This article is a sub in the Lay Betting on Betfair pillar. If you have not yet placed a lay bet on Betfair, read Lay Betting Explained and the lay betting guide before this — the maths below makes sense only after you can place the bets manually.
The site's own trading calculator covers all five operations and is the tool referenced throughout. You can also use a spreadsheet (the formulas are at the bottom of this article) or the in-software calculator that ships with Bet Angel and Geeks Toy. The maths is identical wherever you run it.
Operation 1: stake → liability
This is the single most important number on Betfair. When you place a lay, the amount that gets ring-fenced from your balance is your liability — not your stake. Get this wrong and you put thirty pounds at risk when you intended to put ten.
Formula: liability = stake × (price − 1). A £25 lay at 3.40 carries liability of £25 × 2.40 = £60.00. The Betfair interface shows this clearly before you confirm, but the calculator lets you sanity-check before opening the betslip — particularly useful on mobile where the small print is easy to misread.
Lay: £20 at 4.50.
Calculation: £20 × (4.50 − 1) = £20 × 3.50 = £70.00.
What gets deducted from your balance: £70.00 the moment the lay is matched.
What you win if the selection loses: £20 (the backer's stake) minus commission.
Profit-to-risk ratio: £20 win / £70 risk = 0.29. You need 78% win rate to break even on this trade orientation. That is why laying at long prices is the same uphill walk as backing at short prices.
Operation 2: target liability → stake
Most disciplined lay traders work backwards from a fixed-liability target. "I will risk no more than £20 on any one trade." The calculator then tells you what stake to put on whatever price you are looking at.
Formula: stake = liability ÷ (price − 1). A £20 liability target at 3.40 means £20 / 2.40 = £8.33 stake. The calculator handles the rounding to two decimals automatically; Betfair accepts stakes to the nearest penny.
Bank: £1,000. Per-trade liability cap: 2% = £20.
Lay at 1.60: Stake = £20 / 0.60 = £33.33.
Lay at 2.50: Stake = £20 / 1.50 = £13.33.
Lay at 5.00: Stake = £20 / 4.00 = £5.00.
Three different stakes; identical maximum risk. This is how serious traders size every lay — fixed liability, variable stake. Bankroll Management covers why the 2% cap holds up under realistic losing streaks.
Operation 3: post-commission profit
Commission applies to net market winnings, not gross. A successful £25 lay at 3.40 wins £25 gross; the calculator shows you the post-commission figure based on your account's commission rate (default 2% to 5% depending on your Betfair Points Premium status).
Formula: net win = stake × (1 − commission rate). A £25 win at 2% commission = £25 × 0.98 = £24.50 net. At 5% it would be £23.75. Across hundreds of trades the difference between commission tiers compounds into hundreds of pounds, which is why frequent traders chase the Betfair discount rate at every available opportunity.
Trade: Lay £40 at 2.20. Win.
Gross profit: £40.00.
Commission at 2%: £40 × 0.02 = £0.80.
Net profit: £40 − £0.80 = £39.20.
Note: commission charges apply at market level, so if you also had a losing back on the same market the commission applies to your net position. Per-trade commission like this is an approximation; for exact figures use the calculator which lets you input multiple positions per market.
Operation 4: greening up a winning lay
You laid Liverpool at 3.40 for £20. Liverpool conceded an early goal and the price has drifted to 5.20. You want to lock profit across all outcomes by placing a back at 5.20. How much back stake?
Formula: back stake = (lay stake × lay price) ÷ back price. £20 × 3.40 / 5.20 = £13.08 back stake at 5.20. The calculator does this in one click. The result: £6.92 net profit on every outcome of the market.
Position: Lay £20 of Liverpool at 3.40. Liability = £20 × 2.40 = £48.00.
Now: Price drifts to 5.20.
Green-up back stake: £20 × 3.40 / 5.20 = £13.08 at 5.20.
If Liverpool wins: Lose £48.00 (lay liability), win £13.08 × 4.20 = £54.94 (back). Net = +£6.94.
If Liverpool loses: Win £20 (lay stake), lose £13.08 (back stake). Net = +£6.92.
Locked profit ≈ £6.93 across all outcomes (gross of commission). Green Up Explained covers the full theory.
Operation 5: multi-runner lay dutch
Sometimes you want to lay two or three runners in a race simultaneously, equalising your liability so that whichever one wins costs you the same amount. The calculator handles this; doing it by hand requires three steps and is error-prone under time pressure.
The maths: pick a fixed total liability for the lay dutch, divide that liability across the runners proportionally to their lay prices, then convert each runner's allocated liability to a stake using Operation 1 backwards.
Plan: Lay runners A and B in the same race to a total combined liability of £40.
Prices: A is 4.0, B is 6.0.
Calculation: Liability split ∝ 1/(price-1). A: 1/3 = 0.333, B: 1/5 = 0.2. Total = 0.533. A's share = 0.333/0.533 = 62.5%; B's share = 37.5%.
A allocation: £40 × 0.625 = £25 liability → stake = £25 / 3 = £8.33 at 4.0.
B allocation: £40 × 0.375 = £15 liability → stake = £15 / 5 = £3.00 at 6.0.
If A wins, you lose £25 (A's liability) and win £3 (B's stake). Net = −£22. If B wins, you lose £15 and win £8.33. Net = −£6.67. Almost equal liabilities is the cleanest a two-runner lay-dutch gets — you only equalise perfectly when the prices are equal. Use the calculator to balance more precisely.
Dutching itself is covered fully in Dutching on Betfair. The lay variant above is rarely used standalone — it more often appears as a hedge inside a larger trade.
Bonus operation: lay-to-back swing P&L
Many trades start as a back and close as a lay (or vice versa). The calculator's swing function returns your green-up P&L given any back stake, back price, and current lay price — useful for swing trading where you are deciding in real time whether to take profit.
Formula: profit if greened = (back_stake × back_price / lay_price) × (lay_price − back_price) / lay_price × backProfit. In plain words: take the original back stake, scale it by the price ratio, and the difference is your locked profit. The site calculator does it instantly.
Entry: Back £30 at 5.0 pre-event.
Now: Price has shortened in-play to 3.4.
Lay-off stake: £30 × 5.0 / 3.4 = £44.12 at 3.4.
If selection wins: Win £30 × 4.0 = £120 (back), lose £44.12 × 2.4 = £105.88 (lay liability). Net = +£14.12.
If selection loses: Lose £30 (back stake), win £44.12 (lay stake). Net = +£14.12.
Equal £14.12 across all outcomes — the textbook green-up. Swing Trading covers when to take this profit early vs let it run.
The mistakes the calculator prevents
- Liability shock. Placing a £50 lay at 5.0 and not realising you have just risked £200 of liability is the most common new-trader error. The calculator forces you to see the liability before clicking.
- Stake mis-sizing on long-priced lays. Without the calculator, traders default to fixed stake. Fixed stake plus rising price means rising liability — a £25 lay at 1.6 is £15 risk, a £25 lay at 5.6 is £115 risk. Fixing stake is fixing risk only at one specific price.
- Greening up at the wrong stake. By eye, a green-up looks like "back roughly the same amount". The correct stake is rarely "roughly the same"; it depends on the ratio of lay-to-current price. Getting it wrong leaves a non-flat position you have to manage in two halves.
- Forgetting commission in expected-value maths. A 1% edge before commission is a 0% edge after. The calculator's net-profit field stops you placing trades whose theoretical edge dies under transaction costs.
- Mis-counting liability across multiple positions on the same market. If you have laid two runners in the same race, your total liability is the maximum of the individual liabilities (only one runner can win). Betfair handles this automatically in its risk display; new traders sometimes double-count it.
Calculators are tools, not strategies. Knowing the maths does not make a trade profitable; only positive expected value across many trades does. Use the calculator to size correctly, then use the filters in Strategies That Work to choose trades worth sizing.
Spreadsheet formulas if you build your own
If you want to build your own lay calculator in Excel or Google Sheets, the five formulas above translate directly:
- Liability: =stake*(price-1)
- Stake from target liability: =liability/(price-1)
- Net profit: =stake*(1-commission_rate)
- Green-up back stake: =(lay_stake*lay_price)/back_price
- Lay-dutch liability share for runner i: =total_liability*(1/(price_i-1))/SUM(1/(price_j-1) for all j)
Most traders end up with a tab per market — one row per leg, one column per outcome — that totals net P&L across the matrix. That spreadsheet plus a trading diary is the entire back-office of a serious lay book.
How calculators handle BSP and Starting Price lays
If you set a lay request at the Betfair Starting Price (BSP), you do not know the final price until the off. The calculator handles this by letting you input a max-acceptable price — the BSP cap above which you do not want the lay matched. Liability is calculated against the cap, so you know your worst-case risk even if BSP comes in shorter than the live show price.
BSP lays are heavily used in horse racing pre-off; they let you specify a lay willingly without having to monitor the show movement. The trade-off: you might end up with a worse price than you could have got manually. See Betfair SP Explained for the full mechanism.
Calculator vs ladder display in software
In Bet Angel and Geeks Toy, the ladder display includes the calculator inline. As you hover your stake at a price, a small panel shows liability, P&L if win, and P&L if lose for that exact trade against any open positions. This is far faster than switching to a separate calculator tool — at the cost of needing the software up and your account logged in.
For new traders, learning on the web calculator first builds the muscle memory and the mental model. Migrating to in-software calculators happens naturally once trade frequency exceeds about 20 trades a day. Below that, the web calculator is faster than firing up trading software.
When to upgrade to in-software calculators
The site calculator handles 95% of trade-management situations. Where you genuinely need software-bound calculators is in two scenarios: high-frequency in-play trading where every second matters and clicking between tabs costs trades; and multi-leg correct-score lays where six or seven scores need balancing simultaneously.
For those, the calculators inside Bet Angel, Geeks Toy, and Cymatic Trader are built directly into the ladder interface so green-up is one click. Best Betfair Trading Software 2026 ranks them. For most pre-event and slow-paced in-play trading, a browser tab with the site calculator works perfectly well.
Bookmark the calculator. Open it before every lay you place. After a couple of weeks the maths becomes second nature and you only need the calculator for the multi-leg situations — but the discipline of running the numbers first is what separates traders from punters.
Open Trading Calculator Open Betfair Account →Practising with the calculator before placing real trades
Spend an hour running scenarios through the calculator before placing your first lay. Punch in different prices, different stakes, different commission rates; watch how liability changes; build the intuition before any real money is involved. This is the cheapest training investment in Betfair trading and the one most often skipped.
Better still: keep the calculator open in one tab and a live Betfair market in another. Watch the prices change; do mental calculations for each price; check against the calculator. After a week the maths becomes automatic and you can read the value (or absence of value) at a glance from the price alone.
Frequently asked
Does Betfair show liability before you place the bet?
Yes. The betslip displays "Liability" clearly before you confirm. The calculator is for pre-betslip work — running scenarios, sizing trades, and checking green-up stakes without scrolling through the live betslip.
Does the calculator handle different commission tiers?
The site calculator accepts a commission rate input (default 5%; many traders set 2% if they have earned the discount). The discount rate article explains how the rate is set per account.
Can a calculator help with Premium Charge planning?
No. Premium Charge is calculated lifetime, not per trade. Read Betfair Premium Charge for the formula and planning approach if your account is approaching threshold.
Do I need a calculator if I am only laying small amounts?
Yes — and especially yes. Small lay stakes still produce real liability and small accounts have less room to absorb the surprise of mis-sized risk. The calculator is more valuable on a £100 bank than a £10,000 bank because every error matters proportionally more.
What about lay-the-field calculations?
Lay-the-field on Betfair means laying every selection in a race simultaneously at a fixed maximum price. The calculator does not natively support it; specialist software like Bet Angel handles it cleanly. Lay-the-field strategies are an advanced topic — start with single-runner lays first.
Read next
Inside this cluster: Lay Betting pillar, When to Lay, Strategies That Work, Short Favourites, Lay vs Back, Liability Explained.
Tools and theory: Trading Calculator, Green Up Explained, Dutching, …