Tools
Bet calculator
One bet, one market. Back or lay, your stake, your commission rate — and what actually lands in your account.
| Runner | Back | Lay | If this wins |
|---|---|---|---|
| Home | £98.00 | ||
| The draw | −£100.00 | ||
| Away | −£100.00 |
Best outcome £98.00, worst −£100.00 — a spread of £198.00. Press "level it off" to spread it evenly instead.
Commission is charged on a winning outcome only, and on the net of that outcome rather than bet by bet — which is what the exchange does, and why a levelled book stays level after it.
Two doors into one bet
Over and Under are the same event from opposite ends. This says which side to take, and what each one ties up.
Backing Over and laying Under are the same bet — they win on the same games. The exchange prices them separately, so one door is usually cheaper than the other and nothing on the screen tells you which.
| Line | Back over | Lay under | Lay, as a back | Better door | At risk, backing | At risk, laying |
|---|---|---|---|---|---|---|
| 0.5 | 1.100 | level | £1000.00 | £1000.00 | ||
| 1.5 | 1.250 | level | £400.00 | £400.00 | ||
| 2.5 | 1.568 | level (0.002) | £175.44 | £176.00 | ||
| 3.5 | 3.083 | Back over (0.017) | £47.62 | £48.00 | ||
| 4.5 | 6.556 | Lay under (0.156) | £18.52 | £18.00 |
The last two columns are what you have to put up to win the amount above, each way in. They are close at short prices and nowhere near close at long ones — laying a 12.0 shot to win £100 risks £1,100 — so the better price and the cheaper door are not always the same door. Amber marks a lay that ties up more than 5% extra.
One bet on its own
A single back or lay, and what lands in your account after commission.
- You staked
- £100.00
- At odds of
- 3.90
- Your bet
- won
- Profit before commission£100.00 × (3.90 − 1)
- £290.00
- Commission at 2%
- −£5.80
Back in your account: £384.20 — your stake returned plus the profit.
After 2% commission, backing at 3.90 pays you the same as 3.842 would with no commission. That lower number is the one your model has to beat.
Betfair charges commission on your net winnings per market, not per bet. With one bet in a market this is exact. With several, the winners and losers are netted off first and commission is taken on what's left — so run them together, not one at a time.
Trading in and out
Open on one side, close on the other, and level it off so the result is the same however the game finishes. This is the calculator behind every number in the Trading Centre.
4.20 to 4.00 is 2.0 ticks. One tick here is 0.10, or 2.38% of the price — the hurdle any edge has to clear.
Selection wins: £313.60 · loses: −£100.00Levelling off swaps that for the same result either way.
On commission: it's charged on the net winnings of a market, once — not per bet and never on turnover. Several trades in the same market net off first, so winners and losers inside one market cancel before the cut is taken. Different markets are settled separately, each on its own net.
What each side actually means
Back
You're betting it happens. Risk your stake, win the stake multiplied by the price minus one.
Lay
You're the bookmaker. You take someone's stake and cover their winnings if it comes in. Your risk is the liability, not the stake.
The one that catches people out
Laying at a long price is expensive. Lay £100 at 3.90 and you stand to win £100 — but you're risking £290 to do it. Lay the same £100 at 12.0 and you're risking £1,100. The stake is what you win; the liability is what you lose. Switch to Lay above and drag the price up to see it.
Still to come: getting filled
Both calculators above assume you get the price you asked for. In practice you either take what's offered and pay the spread, or post your own price and wait — and sometimes never get matched. The trades you miss are the ones that moved, which is exactly the thing a backtest can't see.
Also on the list: the hurdle — why the gap between the two prices decides which strategies are even possible.