How the refresh rate test works
Browsers draw a page in step with the display: each time the screen is about to refresh, the browser runs any animation code that has asked for a frame through requestAnimationFrame and passes it a precise timestamp. The gap between two timestamps is one refresh interval — 16.67 ms at 60 Hz, 6.94 ms at 144 Hz, 4.17 ms at 240 Hz. This page records every interval and works from the last three seconds of data.
Individual intervals are noisy: operating systems deliver frames slightly early or late, and some browsers round their timers to protect privacy. So rather than averaging, the test finds the statistical mode — the frame time that occurs most often — and refines it from the frames clustered around it. It then counts how many refreshes each interval really spans (a gap of about two periods is one frame drawn and one missed) and divides total refreshes by total time. That gives a stable figure such as 143.98 Hz even when a few frames were dropped, which is then matched to the nearest standard rate on the ladder.
The live figure uses only the last second, so it reacts straight away when you drag the window to another monitor, and the main figure follows within three seconds.
Reading the results of a 144 Hz or 240 Hz test
| Reading | Meaning | What good looks like |
|---|---|---|
| Hz | Refresh rate the browser is being given, snapped to the nearest standard value. | Matches what you paid for: 144, 165, 240… |
| Avg frame time | Mean interval between frames in the window. | 1000 ÷ Hz, e.g. 6.94 ms at 144 Hz. |
| Jitter ± | Standard deviation of normal (non-dropped) frame times. | Under 0.5 ms. Higher means uneven frame pacing. |
| Dropped frames | Refreshes where the browser didn't produce a new frame. | Zero or a handful per minute. |
| 1% low | Frame rate of the slowest 1% of frames. | Close to the refresh rate. |
On the histogram a single tall bar in the refresh period is perfect. A second bar at twice the period means regular dropped frames; a wide smear means jitter. On the trace, red dots are individual dropped frames — a burst of them when you open another app or scroll is normal, a steady rhythm of them is not.
In the motion test, all three saucers travel at the same speed in pixels per second. The top one moves a small step every refresh; the others move two and four times as far every second and fourth refresh, which is what a display at half or a quarter of the rate would show. On a 144 Hz screen the top saucer should look clearly smoother and sharper. If the top lane stutters, frames are being dropped. Switch the background to dark, grey or light to look for ghosting trails behind the saucer or bright overshoot halos, which depend on the panel's response time and overdrive setting.
Checking for frame skipping with a camera uses the same method as TestUFO. The row of squares lights one square per frame, in order, and there are always more squares than frames in a tenth of a second, so a photo never catches the pattern wrapping round. Wait for the badge to say READY, set your camera or phone to a shutter speed of 1/10 to 1/30 s, and photograph the squares. A healthy display shows a run of several consecutive lit squares with no gaps. A gap in the run means the monitor received a frame and never showed it — typical of an overclocked or mis-timed display mode. A square brighter than its neighbours means the browser itself repeated a frame, which also shows up in the dropped-frames counter.
Why am I not getting my monitor's full refresh rate?
- Windows or macOS is set to a lower rate. On Windows go to Settings → System → Display → Advanced display and choose the highest refresh rate. On a Mac open System Settings → Displays and pick it from the Refresh rate menu. Monitors often ship at 60 Hz until you change this.
- The cable or port can't carry it. 1440p at 144 Hz needs DisplayPort 1.2 or HDMI 2.0; 4K at 144 Hz needs DisplayPort 1.4 with DSC or HDMI 2.1. Old HDMI cables and docks are the usual culprits.
- Power saving is capping frames. Battery saver on Windows, Low Power Mode on iPhone and Mac, and Chrome's Energy Saver can hold pages at 30 or 60 fps. A result of exactly 30 Hz almost always means this.
- The browser is limiting itself. Safari may hold pages at 60 fps on ProMotion displays unless “Prefer Page Rendering Updates near 60fps” is turned off in its feature flags. With two monitors at different rates, some browsers on Windows sync to the primary display — move the window and watch the live figure change.
- Hardware acceleration is off. Without GPU compositing the browser may not keep up. Check it in your browser settings.
A browser can never report more frames than the display shows, so a correct result here confirms the whole chain: settings, cable and panel. To check the panel itself for faults, try the dead pixel test.
G-Sync, FreeSync and variable refresh rate
With variable refresh rate (VRR) — G-Sync, FreeSync, HDMI VRR or Apple's adaptive ProMotion — the display waits for each new frame instead of refreshing on a fixed clock. A browser in a normal window almost always runs flat out at the display's maximum rate, so a VRR screen reads as its top refresh rate here: a 48–165 Hz FreeSync monitor shows 165 Hz. Phones and iPad Pro drop their refresh rate when nothing is moving to save battery; because this page animates every frame, they should run at full speed while it is open, though some phones stay at 60 Hz in the browser whatever the setting. If frame times float instead of locking to one period — for example a game-style load on a VRR monitor, or a phone switching rates — the test says variable refresh detected, shows the average rate and stops counting dropped frames, because a longer frame there is the display waiting, not a miss. Also make sure the browser window is on the monitor you intend to measure, and that the result is marked steady before you trust it. If you are also checking input devices for a gaming set-up, the mouse polling rate checker measures the other half of smoothness.
Frequently asked questions
What Hz is my monitor?
Open this page, keep the tab in front and wait a few seconds: the test measures the rate your screen is actually refreshing at and matches it to the nearest standard figure, such as 60, 75, 120, 144, 165, 240 or 360 Hz. That is the rate you are running now, which can be lower than the maximum your monitor supports. To see the maximum, check the monitor's model number on the manufacturer's site, or look in Windows under Settings › System › Display › Advanced display, or on a Mac under System Settings › Displays.
How accurate is an online refresh rate test?
Very accurate for the rate the browser is actually receiving. Browsers draw in step with the display and time-stamp each frame, and the test averages hundreds of frames, so a 144 Hz monitor typically reads 143.9 to 144.1 Hz. It measures what reaches the browser, so if power saving or a browser setting caps frames, you will see the capped figure — which is usually what you want to know.
Why does the test say 60 Hz when my monitor is 144 Hz?
Most often the operating system is still set to 60 Hz — change it in the display settings. Next check the cable and port: an old HDMI cable or a USB-C dock may not carry 144 Hz at your resolution. Battery saver modes and Safari's 60 fps preference can also cap the browser. Press Reset after each change to measure again.
What are dropped frames?
A dropped frame is a refresh where the browser didn't finish a new frame in time, so the display shows the previous one again. The test spots them as intervals roughly two or more periods long. A few when you switch apps are harmless; a steady stream points to a busy CPU or GPU, background video, or a driver problem, and it makes motion look juddery.
Can a browser show more than 60 fps?
Yes. Chrome, Edge, Firefox and Opera draw at the display's full refresh rate — 120, 144, 240 or even 360 Hz. Safari does too on most Macs, but may hold pages at 60 fps on ProMotion displays unless its feature flag is changed. A browser can never go above the display's rate, because it only draws one frame per refresh.
Does the refresh rate test work on phones and tablets?
Yes. Phones with 90, 120 or 144 Hz screens show their rate as long as the browser allows it; some Android phones limit browsers to 60 Hz in their smoothness settings, and iPhones in Low Power Mode drop to 60 Hz. Keep the screen awake and the tab in front while it measures.
What does the 59.94 Hz reading mean?
Many displays and graphics drivers run “60 Hz” at 59.94 Hz, a timing inherited from broadcast video standards. It is completely normal, and the difference is too small to see. The test recognises it and reports 60 Hz, while still showing the exact measured value.
How do I test my monitor for frame skipping?
Wait for the READY badge under the motion test, then photograph the row of squares with a shutter speed between 1/10 and 1/30 of a second. You should see several consecutive lit squares with no gaps. A missing square means the monitor is skipping frames, which often happens with overclocked refresh rates. Try again at a standard rate to confirm.