Mostly no. Standard online blackjack reshuffles before every hand, so there is nothing to count. Live dealer games deal from a real shoe, but early shuffles and close bet tracking leave a counter little or no edge. The internet is better for practicing than for playing.
RNG blackjack: nothing to count
Most online blackjack is dealt by a random number generator. Each hand comes from a full, freshly shuffled virtual shoe, so the cards from the last hand tell you nothing about the next one. The count starts at zero every hand and never builds up, however well you keep it. No counting system can beat a game that reshuffles before every hand.
Live dealer blackjack: possible, rarely worth it
Live dealer tables stream a real dealer and a real shoe, so in principle you can count them. In practice several things get in the way:
Shallow penetration. Shoes are often cut around halfway. A counter's edge comes from the high true counts that build up deep in the shoe, and a shoe cut early rarely gets there.
Continuous shufflers. Some tables put cards back into a shuffling machine every round, which works like an RNG game: nothing to count.
Every bet is recorded. An online casino sees your entire betting history. Raising your bet only when the count is high stands out quickly, and the casino can cap your bets or close your account.
Terms of service. Most online casinos ban software or apps that help you play. Breaking that rule can cost you your account.
The maths
Each point of true count is worth roughly 0.5% to a Hi-Lo counter, and a good game starts around 0.5% in the house's favour. You need a true count of about +1 just to break even, and the profit comes from counts of +2 and higher, which mostly appear in the last third of a deep shoe. See the true count calculator for your own rules.
Where counting does work
Counting pays in land-based shoe and pitch games that are dealt deep, with good rules: blackjack paying 3:2, the dealer standing on soft 17, and doubling after splits. Casinos know this, which is why they watch for counters and why the law and casino rules matter before you play.
Practice online, play in person
Where the internet does help is practice. You can learn the count, speed and strategy for free in your browser:
Not in standard online blackjack: a random number generator deals every hand from a freshly shuffled shoe, so there is nothing to count. Live dealer blackjack deals from a real shoe, but it is usually cut early or shuffled continuously, which leaves little or no edge.
Can online casinos tell if you are counting cards?
They record every bet you make. A pattern of small bets early in the shoe and big bets when the count is high is easy to spot, and online casinos can limit your bets or close your account.
Is it illegal to count cards online?
Counting in your head isn't illegal. Using software to help you count usually breaks an online casino's terms, which can cost you your account.
Where can I practice card counting online for free?
Right here. The card counting practice trainer runs tag flash cards, timed deck countdowns and true count drills in your browser, and the basic strategy trainer drills every hand on the chart.
Practice on Android
Practice at Home, Not at the Table
Counting Edge is a training app: drill the count, true count, bets and strategy deviations at home. Free and open source.