Stopwatch

Stopwatch - Online Timer with Lap Counter & Split Times

A precise online stopwatch with millisecond accuracy for timing any activity. Features include unlimited lap recording, split times, fastest/slowest lap detection, statistics calculation, fullscreen mode, and keyboard shortcuts. Perfect for workouts, races, cooking, studying, or any activity that requires precise timing.

Can I use the stopwatch in the background while doing other things?

Yes — the elapsed total stays accurate. The stopwatch records a start timestamp from your system clock (Date.now()) when you press Start, and the on-screen total is recomputed from that timestamp on every tick. Modern browsers (Chrome since 2020) throttle background JavaScript timers to roughly once per minute to save battery, so the visual counter may appear to freeze while the tab is hidden, but the moment the tab returns to focus the display self-corrects to the true elapsed time. Note: the running session lives in memory only — reloading the page or closing the tab clears it. If you need the data, record your laps and export them to CSV before navigating away.

How do I record lap times during a session?

Click the Lap button (or press L) at any moment during the run and the current elapsed time is recorded as a lap. The list shows each lap with both its cumulative time (since start) and its split time (since the previous lap), highlighting the fastest and slowest splits in different colors so you can spot consistency at a glance, plus a delta versus the prior split. Laps are stored in a JavaScript array, so there is no practical limit on how many you can capture in one session. When you are done you can copy the laps to the clipboard as text or download them as a CSV file for spreadsheet analysis.

How accurate is the browser stopwatch — can I time milliseconds?

This stopwatch measures elapsed time from a system-clock start timestamp (Date.now()) and displays it to centisecond (10 ms) resolution — the format shown is MM:SS.CC, or HH:MM:SS.CC past one hour. For human-triggered timing (sprint splits, cooking stages, presentation segments, QA cycle steps) this is more than sufficient, because the real limiting factor is your own reaction time on the button, typically 150-300 ms — far larger than the display resolution. Budget for that error by always starting and stopping at the same reference cue, and by comparing relative splits within one session rather than treating any single absolute time as exact. For sub-millisecond scientific reaction-time work you would need dedicated lab hardware.

How do I read split times versus cumulative times for pacing analysis?

Each lap row shows two numbers that answer different questions. The split time is how long that single interval took (since the previous lap) — use it to compare effort lap-to-lap and to spot the fastest and slowest segments. The cumulative time is the running total since you started — use it to check overall pace against a target finish. For pacing analysis, watch the split column: a steadily rising split means progressive slowdown (fatigue), a falling split means a negative split (finishing strong), and a flat split column is the goal for even-pace events. The Consistency (Std Dev) stat puts a single number on how tight that spread is — lower is more even.

What statistics does the tool calculate, and what is the Consistency stat?

Once you have recorded laps, the tool shows Total Laps, Fastest Lap, Slowest Lap, Average Lap, and Consistency (Std Dev). Consistency is the standard deviation of your split times: it measures how far each split typically deviates from the average, so a small value means very even pacing and a large value means uneven splits. It needs at least two laps to be meaningful and shows a dash for a single lap. Athletic coaches use it to track pacing discipline across repeats, and QA or lab users use it to flag variance in repeated cycle measurements. The difference between Slowest and Fastest gives the raw spread; the Std Dev gives the typical spread.

Can I export my lap times to a spreadsheet?

Yes. Click Download CSV to save all recorded laps as a .csv file that opens directly in Excel, Google Sheets, or Numbers. The file has five columns: Lap, Split Time, Cumulative Time, Split (ms), and Cumulative (ms). The two raw-millisecond columns are included specifically so you can chart trends, compute your own averages or variance, and compare sessions without re-parsing the formatted MM:SS.CC strings. The file is generated entirely in your browser (a Blob download) with a UTF-8 byte-order mark so Excel reads the encoding correctly. You can also use Copy All to put a formatted text summary on the clipboard for quick pasting into chat or notes.

Why does my stopwatch sometimes jump when I unlock my screen?

On laptops and phones the OS suspends most processes when the screen is locked or the lid is closed, so browser JavaScript timers stop firing and the visual display freezes. The moment the tab regains focus, this stopwatch reads the current system clock and recomputes the true elapsed time from the original start timestamp, so the display catches up in a single jump. The underlying total is correct — the discontinuity is purely visual. Because timing is based on Date.now(), if the system clock was corrected by NTP during a long suspend, the catch-up reflects the adjusted wall-clock time. For uninterrupted measurement, keep the device awake and the tab in the foreground.

When should I use a stopwatch instead of a countdown timer?

Use a stopwatch when you are measuring how long something takes and do not know the duration in advance — open-ended activities like a training run, a QA cycle, a debate segment, or a lab trial — especially when you need multiple splits to compare. Use a countdown timer when you know the duration and want an alert at the end, like a 25-minute Pomodoro, a 3-minute steep, or a fixed presentation slot. They are complementary: time a workout's work intervals with a countdown timer and measure the actual rest or rep durations with this stopwatch. Rule of thumb — need an alarm at a fixed time, use a timer; need to measure and compare intervals, use the stopwatch.

Key Features

• Precise timing to 10 milliseconds
• Unlimited lap recording
• Real-time lap statistics
• Fastest/slowest lap detection
• Lap time delta calculation
• Average lap time
• Fullscreen mode for better visibility
• Large display in fullscreen (8x size)
• Copy all results to clipboard
• Keyboard shortcuts (Space, L, R, F, ESC)
• Clean, distraction-free interface
• Large, easy-to-read display
• Responsive mobile design
• Dark mode support
• Works offline
• No ads or popups
• No registration required
• 100% free
• Privacy-friendly
• No data collection