No. Counting cards in your head is legal: you are only using the cards everyone at the table can see. But casinos are private businesses that can refuse your play, and using a phone or other device to count is a crime in places such as Nevada.
Counting in your head: legal. Being asked to leave for it: allowed in most places, including Nevada. Using a phone, app or other device to count at the table: a felony in Nevada and against casino rules almost everywhere.
Cheating means changing the game: marking cards, adding to a bet after the cards are seen, working with a dealer, or using a hidden device. A card counter does none of that. They play the game as dealt and remember which cards have gone, using information every player at the table can see. No US state makes doing that in your head a crime.
Legal doesn't mean welcome. Blackjack is only profitable for a casino if players don't have an edge, so casinos watch for counters and act when they find one. Common responses, from mild to strict:
Resorts International in Atlantic City barred the card counter Ken Uston in 1979. He sued, and in 1982 the New Jersey Supreme Court held that the casino could not exclude him for counting. The state's Casino Control Act gives the Casino Control Commission, not individual casinos, the power to set the rules of play, and Uston had broken none of them. Atlantic City casinos responded with game changes instead: more decks and earlier shuffles.
The line the law draws is between your head and a machine. Nevada's NRS 465.075 makes it illegal to use, or possess with intent to use, any device in a licensed casino to predict outcomes, keep track of cards played, calculate probabilities or analyze betting and playing strategy, unless the Gaming Commission allows it. Under NRS 465.088 that is a felony. A phone running a counting app is a device.
Counting Edge is a training app. Do not use it, or any other app, at a casino table. Practice with it until you can keep the count in your head.
The picture is broadly the same in most countries: counting in your head is not a crime, and casinos may still refuse your custom or change the game. Rules on devices and on who a casino may exclude vary by country and state, so check the rules wherever you play.
This page is general information. Laws differ between states and countries and change over time. If gambling stops being fun, call the US National Problem Gambling Helpline on 1-800-GAMBLER.
Counting Edge is a training app: drill the count, true count, bets and strategy deviations at home. Free and open source.
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