Look at the circle first, with the slider at zero.
It should already look perfectly round. If it looks like an oval right away, your display is stretching something.
Go fullscreen.
Overscan and scaling issues are often invisible in a small window and only show up filling the whole screen.
If it looks stretched, use the slider to cancel it out.
Drag until the circle looks genuinely round to your eye — the percentage shown is your rough measured distortion.
Check whether it's one app or system-wide.
Open this same page in a different browser, or compare against your desktop wallpaper. If only one thing looks stretched, that thing has its own scaling setting.
Check for overscan if the whole system looks stretched.
This is a very common, fixable cause, especially over HDMI.
How this test actually works
The circle, grid, and crosshair are drawn on a canvas using your actual pixel dimensions, so at zero correction they're mathematically exact — a true circle, true squares, nothing pre-distorted to compensate for anything. If your display's pipeline is genuinely undistorted, that's exactly what you should see without needing to touch anything.
The slider works by deliberately stretching that same pattern by a known percentage in one direction. If your actual display is already stretching the image the opposite way, dragging the slider to cancel that out will make the circle look round again — and the percentage where that happens is a rough, honest estimate of how much distortion your setup is currently introducing. It won't match a lab measurement to a decimal point, but it turns a vague "this looks off" into an actual number you can compare before and after changing a setting.
What causes a stretched image
By far the most common cause on a modern monitor connected over HDMI is overscan — a setting left over from CRT television technology that zooms the image in slightly and crops a few percent off every edge. It can switch itself on without you doing anything, simply as a result of how some monitors and TVs default their HDMI input, and the resulting crop often reads as horizontal or vertical stretching in a test like this one.
Beyond overscan, a resolution that isn't your display's true native resolution forces scaling somewhere in the pipeline, which can distort proportions if that scaling isn't done carefully. An incorrect scaling mode — Stretch instead of Aspect or 1:1, for instance — does this deliberately. And on a monitor that's been rotated to portrait orientation, the operating system or a specific app can sometimes keep assuming the old landscape dimensions until the settings are updated to match.
Fixing overscan, by graphics card
NVIDIA
Open the NVIDIA Control Panel → Display → Adjust desktop size and position, and select No Scaling. This is usually the single fix that resolves HDMI overscan on an NVIDIA setup. If the option is missing entirely, grab the latest driver from NVIDIA's official download page first.
AMD
Open Radeon Settings → Display, enable GPU Scaling, and set the Scaling Mode to Preserve aspect ratio. AMD's support site has the current Radeon driver if this section isn't showing up at all.
The monitor or TV itself
If the graphics-card setting doesn't fully resolve it, check the display's own on-screen menu for an option usually named Image Size, Scan, or Just Scan, and set it to show the image at its full, uncropped size.
Common aspect ratios
Ratio
Decimal
Typical resolutions
4:3
1.33:1
Legacy standard for CRTs and old TV broadcasts
16:10
1.6:1
Common on some laptop screens
16:9
1.78:1
1920×1080, 2560×1440, 3840×2160 — the dominant modern standard
21:9
2.33:1
2560×1080, 3440×1440 — standard ultrawide
32:9
3.56:1
5120×1440, 3840×1080 — super ultrawide, equivalent to two 16:9 screens side by side
Is it my monitor or just one app?
This distinction changes what you should actually go fix, so it's worth checking directly. Compare this page's circle against your desktop wallpaper, a photo viewer, or a second browser — if the stretching shows up consistently everywhere, the cause is system-wide, and the resolution, scaling mode, or overscan settings above are the right place to look. If only one specific app or game looks distorted while everything else on the same screen looks fine, that app almost certainly has its own separate aspect ratio or stretch setting, usually in its own display or video options, that's set independently of your operating system. If you're setting up a new monitor generally, it's worth running through our Refresh Rate Test and Dead Pixel Test too, while everything's still fresh in your mind.
Frequently asked questions
What does it mean if the circle looks like an oval?
It means something in the display pipeline is stretching the image horizontally or vertically — commonly overscan, a non-native resolution, or an incorrect scaling mode.
What is overscan and why does it cause this?
Overscan is a leftover setting from old CRT TVs that zooms the image in slightly, cropping a few percent off every edge. It can switch on by itself over HDMI, and the cropping often reads as stretching.
How do I turn off overscan?
NVIDIA: Control Panel → Display → Adjust desktop size and position → No Scaling. AMD: Radeon Settings → Display → GPU Scaling → Preserve aspect ratio. On the monitor/TV itself, look for Image Size, Scan, or Just Scan.
How do I know if it's my monitor or just one app?
Compare this page against your desktop background or another app. If the distortion shows up everywhere, it's system-wide. If only one app looks wrong, that app has its own setting to fix.
My vertical or portrait monitor looks squashed — is that the same issue?
Usually yes. Rotating to portrait sometimes leaves the OS or an app still assuming landscape dimensions, producing this same kind of stretch until settings are corrected.
Does this tool record or upload anything?
No. Everything is drawn locally on a canvas element in your browser — nothing is captured, recorded, or sent anywhere.
More tools on ProDeviceTest
Geometry is one piece of overall display quality. These cover the rest.
ProDeviceTest