Moving to Expert and Deep Leagues: AL-Only, NL-Only and 15-Team Formats
How fantasy baseball changes in AL-only, NL-only and 15-team leagues: a smaller talent pool, a lower replacement level, harder catchers and closers, and a different approach to auctions, ratios and FAAB.
Most fantasy players learn the game in a mixed league of ten or twelve teams, where every player in the major leagues is available and the waiver wire is a reliable supply of useful hitters and starters. Move to an AL-only league, an NL-only league or a mixed league with fifteen teams, and the same skills stop working in the same way. The rosters are the same size, but the pool of players you are drawing from is much smaller, and the player you could pick up for free in a standard league now belongs to someone else.
That single change, the fall in replacement level, rewrites most of the strategy. Playing time becomes more valuable than talent. Catchers and closers become scarce in a way they never were. Injuries to stars cannot be patched with a free agent, and a bad pitcher cannot be dropped for a better one. The auction dollars you paid for the middle of your roster matter as much as the dollars you paid for stars.
This article explains what the deep and expert formats are, shows with arithmetic why the pool is tighter, and walks through the changes in drafting, bidding, pitching and in-season management. It is general strategy, not advice for a particular league: your league's constitution decides the categories, roster slots and rules, so read it before you apply anything here.
What the formats are
The common starting point is the mixed league: players from both leagues are eligible, usually with ten or twelve teams, and a roster of around 23 active players. In the traditional rotisserie version, hitters are scored in five categories, batting average, home runs, runs, runs batted in and stolen bases, and pitchers in five more: wins, saves, strikeouts, earned run average and WHIP.
An AL-only league or an NL-only league keeps the same categories and roster structure but limits the pool to players on the fifteen clubs in one league. Industry competitions have long used these formats; contests such as LABR and Tout Wars, run by fantasy writers and analysts, are well-known examples with AL-only and NL-only auctions. These leagues usually use an auction draft with a fixed budget, and often keep reserve and minor-league rosters.
A deep mixed league keeps both leagues in the pool but adds teams, commonly up to fifteen, and sometimes adds roster spots. It is still a mixed league, but each team needs more players, so the free-agent list is thin quickly.
Two details vary by league and deserve a careful read. First, what happens when a player moves from one league to the other in the middle of the season: some AL-only and NL-only leagues let you keep him and count only the stats he earns while he is in your league's pool, and others require you to drop him. Second, whether a league uses reserve or farm slots for players who are not yet in the majors. Both change how you value prospects and injured players.
The arithmetic of scarcity
The easiest way to see the difference is to count roster spots. Major-league clubs carry 26 players on the active roster as of the 2025 season, which means 780 active roster spots across 30 clubs and 390 across the 15 clubs in one league. A fantasy team with 23 active players takes up a share of those. The table below assumes 23 active players per team and counts only active spots, so it ignores reserves and minor-league slots.
| Format | Teams | Players rostered | Active MLB spots in the pool | Share rostered |
|---|---|---|---|---|
| Mixed | 10 | 230 | 780 | 29% |
| Mixed | 12 | 276 | 780 | 35% |
| Mixed | 15 | 345 | 780 | 44% |
| AL-only or NL-only | 12 | 276 | 390 | 71% |
These are rough, because many real major leaguers are bench players who are not fantasy-relevant, and some fantasy-relevant players are in the minors. But the pattern is clear. In a twelve-team AL-only league, nearly three of every four active players in the league sit on a fantasy roster. In a twelve-team mixed league the figure is about one in three. The free-agent pool in the AL-only league contains very little that a standard league would call useful.
A second piece of arithmetic is positional. A twelve-team AL-only league with two catcher slots needs 24 catchers. The fifteen AL clubs carry about two catchers each, so roughly 30 are on active rosters, and most of them, including the backups who start twice a week, will be rostered. A closer is a similar story: if each of 15 clubs has one closer and 12 fantasy teams want saves, almost every closer is taken, and teams with an unsettled ninth inning produce no saves for anyone.
Replacement level: the idea that changes everything
Replacement level is the production you can get for free. In a standard twelve-team mixed league, a free agent might be an everyday outfielder at a modest level, or a starting pitcher with a decent matchup. That is the benchmark against which you measure what a player is worth to you. A player who hits for slightly more than the free agent is worth a small amount; a player who hits for far more is worth a lot.
In a deep or single-league format, that benchmark is lower. The best unowned hitter in an AL-only league may be a bench bat or a minor-league call-up with no clear role. A player who simply plays every day and hits .260 with a modest number of home runs and steals can be worth many auction dollars, because the alternative is so much worse. That is the reason experienced deep-league players say that playing time is the most important attribute: an everyday job produces counting stats, and a part-time job does not.
The way to apply this is to compute values against the right pool. A common method in rotisserie leagues is to estimate how many standings points a unit of each category earns, then price players by the standings points they add above a replacement player drawn from your actual league pool. The site's explainer on how fantasy rankings are built describes the idea and how to adjust a generic ranking for your own league. A ranking built for a standard mixed league will overrate the stars and underrate the dependable regulars in an AL-only league.
Catchers, closers and the thin positions
Catcher is the clearest case. Because two catchers are required, the position acts like a tax. A team that leaves the draft with a catcher who plays part-time gives up counting stats in every category. In a deep league, the bargain is to find a catcher who plays most days, even if his skills are modest, because a catcher who plays 120 games produces far more than one who plays 80.
Other positions are less scarce than catcher, but extra utility, middle-infield and corner-infield slots matter. The more flexible slots a league has, the more it rewards players who qualify at more than one position, since they let you fill an injury without going to a thin free-agent list.
Closers are the second thin position. Saves are limited by how many games a club wins by a small margin, and which reliever gets them is largely the manager's decision. In deep leagues, teams spend heavily on the safest closers and then scramble to hold on to the second-tier ones. The bullpen strategy article explains holds, middle relievers and ratios, which matter more when saves are hard to buy.
Pitching: ratios, innings and the worst pitcher you must carry
Pitching in a deep league has a problem that mixed leagues avoid: you cannot easily cut a bad pitcher, so the worst pitcher on your staff drags the ratio categories. Earned run average and WHIP are rate statistics, so each pitcher's innings are the weight in the team total.
Here is a worked example. Suppose your staff has thrown 1,200 innings with a 3.80 ERA, which means 507 earned runs. Add 100 innings of a pitcher with a 5.50 ERA, who allows 61 earned runs. Your staff now has 1,300 innings and 568 earned runs, for an ERA of 3.93. Those 100 innings cost you 0.13 of ERA, and in a competitive league that can be a place or two in the standings. The lesson is that a rostered pitcher who throws many innings and pitches badly is more costly than a rostered pitcher who is simply used less.
Many leagues set an innings cap or minimum. A cap means that a team cannot gain categories by simply piling up innings, and a minimum means a team cannot hide by pitching too little. If your league has either, plan around it: it changes the value of the starters with high strikeout totals, because they add strikeouts in a limited number of innings, and of pitchers who pitch many innings but have a mediocre ERA.
In deep leagues, streaming, the habit of picking up a starter just for a good matchup, is less available because the starters in the pool are already taken. The substitute is to carry a balanced staff, avoid pitchers with a clear risk of a rotation demotion, and accept that you will have to hold a weaker arm in order to meet your innings.
Auction changes: budgets, nominations and the $1 player
A standard auction budget in rotisserie leagues is $260 per team for 23 players, an average of about $11 each, with a $1 minimum. Total dollars are the same in an AL-only league as in a mixed league of the same size, but the structure of the room differs. The auction strategy article covers the fundamentals. In single-league auctions, a few adjustments follow.
- Know every player in the pool. In a twelve-team AL-only auction there are about 276 players to buy, and an owner who has prepared a price for each of them has an advantage over one who has prepared only for the top 150.
- Bid up the dependable regular. The everyday player with a stable role is worth more than a standard-league ranking suggests.
- Treat $1 players as real decisions. The last ten spots on your roster are filled with $1 players, and a good $1 player, a platoon bat with a favorable split or a closer in waiting, can return many times his price. Spend time on the end of the auction as well as the beginning.
- Watch the room. If other owners are paying more than your values for stars, the extra dollars they spend leave bargains in the middle.
- Reserve dollars for the end. Because late bargains are real, holding back a small amount makes sense, though not so much that you cannot fill your roster.
If your league uses a snake draft in a deep format, the principles are similar, but the draft is less flexible. The snake draft guide explains the structure, and in a deep league the late rounds, where you take part-time hitters and speculative pitchers, matter more than they do in a twelve-team league.
In-season management: FAAB, injuries and call-ups
Many deep leagues use FAAB, a free-agent acquisition budget, in which owners submit blind bids for available players. The currency is a season-long budget, so the decision about how much to spend on a call-up or a new closer competes with every other bid for the rest of the year. Two guidelines help. Do not spend the budget in April on marginal gains, because more valuable players, such as a prospect who is called up or a reliever who takes over the ninth inning, appear later. And remember that the bids are decided by an opponent's valuation, not your own.
Injuries hit harder in deep leagues because the replacement is weak. The practical answer is depth: the bench players you carry should be ones who can fill a role and, if possible, have value of their own. If your league has a reserve list or injured-list slots, use them, since they let you hold a hurt star without losing a roster spot.
Minor-league call-ups are the main source of new value. Following a club's prospects and bullpen roles, and watching roster moves around the league, tells you who might be next. When a club's circumstances change, such as a new home park, as with the Athletics' move to Sutter Health Park for the 2025 season, an AL-only owner has to re-evaluate every hitter and pitcher in that club, since the whole league is the pool.
Common mistakes in the move to deep leagues
- Using a standard ranking unchanged. It assumes a replacement level that does not exist in your league.
- Overpaying for stars at the cost of the middle of the roster. A star who is injured cannot be replaced, and a $1 slot filled with a nonplayer is a lost category.
- Ignoring playing time. A talented bench player produces nothing.
- Chasing saves late. Closers are scarce, and the ones available on waivers are usually unsettled.
- Cutting pitchers too quickly. A pitcher who has a bad week still counts toward your innings, and the alternative may be worse.
- Skipping the league's rules. Innings caps, player-eligibility rules and trade deadlines change what is worth doing.
Common questions
Is AL-only or NL-only harder? Neither is harder in principle. The NL has had a designated hitter since 2022, so the pools are more similar than they were before. Differences come from the clubs and parks involved.
How many teams make a league deep? There is no official definition. Fifteen-team mixed leagues and single-league formats are commonly called deep.
Should I use different stats in a deep league? Your league sets the categories, and many deep leagues use the same five-by-five categories as standard ones. The adjustment is to your player valuations, not the stats.
Do the same rules apply in points leagues and head-to-head leagues? The scarcity logic applies, but the details differ. The roto, head-to-head and points comparison explains the formats.
Where to go next
If you are new to the deeper formats, start with the auction draft strategy and the guide to how rankings are built and adjusted, then read the snake draft guide if your league drafts in rounds. For the pitching side, see the bullpen strategy article. The site's fantasy pages collect tools for managing a roster.