How Non-Runners Affect Starting Price (SP) in Horse Racing

A non-runner shortens the remaining field — and the Starting Price moves before the stalls open. The odds you took at 10am are not the odds the market settled on at 3:15pm. Whether that matters to your payout depends on the type of bet you placed, the price you locked in, and whether the SP shifted in your favour or against it.

Starting Price is the official odds of a horse at the moment the race begins, determined by the on-course betting market. It is the price that governs bets placed at SP and, in some cases, the benchmark against which early-price bets are measured under Best Odds Guaranteed promotions. When a non-runner is declared, the probability it carried is redistributed across the remaining runners, and the SP of every surviving horse adjusts accordingly.

The effect is proportional to the non-runner’s market position. Average field sizes on Core Flat fixtures in 2025 stood at 8.54 runners, with Core Jumps down to 7.63, according to the BHA Racing Report Q3 2025. In fields of that size, a single non-runner removes a meaningful slice of the total probability — and the remaining runners’ SPs shorten to absorb it. In bigger fields, the redistribution is spread more thinly and the movement is smaller. In smaller fields, every withdrawal creates a visible compression.

How SP Forms and Why Non-Runners Move It

The Starting Price is not set by a single entity. It is derived from the on-course betting ring — the collection of bookmakers who stand at the racecourse and offer odds to punters in person. An official SP reporter, employed by the Racing Post, records the prices available in the ring at the moment the race starts and calculates the SP for each runner based on the best generally available odds.

When a non-runner is declared, the on-course bookmakers adjust their boards. The withdrawn horse is removed, and the probability it occupied — expressed as implied odds — is spread across the remaining field. If a 4/1 shot is scratched from an eight-runner race, the remaining seven horses each pick up a portion of that probability. The favourite might shorten from 2/1 to 7/4. A mid-range horse might go from 6/1 to 5/1. An outsider might barely move at all, because the redistributed probability is weighted towards the horses most likely to benefit from the non-runner’s absence.

A total of 18,452 individual horses ran at least once in Britain in 2024, a one percent decline from the year before. The shrinking population means that the horses entering the ring at each meeting carry more individual weight in the SP calculation. Fewer horses in training means fewer entries, smaller fields on average, and larger SP swings when a non-runner reduces the field further.

The timing of the withdrawal matters for the SP. An overnight non-runner is absorbed into the morning tissue prices and the on-course market opens with the field already adjusted. The SP reflects a market that never included the withdrawn horse. A late non-runner — declared thirty minutes or less before the off — forces the on-course bookmakers to reprice in real time, and the SP that results may not fully reflect the new market equilibrium because there was not enough time or liquidity in the ring to settle on accurate prices. Late non-runners produce the most volatile SP adjustments.

Early Price vs SP — Which Protects You Better After a Non-Runner

The odds you took aren’t the odds the market settled on. If you took an early price at 6/1 and the SP after a non-runner is 4/1, you are in a better position — your locked-in price is higher than the market’s final assessment. If you took 6/1 and the SP drifted to 8/1, you are in a worse position — but a non-runner is unlikely to cause a drift. In almost every case, a non-runner shortens the remaining field and the SP comes in.

For bettors who take early prices, a non-runner in the same race generally works in their favour on the price side. The early price was set with the withdrawn horse in the field; the SP is set without it. The early price is typically longer because the probability was spread across more runners when the bet was struck. After the withdrawal, the SP reflects a smaller, more concentrated field — and your early price looks generous by comparison.

The catch is Rule 4. Your generous early price is subject to a deduction calculated on the non-runner’s price at the time of withdrawal. The deduction may negate some or all of the advantage you gained by taking a longer price early. If you took 6/1 and the SP is 4/1, but a 25p Rule 4 deduction applies, your adjusted return on the 6/1 price may not be much better than a clean return at 4/1 SP. The maths depends on the specific prices and deduction, and it varies case by case.

For SP bettors, the picture is simpler. You bet at whatever price the market settles on at the off. If a non-runner shortened the field, the SP already reflects that change. No Rule 4 applies to your bet because you did not lock in a price before the withdrawal — your bet was always going to be settled at the market’s final assessment. SP bets are immune to Rule 4 in this sense, because the deduction only applies when the bettor’s price was set before the non-runner was declared.

The Overround After a Withdrawal — Bookmaker Margin in Motion

The overround is the bookmaker’s built-in margin — the amount by which the total implied probabilities of all runners in a race exceed 100%. In a perfectly fair book, the probabilities would sum to exactly 100%. In practice, they sum to 110%, 115%, or more, depending on the race and the bookmaker. That excess is the bookmaker’s edge.

When a non-runner is declared, the overround shifts. The withdrawn horse’s probability is removed from the book, but the bookmaker does not redistribute it evenly. Instead, the remaining runners’ prices are adjusted in a way that maintains or even increases the overround. The bookmaker uses the non-runner as an opportunity to tighten the book — shortening the remaining prices slightly more than the pure probability redistribution requires, thereby preserving or growing their margin.

For bettors, this means that the SP after a non-runner is slightly less generous than it would be in a perfectly efficient market. The on-course bookmakers are repricing under time pressure, and the adjustment tends to favour the layer rather than the backer. The effect is small — typically a fraction of a point on each remaining runner — but it compounds across a season’s worth of bets. On a single race, the difference between a fair-redistribution SP and the actual SP might be the gap between 4/1 and 15/4. Over hundreds of bets, those fractions add up.

Understanding the overround after a withdrawal is not about avoiding the bet. It is about recognising that the market’s response to a non-runner is not neutral. The remaining runners shorten, which is expected. But they shorten slightly more than the maths requires, which is the bookmaker’s edge in action. If you are comparing SP returns against early-price returns after a non-runner, factor in that the SP already includes a margin adjustment that the early price did not. The early price may look worse after Rule 4, but it was set in a market with a different overround — and sometimes that difference works in the early-price bettor’s favour.