05 Screen test
Refresh rate test
How many frames per second your browser actually draws on this screen, how steady they are, and whether motion looks as smooth as it should.
Frame rate
Measuring… keep this tab in front for a few seconds.
- Frame time
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- Average fps
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- Jitter
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- Missed frames
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Moving block
Frame skipping (camera check)
How the measurement works
Browsers redraw a page once per screen refresh and run animation code just before each redraw. The test times those calls: the median gap gives the refresh rate, and gaps of one and a half frames or more are frames the browser missed because the computer was busy. If you see 60 on a 144 Hz monitor, check the refresh rate in your system's display settings, the monitor's own menu and the cable: older HDMI cables and ports often cannot carry high refresh rates at high resolutions.
Browser limits
Safari has long held pages to 60 frames per second on 120 Hz ProMotion screens; its Feature Flags in the Develop menu include "Prefer Page Rendering Updates near 60fps". Battery-saving modes can also cap the frame rate, and with two monitors the browser may follow the slower one.
Moving block and frame skipping
The upper block moves on every frame, the lower one only on every second frame, so you can see what halving the frame rate looks like. The strip lights one cell per frame. Photograph it with a shutter speed of about 1/15 s: an unbroken run of lit cells means every frame reached the screen, and a gap means the monitor dropped one, a common sign of an unstable overclock.
Sources
- MDN Web Docs, Window: requestAnimationFrame() method: callbacks generally match the display refresh rate and pause in background tabs.
Questions
Why is the number not exactly 144?
Real refresh rates are rarely round: 143.9 or 59.94 Hz are normal. The test shows the nearest common rate when it is within 4%.
Does this measure response time or input lag?
No. It measures how often frames are delivered, not how quickly pixels change or how long a click takes to show.
Why do I see missed frames?
Something else needed the computer at that moment: another tab, a background app or the browser's own housekeeping. A few are normal; a steady stream on an idle machine is worth looking into.