looking for a formula to calculate betting size

i dont get it
can you show an example?

So lets say your losses are $150 so far (X=150)

You get a sure tip on a dishlicker at 5/2 (Y=5, Z=2)

So you need to win $150 to break even. That means you need to win $150 at 5/2. Correct?

If you put $60 on at 5/2 you win 2 and a half times your stake. Correct?

So 2 and a half times $60 is $150 (your losses), so you have broke even.

How did i get that $60? Well, $150 divided by 5 (=$30) multiplied by 2 =$60. ie. X/Y*Z.

Disclaimer: Bet responsibly. Don't chase your losses. Isn't that what they say on the ads?

thank you i got it now

Not quite sure what you mean here but I'll try to work something out...my initial formula didn't work out when I wrote a spreadsheet to test it.

Give me a mo...and I'll get back to you.

OK I've attached a worksheet that will tell you how long your initial bankroll will last given odds, your bet size, and your expected win/loss ratio

I find a spreadsheet much easier to solve these types of problem than a 'formula' and this is an Excel forum after all, not a maths forum (note the cop out justification )

(If I get keen, I'll try to work out the actual formula).

Put your inputs in the green cells.

OK. I've had a think about this and come up with the following;

Your expected outcome for each bet = Probability of winning X Amount you win - Probability of Losing X Amount you lose

So the amount of bets you can expect to make with your initial bankroll is your bankroll divided by that number above (multiplied by -1 because your expected outcome will be less than zero if you are being realistic)

If your expected outcome is positive, your money will last forever of course.

Another consideration is that your wins will not be nice and uniform over all of your bets, so the ACTUAL number of bets you get to make will depend on when your wins occur, as demonstrated by the worksheet I put up.

Hope that explains it in a maths sense?

Note: If you randomize your wins (as expected in the real world) by putting this formula in E2 in the worksheet I posted, every time you recalculate (F9), you will get vastly different results.

=IF(RANDBETWEEN(1,$H$2)=$H$2,"W","")

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OK...I THINk this is what you mean.
I made a change to your initial bet to give you your target profit, and made the following calculation for your next bet, depending on whether you won or lost on your last bet.
This will cover your previous losses AND give you your target profit if you win.

=IF(F2>0,MIN($D$2,B3),MIN(B3,$D$2-MIN(G2,F2)/C3))

Change the profit from whatever I put in the attachment to test it, to 2.40 then!

Of course the worksheet I gave you works on a (more realistic) random win ratio.

If you want a uniform win every so many bets, then use this formula in E2 and drag down

=IF((ROW()-1)/$I$2=INT((ROW()-1)/$I$2),"W","")

Then just increase your win loss ratio until your bankroll is "Gone with the Wind".

The answer in that scenario for your given data is 15 bets.

If that doesn't give you what you want, then maybe you should ask Margaret Mitchell.