Work out your lay stake, the liability the exchange will hold, and what you actually keep. Free, no signup.
Money tied up: £20.30 — £10.00 at the bookmaker plus £10.30 held by the exchange, unavailable until the match settles.
The arithmetic is fixed in advance, which is not the same as risk-free: a mistake in either leg leaves you with an ordinary, uncovered bet. Place the back bet first — the bookmaker price moves more readily than the exchange. 18+ · BeGambleAware.org
Most calculators lead with the profit and mention the liability in passing, or not at all. That's the wrong emphasis, and it's the single most common reason people give up in week two.
A £20 free bet at 5.00 pays you around £16 — but the exchange will hold roughly £65 while the match runs. If you started with £50 of float, you did the first offer fine and then simply couldn't place the second. Nothing went wrong; you were never told the number that mattered.
Qualifying bet.Your stake comes back if it wins, so you're aiming to lose as little as possible — a small negative is normal and correct. That's the cost of unlocking the free bet.
Free bet.The stake isn't returned, so you keep a proportion of the face value whichever way it lands. Higher odds usually retain more, which is why the advice is to use free bets on longer prices — and also why they tie up more liability.
Place the back bet first, then the lay immediately. Bookmaker prices move more readily than exchange lay prices on liquid markets, and holding a lay with no back bet behind it is an open, uncovered position.
And check the selection matches on both sides before you confirm. Backing one team and laying another is the fastest way to lose your stake twice.