Stopwatch
Online stopwatch with 1/100-second precision: start, pause, reset and lap recording, showing per-lap and cumulative times at a glance. Great for sports training, cooking and productivity timing.
How to use
- Click "Start" to begin: the big digits tick in real time with 1/100-second precision. Click the same button again to "Pause".
- While running, click "Lap" any time to record the current split. The lap list shows both the per-lap time and the cumulative time.
- "Reset" stops the watch and clears the time and all laps. You can also reset while paused.
- The browser tab title shows the running time, and accuracy is based on timestamp differences, so switching tabs never makes the watch drift.
FAQ
How accurate is it? Does it slow down over time?
What is the difference between Lap and Reset?
How many laps can I record?
Is my timing data uploaded anywhere?
How to use
- Start the watch
Click Start and the large digits tick in real time with 1/100-second precision. Clicking the same button again pauses, and the status line follows along.
- Record laps on the go
While running, click Lap at any split point. Each click adds a row showing both the per-lap time and the cumulative total up to that moment.
- Lap versus reset
Lap never interrupts timing, which suits per-kilometer pacing. Reset stops the watch and clears the total time along with every recorded lap.
- Read the lap list
Laps are unlimited and listed in order, each row pairing the split with the running total. The fastest segment is easy to spot afterwards.
- Track from the tab title
The browser tab title mirrors the running time, so you can watch the clock from another tab. All data stays local and vanishes when the page closes.
About this tool
The stopwatch derives elapsed time from system timestamp differences and uses periodic refreshes only for display. Background throttling or a frozen tab never causes the classic setInterval drift, and timing stays accurate to 1/100 of a second. The lap feature splits the run at any moment, recording per-lap and cumulative figures side by side for every segment you mark.
Use it for running and gym intervals, cooking, speech rehearsals or timing slow processes. Press Lap at each kilometer of a 5K and the finished list shows exactly where you slowed. Unlimited laps handle multi-set workouts just as well. Everything is computed locally in the browser with no network requests. Closing the page wipes both the time and the lap history.
How it works
A stopwatch's accuracy has always been bounded by mechanics and by the human finger. Mechanical watches leaned on a balance wheel, and pressing start and stop adds roughly 0.2 seconds of reaction error at each end. Athletics rules therefore convert hand times by adding 0.24 seconds, and since 1977 official records must come from fully automatic electronic timing. This tool aligns its 1/100-second display with that electronic tradition, but the finger remains in the loop. Reaction adds about 0.15-0.25 seconds at each press, so for sub-second intervals the start-stop error can exceed the interval itself. Several lap readings give a cross-check that a single reading cannot.
The lap feature is a prefix-sum ledger. Every row carries the per-lap split beside the cumulative total, and any lap's split equals the difference between adjacent cumulative entries. One missed lap breaks the reconciliation instantly. That two-column layout is the standard exchange format of professional timing as well. Pace tables in running apps and intermediate checkpoints at races export exactly this pair. Reaction-time experiments and machine-cycle studies fit the same logic. Record laps indiscriminately first, then recover each segment by subtraction afterwards, which beats segmenting from memory at the scene.
When to use it
- Post-run pace review
Tap Lap each kilometer, then read the split column. The lap that suddenly jumps ten seconds maps to the climb or headwind you remember. The cumulative column cross-checks the total time better than a phone's health estimates.
- Interval training logs
For HIIT rounds of 40 seconds hard and 30 easy, hit Lap on every transition. That captures each round as it actually ran. Paste the copied results into the training log and compare the same rows weeks later, not the feeling.
- Experiments and process steps
A 6-minute-30 egg, a reaction from dosing to color change: combine Pause and Lap to mark every stage. Hundredth-second precision is redundant for most flows and exactly right for locating which step is slow.
Tips & tricks
- Reset also works while paused. Pause before grabbing a screenshot of a result you want to keep.
- Each lap row shows split and cumulative times, and subtracting them verifies that no lap was missed.
- For repeated intervals, tap Lap at the start of every round so the list is easier to review later.
Things to watch
- Your finger is the bottleneck
Each press carries about 0.15-0.25 seconds of reaction error. Timing a sub-second event mostly measures the finger. Reaction-time work needs light-and-sound rigs. Keep this stopwatch for spans of a second and up.
- Nothing is saved: copy first
The running time and lap list live only in page memory. A refresh or an accidental reset wipes them. Copy the result every few laps during long sessions. Reset only when the record is safely archived somewhere else.
- One button, two outcomes
Start and Pause share the same button, and a careless double tap stops the clock mid-run. Put the page where a stray touch cannot reach it. The Lap key never interrupts timing, so let it do the recording.