Tally counter
A big, easy click counter: count with a tap, with your keyboard, or create multiple named counters. Everything stays saved in your browser.
Counter 1
Shortcuts: Space or ↑ for +1, ↓ for −1
What a tally counter is good for
It's the digital version of the handheld clicker ushers and doormen use: perfect for counting people entering an event to stay within capacity, taking warehouse inventory, keeping score in a card game, tracking laps around a running track, or counting stitches and rows while knitting. You can create a separate counter for each thing you're tracking (say, "Adults" and "Kids") and update them side by side: just tap a counter's chip to make it the active one.
Keyboard shortcuts and automatic saving
When you need to count fast without looking at the screen, use your keyboard: the spacebar or the up arrow adds 1, and the down arrow subtracts 1. Every change is saved instantly to your browser's local storage (localStorage), so you can close the page and find all your values right where you left them next time, even with no internet connection.
Counting in your head fails past a few dozen
The limit is not memory, it is interruption: anyone counting people at a door loses the number at the first question, the first phone call, the first greeting. With a counter the number lives outside your head, and picking up again costs nothing.
The other gain is that you can count without looking: the space bar or one large tap on the screen, while your eyes stay on what is being counted. It is exactly the gesture of the mechanical hand tally counter, used at museums and doors for sixty years for this very reason.
Separate counters for different things
Where mental counting collapses is when there is more than one category: adults and children, cars and vans, good items and faulty ones. One counter per category turns an impossible task into three different gestures.
It is the method used for stocktaking and quality control: you count by type while you look, rather than taking notes and reclassifying afterwards. Reclassifying afterwards means counting again, and counting again at the end of the day is when errors get in.
A count has to be kept, not just made
A number counted and not recorded vanishes the first time the page closes, which is the classic frustration of somebody who has been counting for two hours. A count saved in the browser lets you close it, change room and pick up where you were.
It is also worth resetting deliberately: a counter left full from the previous shift is worse than an empty one, because the number looks plausible. Resetting at the start of a count is the habit that avoids most of the errors.
Nearby tools
To count time rather than objects there are Online stopwatch and Online timer. If the count concerns a text, the way through is Word counter.