Why do ATMs only give out $20 bills?

Open the back of an ATM and the answer is sitting right there: locked steel trays called cassettes — each one loaded with a single denomination. Most U.S. operators fill every tray with twenties, because the $20 is the compromise that makes the whole machine work. Load tens and the machine empties twice as fast, which means twice the armored-truck visits. Load fifties and a customer who wants $30 is out of luck. Twenties keep the vault dense, the withdrawal menu flexible, and the refill truck away.

The cassette constraint

An ATM is not a cash register with a brain. The dispenser picks notes off the top of a stack, one tray at a time, and each tray holds only one bill type. Every extra denomination costs a whole tray — space that could hold more twenties. So operators run the trade study: a cassette of twenties holds twice the dollar value of a cassette of tens in the same physical slot, and still serves every request from $20 to $500 in clean steps. That is why your withdrawal options come in multiples of twenty — the menu is drawn by the hardware.

Refills are the real cost

Cash doesn't walk to the machine. Armored carriers restock ATMs on service runs, and every run costs the operator real money whether the machine took in one refill or five. Denser cassettes mean longer intervals between visits. Twenties also circulate hard and hold up well — the Federal Reserve estimates a $20 note lasts about 11 years in service, and roughly 11 billion of them were in circulation in 2024. Plentiful, durable stock means fewer jams and fewer emergency calls.

The exceptions prove the rule

Plenty of modern machines do dispense $5s, $10s, $50s, even $100s — bank-lobby ATMs with four or more cassettes can afford the slots. The limitation was never the technology. It was always an operator's line-item decision: capacity per tray versus flexibility per customer, settled in favor of the twenty almost everywhere the machine stands alone.