Lay Stake Calculator: How to Work It Out Yourself

A matched betting lay stake calculator tells you exactly how much to lay on the betting exchange so your back bet at the bookmaker and your lay bet on the exchange combine to give you the same outcome whether the selection wins or loses. Paid tools like OddsMonkey hide the maths behind a clean interface, but the formulas are short, public, and easy to drop into a spreadsheet you control.

This guide walks through the three formulas every matched bettor needs — qualifying bet, stake-not-returned (SNR) free bet, and stake-returned (SR) free bet — with worked examples for each. By the end, you will be able to calculate your own lay stakes from scratch and audit any calculator output before you place a bet.

If you are brand new to the technique, start with our walkthrough of how matched betting works first, then come back here for the maths.

Every lay stake calculation depends on three numbers. Get these right and the formula does the rest.

• Back odds — the decimal odds offered by the bookmaker on your selection winning (for example 5.0).
• Lay odds — the decimal odds offered by the betting exchange on the same selection losing (for example 5.2). The closer lay odds are to back odds, the smaller your qualifying loss or the higher your free bet retention.
• Lay commission — the percentage the exchange charges on your winning lay bets, expressed as a decimal. Smarkets charges 2% (0.02), Betfair charges 5% on the standard market (0.05), and Matchbook charges 1% (0.01).

The back stake is given by the offer (often £10, £20, or £50 for sign-up bets). The unknown you are solving for is the lay stake: how much to risk on the exchange so your overall position is balanced regardless of which side wins.

For a qualifying bet, you are placing real money at the bookmaker and matching it on the exchange. The goal is the smallest possible loss across the two bets so you unlock the free bet that follows. The formula is:

Lay stake = (back stake × back odds) ÷ (lay odds − commission)

The numerator is the bookmaker payout (your back stake plus winnings). The denominator is the lay liability multiplier, adjusted downward by the exchange fee so the after-commission proceeds match.

Worked example. A £20 qualifying bet at 5.0 back odds, lay odds of 5.2, and Smarkets commission of 0.02:

• Lay stake = (20 × 5.0) ÷ (5.2 − 0.02) = 100 ÷ 5.18 = £19.31
• Lay liability = 19.31 × (5.2 − 1) = £81.10
• If the selection wins: bookmaker pays 20 × 5.0 = £100 (£80 profit), exchange loss is £81.10. Net: −£1.10
• If it loses: bookmaker keeps £20, exchange pays 19.31 × 0.98 = £18.92. Net: −£1.08

The qualifying loss is around £1.10 either way — close enough that you can treat the two outcomes as identical. That £1.10 is the cost of unlocking the free bet, and you make it back many times over on the next bet.

When the bookmaker gives you a free bet where the stake is not returned with winnings (the standard sign-up bonus from Bet365, William Hill, Coral, and most major books), you are only collecting the winnings portion if the back wins. The formula adjusts to match that:

Lay stake = (back stake × (back odds − 1)) ÷ (lay odds − commission)

Note the (back odds − 1) — that subtracts the stake out of the payout because you do not get it back. The whole formula otherwise mirrors the qualifying bet calculation.

Worked example. A £20 SNR free bet at back odds of 5.0, lay odds of 5.2, Smarkets commission 0.02:

• Lay stake = (20 × (5.0 − 1)) ÷ (5.2 − 0.02) = 80 ÷ 5.18 = £15.44
• Lay liability = 15.44 × (5.2 − 1) = £64.85
• If back wins: free bet pays 20 × 4 = £80 winnings, exchange loss = £64.85. Net: +£15.15
• If lay wins: free bet returns nothing, exchange pays 15.44 × 0.98 = £15.13. Net: +£15.13

Profit is £15.13 either way — a 75.7% retention rate on a £20 free bet, which is a typical ratio at back odds in the 5.0–7.0 range.

A stake-returned free bet behaves like a real-money bet for the back side: if the selection wins, you get the stake plus winnings. SR free bets are usually money-back-as-cash refunds or specific promotions where the bookmaker explicitly says the stake is returned. The formula is the same shape as the qualifying bet:

Lay stake = (back stake × back odds) ÷ (lay odds − commission)

This is exactly the qualifying-bet formula, because the back side now pays out the same way a real-money bet would.

Worked example. A £20 SR free bet at 5.0 back odds, 5.2 lay odds, 0.02 commission:

• Lay stake = (20 × 5.0) ÷ (5.2 − 0.02) = £19.31
• Lay liability = 19.31 × 4.2 = £81.10
• If back wins: bookmaker pays £100, free bet did not cost real money, exchange loss £81.10. Net: +£18.90
• If lay wins: bookmaker keeps nothing of yours, exchange pays £18.92. Net: +£18.92

Profit of around £18.90 — about 94.5% retention. SR free bets are roughly worth double an SNR equivalent, so always check the terms before placing.

A risk-free bet — sometimes called a refund or money-back special — refunds your stake (usually as an SNR free bet) if your first bet loses. To extract maximum value, you need to lay-off the qualifying portion at low back odds and the prospective free bet at high back odds. This is two separate calculations using formulas 1 and 2 above, applied to two different selections (or the same selection at two different points).

Many matched bettors use the simpler approach: place the qualifying bet, lose it, then convert the resulting SNR free bet using formula 2. Expected value is roughly the same once you account for the probability of winning the qualifier, but the variance pattern differs.

For a deeper dive into squeezing the maximum profit out of these offers, read our guide to free bet conversion methods.

If you do not want to pay for a tool, you can replicate the core lay stake calculator in a free spreadsheet. Set up the following layout in any blank Google Sheet or Excel file:

• A1: Back stake — input cell
• A2: Back odds — input cell
• A3: Lay odds — input cell
• A4: Commission (e.g. 0.02 for Smarkets) — input cell
• A5: Qualifying / SR lay stake — formula =A1*A2/(A3-A4)
• A6: SNR free bet lay stake — formula =A1*(A2-1)/(A3-A4)
• A7: Lay liability — formula =A5*(A3-1) (or use A6 for free bet liability)

That seven-row sheet covers every common matched betting situation. Copy it across tabs for different exchanges (Smarkets at 0.02, Betfair at 0.05, Matchbook at 0.01) so you do not have to retype commission each time.

For your first ten or twenty offers, a spreadsheet with the formulas above is genuinely all you need — you save the £17.99 monthly fee and learn the underlying maths. Once you start working through reload offers, casino bonuses, or accumulator refunds at pace, the limitations show up:

• Spreadsheets do not surface the live odds matchers that show you which selections currently have the lowest back-to-lay spread.
• You have to manually log every bet for tax records and EV tracking — paid tools do this automatically.
• Spreadsheets cannot warn you about a bookmaker mug-betting requirement or alert you to a new offer.

If you reach that point, the comparison between the two main UK paid options is covered in our OddsMonkey vs Profit Accumulator review. For now, a free spreadsheet plus the three formulas above will run every calculation a paid tool runs — just without the automation.

The numbers in this guide assume you are placing real lay bets on a regulated UK exchange. Always double-check your stake on the exchange screen before confirming, and never assume a calculator's output without spot-checking the formula yourself. Bet within your means and remember matched betting is taxable... no, just kidding, profits from genuinely matched bets are not taxed in the UK — but read our matched betting and tax primer if you want chapter and verse.