; mtgstuff reads neither.

Sources

About Prep

What it does

  • The field desk: your decks against the archetypes you expect, the shares from MTGO lists (every list, Challenge lists or Karsten's winners' metagame) or your own, each matchup pulled toward a starting point you choose (Karsten's Beta(10, 10), a weight this format's MTGO results fit, or MTGO's own record for the archetype your deck plays like) with its 95% range, and each deck's expected match win rate against the field with its range and how often it is the highest across 4,000 draws.
  • What would change the order: the share any one archetype would need for the next deck to catch up.
  • What to test next: for every matchup, the chance that more matches there change which deck rates highest, and the expected gain in match win rate, so testing time goes where it can still matter.
  • If the field moves: your decks against one replicator step of the field and against the field's Nash mix, from MTGO's recorded matches, with Karsten's caveat.
  • Team codes: share your matrix as a link (no account, nothing uploaded) and switch teammates' matches on and off.
  • Best of three, exactly: four game rates (before and after sideboarding, on the play and on the draw) and who chooses the seat, from what you type or from matches you logged game by game, against the common 3p squared shortcut.
  • Event: the Swiss rounds and cut for any number of players (Magic Tournament Rules Appendix E, with byes), your chances of a top cut and an invitation from thousands of simulated events with real pairings and tiebreakers, the record that usually makes the cut with and without intentional draws, the last round from standings you type, your own tiebreakers, prize splits by the rules, the day's timetable, and a printable match pad and RCQ pocket card.
  • The season: what's coming up, RCQ rounds with their dates and days left, the Regional Championships against Standard's rotation, the Pro Tours with their formats, Arena Championships and Qualifier Weekends, the Eternal Weekends with their proxy rules, and every format's next banned list date, each with its source.
  • The pocket card one press from the installed app's icon.
  • Run an event on one device, no server: Swiss (round one at random, then by match points, no rematch while another pairing works), round robin or two gauntlet pods; results by games, drops, late entries, byes, a round timer with the end-of-round rule, standings with the Appendix C tiebreakers and the 33% floor, a view for a room, and the standings as text or a link.
  • The event ledger: every event you play, with its date, format, deck, store, entry fee, record, place and prizes; totals per format or deck; a CSV; the same events as the Game Diary's.
  • The season in your calendar: every dated item on the Season tab, or one, and each of your own events, as an .ics file with its source; and links to Wizards' event locator and the Companion app to find an event.

Why

Teams prepare in spreadsheets: they log games, smooth small samples by hand and argue about the field. No tool takes your own games, splits them before and after sideboarding, weighs them by the field you expect and shows how sure it can be. And RCQs run in EventLink, which no standings tool reads, so the last-round question (can I draw in?) is still done on a phone calculator. Prep does both, and leaves the choice to the player.

Changelog

  • Run an event: Swiss, round robin or gauntlet pods on one device, with standings by the Magic Tournament Rules' tiebreakers (3.1 and Appendix C), drops, byes, a round timer, a view for the room and the standings as text or a link. The event ledger: fees, prizes and records with totals per format and deck, on the Game Diary's events. The season and your own events as calendar files, and where to find an event.
  • The pocket card is a shortcut on the installed app's icon (/prep/?t=event&pocket=1). Prep's saved state shows on the home page's Pick up where you left off.
  • Thirteen formats: Duel Commander from its MTGO lists, and Premodern, Historic, Penny Dreadful, Canadian Highlander, 7 Point Highlander and Old School with a field you type. The Season tab adds what's coming up, the 2027 Pro Tours with formats, Worlds 33, the Limited Championship, Arena Championships 13 to 16, the Eternal Weekends with each organizer's proxy rule, which Standard each winter Regional Championship uses (or that no source says), and each format's next banned or points list date.
  • A Season tab: the RCQ rounds with dates and days left, the Regional Championships by region, the Pro Tours and the qualification facts, each with its source; the pocket card adds MTR section 4 (talking during a game: shortcuts, out of order play, loops, triggers) in plain words.
  • Live. The field desk with ranges, what to test next, the field's moves, team codes, best of three from four game rates, and event math: rounds by Appendix E, simulated chances, the record that makes the cut, the last round, tiebreakers, prize splits, the day, a match pad and an RCQ pocket card.