Back and lay
Back is the bookmaker side: you want the selection to win. Lay is the exchange side: you are the book. Mix the formats — back 5/2 against a 3.40 lay is a normal ticket.
Back
Lay
Commission
Taken from exchange profit when the lay wins — the consumer rule on Betfair, BetDAQ, Matchbook and Smarkets Standard. Not taken from the stake, and not a weekly fee.
Back decimal3.500
Lay decimal3.400
Back return£35.00
Lay stake£10.29
Lay liability£24.71
If the back wins£0.29
If the lay wins£0.29
Work it by hand
Convert both prices to decimal. Back return is back stake × back decimal. The lay stake that matches that return is back return ÷ lay decimal. Liability — what you lose on the exchange if the selection wins — is lay stake × (lay decimal − 1).
Example. £10 at 5/2 (3.50) returns £35. To lay off at 3.40 decimal, lay stake is 35 ÷ 3.40 = £10.29. Liability is 10.29 × 2.40 = £24.71. If the selection wins you collect £35 and pay £24.71: £0.29 after the £10 stake. If it loses you keep the £10.29 lay stake and lose the £10 back: the same £0.29. A longer lay than the back (3.60 against 3.50) flips that into a small red both ways.