Online Stopwatch
A stopwatch and a countdown that stay right when the tab is in the background. Most web stopwatches count timer ticks, and a browser stops delivering those to a hidden tab, so they quietly run slow; this one records the moment you pressed Start and subtracts, so the reading is correct the instant you come back. Laps with the fastest and slowest marked, hundredths or whole seconds, a full screen mode you can read from across a room, keyboard control, and a countdown that tells you how long ago it finished if it ran out while you were away.
Stopwatch · readypress Start, or the space barSpace start and stopL lapR resetF full screen
Ready. Press Start, or the space bar.
The shortcuts wake up once you have clicked or tabbed into the tool, and only while the clock is on screen. Until then the space bar pages the article down, the way it does everywhere else.
Accurate in the background, which most web stopwatches are not. A hidden tab is not drawn at all, and its timers are slowed to roughly one a second, so a stopwatch that counts frames or ticks loses whatever it was not given. This one records the moment you pressed Start and subtracts, so the display can freeze while you are away and still be right the instant you come back. The countdown works the same way: if it ran out while you were in another tab, it says how long ago rather than pretending it has just gone off.
Accuracy. Time is measured from timestamps, not counted in ticks, so the reading stays right when the tab is in the background and hundredths are true hundredths. A countdown that ends while you are away reports how long ago it finished. The alarm needs this page to still be open: a closed tab cannot ring, and a browser that has put a hidden tab to sleep may deliver the beep a little late, which is why the message says when the time was actually up. Sound at lap adds a beep to each lap you record; the countdown alarm sounds whatever that switch is set to, as long as the page has been clicked at least once, which is what browsers require before any page may make a sound.
Common questions
- Is an online stopwatch accurate in a background tab?
- This one is. The usual way to build one is to add ten milliseconds to a counter every ten milliseconds, and browsers slow a hidden tab down to roughly one timer a second, so a counting stopwatch loses whatever it was not given: leave the tab for five minutes and it can come back minutes short. Here the only thing stored is the clock reading at the moment you pressed Start, and the display is the difference between that and a fresh reading. Nothing is accumulated, so nothing can be lost. The display does freeze while the tab is hidden, because the browser stops drawing it, and the first frame after you return is already the right number.
- What are the keyboard shortcuts?
- Space starts and stops, L records a lap, R resets to zero, and F switches full screen on and off (Escape also leaves full screen). The keys are listed under the buttons. They are ignored while you are typing in the countdown length box, so you can type 1h 20m without starting the clock.
- How do the laps work?
- Press Lap, or L, while the stopwatch is running and it records a split. The table shows each lap's own time and the total on the stopwatch when it was recorded, most recent first, with the fastest and slowest marked. Ties are marked too, and if every lap is the same length nothing is marked, because there is no fastest. Copy laps puts the table on your clipboard as tab separated text, which pastes into a spreadsheet as columns. Laps belong to the stopwatch; the countdown does not have them.
- Does the countdown alarm work when the tab is hidden?
- Yes, as long as this page is still open somewhere. The alarm is not a countdown that ticks: it is a timer set for the time that is left, and each time it wakes it reads the clock rather than assuming. A browser throttling a hidden tab delays it rather than losing it, usually by well under a second, though a tab that has been hidden for several minutes can be held to about one wake-up a minute, and a phone with the screen locked can hold it longer still. Coming back to the tab fires it immediately if the moment has passed. That is why the page says how long ago the time was actually up instead of pretending the beep was on time. It cannot ring if you close the tab or the browser: nothing here runs outside the page, and there is no notification and no account. A countdown picked back up after a page reload can only flash the tab title until you click the page once, because browsers do not let a page make a sound before it has been touched.
- What can I type in the countdown length box?
- 5:00 for five minutes, 90 for ninety seconds (a bare number is seconds), 1h 20m, or 00:00:30. Words work too: 45 sec, 25 minutes, 2 hours, 1.5h, 2m30s. A unit form can go over sixty, so 90s is a minute and a half. The seconds in a colon form cannot: 1:75 is refused, with the time you probably meant, 02:15, offered instead. Minutes can, as long as you are not also writing hours, so 90:00 is an hour and a half.
- Does it keep running if I reload the page or close the tab?
- A running stopwatch survives a reload. The reading and the moment it was written down are saved in this browser, so when the page comes back it adds the time that passed while it was away and carries on. It is not running while the tab is closed in the sense of doing anything, but the arithmetic still comes out right, which is the part that matters. Your mode, precision, lap sound and countdown length are remembered the same way. Nothing leaves your browser.
- Why does it show hundredths, and are they real?
- They are measured, not decorated. Each frame reads the clock and truncates, so every hundredth shown is one the stopwatch actually reached, and 59.99 seconds never rounds up to a minute that has not happened. What hundredths cannot promise is reaction time: the gap between what you are timing and your thumb on the button is tens of milliseconds, far larger than anything here. Switch the display to Seconds if the extra digits are noise for what you are timing.
Time is measured from timestamps, not counted in ticks, so the reading stays right when the tab is in the background and hundredths are true hundredths. A countdown that ends while you are away reports how long ago it finished.