The below bit of Javascript is a calculator to help with the wonderful way of making free money known as matched betting. It is based entirely on the information and files gleaned from the moneysavingexpert forums so all credit to the extremely helpful and generously informative netizens who hang out there.

Use at your own risk - please let us know of any errors and omissions. A sample runthrough is below in case the form is not quite as self explanatory as it might be.

Usage example

Horray! The Poorhouse spots a free £10 stake offer at a bookies called Bet-u-like.

1. We are going to stake it all on an event, so fill the "How much stake are you betting with?" box in with "10".
2. The offer says that the stake is to be returned, so we enter 0 in the "stake not returned" box (if it was a stake not returned offer, we would end 10 here to correspond to the £10).
3. Bet-u-like are offering odds of 5/2 on Accrington Stanley winning a match today. Using the converter (or dividing the first number by the second and adding one) we see this is equivalent to decimal 3.5 odds. This is what we enter in the odds on the bet box.
4. As is usual, this bookies does not take commission on their own winnings. We therefore enter 0 in the commission on bet box.
5. Looking around on Betfair, we see that we can lay on Accrington Stanley winning - that is to say we are putting money on them not winning - at odds of 3.7. This is entered onto the decimal lay odds box.
6. The small print in Betfair tells us it takes 5% commission on any winnings. We therefore enter 5 in the commission on the lay box.
7. Looking at the results (press "Calculate" if you can't see them) we can see that:
   • We should lay against a stake of £9.59 at Betfair - that is accept £9.59 of a punter's bet.
   • Our Betfair account needs to have at least £25.89 in case the team wins and we need to pay out in Betfair
   • Either way we will come out of this winning £9.11 of the £10