Less than its face value, always. A £20 free bet is not £20, because the stake is not returned — win at 6.0 and the bookmaker pays you £100, not £120. The £20 was never yours.
What it is worth depends entirely on what you do with it, and the arithmetic below is the part worth understanding.
Why the two outcomes match
The calculator sets the lay stake so that the money is the same whether the selection wins or loses. That is the whole trick, and it is one line of algebra.
If the free bet is stake-not-returned, backing at odds B with a free bet of F returns F(B − 1) when it wins. Laying the same selection at odds L for a stake of S on an exchange charging commission c costs you S(L − 1) when it wins, and pays S(1 − c) when it loses.
Setting the two outcomes equal:
F(B − 1) − S(L − 1) = S(1 − c)
F(B − 1) = S(L − 1) + S(1 − c)
F(B − 1) = S(L − c)
S = F(B − 1) ÷ (L − c)
That is the formula the calculator uses. The commission sits in the denominator rather than being taken off at the end, because the exchange only charges it on a winning lay.
For a stake-returned bet — a normal qualifying bet with your own money — the bookmaker gives the stake back, so the balance is S = F × B ÷ (L − c) instead, and the result is usually a small loss rather than a profit. That loss is the price of qualifying for the free bet, and it is why the calculator labels it a cost rather than a profit.
What a good result looks like
The proportion of the free bet you keep is (B − 1)(1 − c) ÷ (L − c). Two things drive it, and only two.
The gap between back and lay odds. This is the one that matters. Back at 6.0 and lay at 6.2 and you keep around 78%. Back at 6.0 and lay at 7.0 and you keep about 68%. A tenth of a point at short odds costs more than it looks.
Commission. At 2% the effect is small. At 5% it is not, and on a marginal price it can be the difference between a decent extraction and a poor one.
Higher odds keep a higher proportion of a stake-not-returned free bet, which is why free bets are usually placed on longer-priced selections than qualifying bets. The trade-off is that liquidity thins out as the price lengthens, so the lay odds drift further from the back odds and give back what the longer price gained.
What this calculator does not know
It has no view on whether a particular offer is worth taking. It does not know the terms, the minimum odds, the expiry, whether the free bet is split into pieces, or whether the bookmaker will settle it the way you expect. Those are all in the terms, and the terms are the part people skip.
It also assumes you can get the lay matched at the odds you typed. If the market is thin, the price you actually get is the one that counts.