How Does the MLB Run Line Work, and What Is a First Five Innings Bet?

A run line is a baseball point spread fixed at 1.5 runs. The favourite at -1.5 has to win by two or more; the underdog at +1.5 wins the bet if it wins the game or loses by exactly one. Because the handicap does most of the work, the prices move toward even money: in an illustrative market, a -170 moneyline favourite might be +130 on -1.5 while the +150 underdog becomes -150 on +1.5. Every book sets its own numbers.

A first five innings bet (F5) is settled on the score after five innings and ignores everything that follows. Regular starting pitchers averaged about five and a half innings a start in 2025, so an F5 price is a bet on the two starters and the lineups facing them, with the bullpens removed from the question.

Why is 1.5 the number, and why does it matter who is at home?

Baseball has no ties in the regular season, so a full-game margin can't be zero, and a line of 1.5 splits the result cleanly: win by one, or win by more. The one-run game is where the run line lives. In 2025, 29.4% of completed nine-inning MLB games were decided by exactly one run, by our count of the league's own line scores.

The home team gets fewer chances to win big, and the rulebook is the reason. Rule 7.01(a) of the Official Baseball Rules shortens a game when "the home team needs none of its half of the ninth inning." If the home side leads after eight and a half, it never bats again. Those 2025 line scores show 1,095 home wins, 83% of them, ended without a bottom of the ninth. A team leading 3-2 after eight and a half is a 3-2 winner, and a -1.5 ticket on it loses.

The other route to a home win is a walk-off, and a walk-off ends the game when the winning run scores. The one exception is a game-ending home run, where Rule 9.06(g) lets the batter and every runner score. Of 222 walk-off wins in 2025, 200 were by one run. Extra innings push the same direction: under Rule 7.01(b)(2) every extra half-inning starts with a runner on second, so games that reach the tenth tend to end close. 8.6% of 2025's games went past nine.

Put together: 34.3% of home wins in 2025 were by exactly one run, against 23.6% of road wins. The road team always gets its bottom-of-the-inning chance to pad a lead; the home team often doesn't.

Worked example: the same favourite, home and away

Say you believe a team wins 60% of the time. Using the 2025 league-wide one-run shares as a rough guide:

	Home favourite	Road favourite
Wins the game	60%	60%
Share of wins by one run	34.3%	23.6%
Wins by 2+ (covers -1.5)	60% × 65.7% = 39.4%	60% × 76.4% = 45.8%
Fair price on -1.5	+154	+118

The arithmetic for converting a probability p below 50% to American odds is 100 × (1 − p) ÷ p. For 39.4%: 100 × 0.606 ÷ 0.394 = +154.

Same team, same 60% to win, and the fair price on -1.5 is +154 at home but +118 on the road. A book quoting both at +130 would be short-changing one bettor and overpaying the other. The underdog's +1.5 at home is the mirror image: the home dog covers when it wins (40%) or loses by one (60% × 34.3% = 20.6%), a 60.6% proposition, fair at about -154.

These are league averages, not a model. We would expect the one-run share to be lower when the favourite is heavy or the total is high, since bigger run environments leave room for bigger margins; we have not measured that split here. Treat the table as the shape of the effect, and price a specific game with the specific total in front of you. Our guide to implied probability covers the conversions if the formula above is new.

Run line or moneyline: which one?

Neither is better in the abstract; they are two prices on overlapping outcomes. The run line on a favourite pays more because it wins less often, and the question is only whether the extra payout is enough for the extra risk.

Two practical points. The vig is charged on both, so compare them on the same footing: convert each side to implied probability, strip out the margin (our devig guide shows how), and see which one leaves you a better fair price against your own number. And the underdog +1.5 is usually priced as a favourite itself, often a steep one, which means risking a lot to win a little on a team that is, by definition, expected to lose.

What a first five innings bet actually measures

The average starting pitcher doesn't finish the game. The 117 pitchers who made at least ten starts and no relief appearances in 2025 averaged 16.4 outs per start, about five and a half innings, on 88.8 pitches. Innings six through nine mostly belong to relievers, who are chosen on the day by the manager based on the score and who is rested.

F5 markets exist to cut that off. The bet is settled on the first five innings only, so you are pricing the two listed starters, the two lineups, and the park, and leaving out the part of the game that is hardest to forecast before first pitch.

That makes F5 the natural market when your view is about a starter rather than a team. A club with an excellent starter and a thin, overworked bullpen can be a fair price on the full game and a stronger one through five. The reverse also holds: a team whose starter is shaky but whose bullpen is deep is worth less through five than its full-game number suggests.

Ties, 0.5 lines, and three-way prices

Unlike the full game, five innings can end level. In 2025, 15.9% of games were tied after five. Books handle that in one of three ways, and they are different bets:

• F5 moneyline, two-way: a tie after five is usually refunded as a push. The fair price is conditional on there being a winner.
• F5 three-way: home, away and tie are all offered, and a tie loses both team bets.
• F5 run line -0.5 / +0.5: the favourite must lead after five; the underdog +0.5 wins on a lead or a tie.

The numbers show why the labels matter. Suppose the favourite leads after five 48% of the time, the dog 36%, and they are tied 16%. The three-way fair price on the favourite is 100 × 0.52 ÷ 0.48 = +108. On a two-way line that refunds the tie, the favourite's chance among decided games is 48 ÷ 84 = 57.1%, fair at about -133. Same team, same five innings; +108 and -133 are both fair because they are settling different things. Read which one you are buying before you compare prices across books.

When does an F5 bet count, and what about pitcher changes?

F5 markets are official once five innings are complete; the rest of the game doesn't matter. The STN Sports house rules in Nevada, for example, say partial-game markets such as the first five innings "are considered 'Official' upon the completion of the specified market." If rain stops a game in the fourth, the F5 bet has no result, and the usual outcome is a refund. Full-game bets lean on Rule 7.01(d): a called game becomes regulation after five innings, or four and a half with the home team ahead. Books do not all follow the league here. STN grades a moneyline on a called game once it passes five innings, but requires nine innings (eight and a half) before a run line or total has action.

Starting pitchers are the other trap. Because an F5 price is a price on the starters, some books tie F5 wagers to the listed pitchers, so a late scratch voids or reprices the bet; others treat them as action. We have not surveyed every book's default, so read the rule where you bet. Full-game bets can be taken as "action," standing whoever starts, or tied to one or both named pitchers. Our Academy guide to listed pitchers and action bets walks through how each option settles when a starter is scratched. Check which one your book defaults to before you bet.

What the house is charging you

Every one of these markets carries a margin. A -170 / +150 moneyline implies 63.0% + 40.0% = 103.0%, so the book is holding roughly 3% of the pair before anyone places a bet. Run lines and F5 lines are separate markets with their own margins, which need not match the moneyline's, so check each one. Books set these prices with more information than most bettors have, and they limit accounts that beat them consistently. Most bettors pay that margin season after season and finish behind. The run line and F5 markets are tools for expressing a specific view at a fair price, not a way around that.

The next guide in this series covers pitcher strikeout, outs and earned-run props, where the same starting-pitcher logic gets applied to one player at a time.