Rule 4 Calculator

The Rule 4 Calculator will help you determine the new odds of a horse when a Rule 4 deduction has been applied.


What is a Rule 4 Deduction?

In horse racing, if a horse withdraws from the race before it has started, the odds of the remaining runners will be reduced. This is because as there are fewer runners, each horse now stands a better chance of winning.

Bookmaker deductions are made according to Tattersalls Rule 4 which provides a table with the amount to be deducted from winnings according to the price of the non-runner. The bigger the price of the non-runner, the lower the deductions made and when it comes to odds of 14/1 or more, no deductions are made.

Odds are banded together with deductions made in 5p increments, so for instance, odds from 7.0 (6/1) to 10.0 (9/1) will cause a 10p deduction.

rule 4

Betting exchanges calculate deductions differently, they give each horse a reduction factor based on its forecast price. The reduction factor can be seen by clicking on the graph icon beside the horse’s name when looking at the market. Each price has its own reduction factor and is more precise, so you’ll seen deductions like 27.78% and 11.69%.

How will you be affected by rule 4 deductions and exchange reduction factor?

If you’ve already matched your back and lay bets, you’ll find there is very little difference in the cost of your qualifying bet or the profit retained from your free bet. The difference is usually pennies and nothing to be concerned about.

If you’ve placed a back bet and have unmatched bets on the exchange, you can work out the adjusted back and lay prices and use the matched betting calculator to see if you need to make any adjustments.

Rule 4 Calculator

You can use the Rule 4 Calculator to determine the new odds of your horse after a Rule 4 deduction has been applied.

The fields you need to populate are:

  • The original odds of your horse before the Rule 4 deduction
  • The original number of runners in the race
  • Your original back stake
  • Whether or not your bet was an each way bet
  • Whether or not the race is a handicap race
  • The odds of the withdrawn horse (click ‘Add Non Runner’ to add multiple withdrawals)
  • The deduction percentage (optional)


rule 4 calculator

The image above shows us that we originally placed a £10 win single bet on a horse at odds of 9/1 (10.0) in a race with 8 runners and that one horse withdrew which had odds of 3/1 (4.0).

Our original odds of 10.0 have been reduced to 7.75 – A deduction of 25%.

If you haven’t placed your lay bet yet, you can use these new odds in the matched betting calculator to determine the ideal lay stake for your bet.