Definition of hand improvement odds
Let's go back to our colour draw above. Let's say the pot is €10. Our opponent bets €10 in 10. To put it simply, we'll say that we don't have the best hand at this stage and we need to complete our flush to win the hand. Here we know the 3 cards of the flop plus our two cards, so 5 cards out of 52. We know that 4 spades are already out (for our flush draw). This leaves 9 cards out of 47 (the unknown cards) that can help us. So, 1 time out of 5 (47/9 = 5.22 rounded up to 5) the turn will give us our flush, i.e. odds rounded up to 4:1 (1 time we hit, 4 times we don't hit). As our opponent bets 10 in 10, we will lose 4 x 10 euros, i.e. 40 euros, before making 1 x 20 euros, i.e. 20 euros. A total loss of 160 -20 or 140 euros in the long run. This is a bad call according to the hand improvement odds.