The ability to count cards needs one further step to make it useful to a card counter.
So far we know how to work out the running count by using the Hi-Lo method of counting. This tells us if the deck is rich in 10's and A's, or is depleted of them. We need to take this one step further by looking at how many decks are left, and determining the advantage/disadvantage per deck left to be played. This is called the True Count.

True Count = Running Count/No. of Decks Left.

True count gives you the percentage advantage you have over the casino. At a true count of 0, the casino has a 0.5% advantage. For every additional +1, the advantage swings by 0.5% towards the player. So a true count of +5 means the player has a 2% advantage.

True Count vs Running Count
The running count merely tells you how many more (or less) high cards there are than low cards. If the running count is +6, this means there are 6 more high cards than there are low cards left in the deck. Of course, if there are only 6 cards left to play, we can confidently say that there will all be high cards. But if we have six decks left, then that knowledge is greatly dispersed. That is why the True Count is needed. It tells us the proportion of high cards per deck and we can use this to make strategic decisions at the blackjack table.

A true count of +1 or lower means the casino has the advantage. A true count higher than +1 means that we, the players have the advantage. True count is the basis of any card counting strategy. Knowing when you have the advantage and how to use it, is how we make money as card counters.