A page sees the signal the computer sends, not the panel. The patterns below show the rest.
One-pixel vertical lines, black and white
One-pixel vertical lines, black and white
Preview · enlarged so the lines show
Black and white columns, each one device pixel wide: 1,920 white lines across a 3840 wide screen. Seen close at native resolution, every line is separate and the field is even. Wide bands, ripples or soft grey patches mean something is rescaling the picture.
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Click, tap or press the right arrow for the next one. The left arrow goes back.
One-pixel vertical lines, black and whiteBlack and white columns, each one device pixel wide: 1,920 white lines across a 3840 wide screen. Seen close at native resolution, every line is separate and the field is even. Wide bands, ripples or soft grey patches mean something is rescaling the picture.
One-pixel horizontal lines, black and whiteThe same grid turned on its side: 1,080 white rows down a 2160 high screen. Rescaling does not always treat width and height alike, so a picture can be clean in one direction and soft in the other. Check both before you trust either.
Single-pixel checkerboardEvery pixel differs from the four around it, the finest detail a screen can be asked to draw. At native resolution it reads as even mid grey from arm's length and a fine mesh up close. Crosshatch, blotches or crawling bands mean pixels are being blended.
Show all 31 patterns and checks (28 more)
One-pixel patterns, continued
Line pairs at 1, 2, 3 and 4 pixelsFour blocks of stripes, from one pixel wide up to four. Find the finest block where the lines are still separate and even. On a true 4K picture that is the first. If only the two-pixel block is clean, the picture is arriving at half resolution.
Device pixel lines beside CSS pixel linesTwo grids side by side. The left is drawn in device pixels, the right in CSS pixels, the unit web pages are laid out in. At 100 percent scaling they match. At 150 percent each CSS line covers 1.5 device pixels, so the right grid turns soft or uneven.
Red and blue one-pixel linesRed and blue columns one pixel wide, beside a black and white block for comparison. On a full-colour link both blocks are equally sharp up close. A link using 4:2:0 colour keeps one colour sample for every four pixels, so the coloured block smears while the other stays crisp.
Dot-matrix text at 1, 2 and 3 pixels a dotLetters built on a 5 by 7 dot grid and drawn with no smoothing, using dots of 1, 2 and 3 device pixels. The smallest line is 7 pixels tall, about 1.1 mm on a 27 inch 4K monitor. Clean strokes mean native resolution. Thickened, uneven strokes mean rescaling.
Browser text from 8 to 16 CSS pixelsOrdinary text set by the browser at 7 sizes, with its usual smoothing. This is the everyday view: how sharp small type looks at your current scaling. Faint red and blue edges on the letters come from subpixel rendering, a text technique, not from a fault.
One-pixel border on all four edgesIn full screen, a white line one pixel wide runs round the outermost pixels, with a mark in each corner. All four sides should show. A missing side means the screen is cropping and enlarging the picture, which rules out one-to-one pixels everywhere else too.
Pixel ruler with a tick every 100 pixelsRulers along the top and left edges count device pixels. In full screen on a 4K picture the last numbered tick across reads 3,800 with 40 pixels beyond it, and the last one down reads 2,100 with 60 beyond. Lower numbers mean fewer real pixels are in use.
What the numbers mean on a 4K screen 8
3840 by 2160 at 100 percent scalingThe browser reports 3840 by 2160 with a pixel ratio of 1, so one CSS pixel is one screen pixel. That is 8,294,400 pixels, exactly 4 times Full HD. Nothing is scaled, which suits these patterns and makes text very small on a desk monitor.
4K at 125 percent: the browser sees 3072 by 1728A pixel ratio of 1.25. The operating system draws everything a quarter larger, so the browser is told the screen is 3072 by 1728. Multiply back by 1.25 and you get 3840 by 2160. All the pixels are in use. Only the layout units changed.
4K at 150 percent: the browser sees 2560 by 1440A pixel ratio of 1.5. The browser reports 2560 by 1440, which can make a 4K monitor look like a 1440p one in a simple resolution readout. Multiply by 1.5 and the full 3840 by 2160 comes back. Every pixel is in use, with text drawn half as large again.
4K at 175 percent: about 2194 by 1234A pixel ratio of 1.75 does not divide 3840 evenly: the exact answer is 2194.29. Browsers report whole numbers, so multiplying back gives 3839.5, not 3840. Real pixels within 4 of 3840 by 2160 are still a 4K picture.
4K at 200 percent: the browser sees 1920 by 1080A pixel ratio of 2. Each CSS pixel is a block of four screen pixels, a whole-number fit, so page layout lands exactly on the pixel grid. The reported size matches Full HD, but the ratio of 2 shows that all 8,294,400 pixels are in use.
1920 by 1080 with a pixel ratio of 1If this is a 4K screen, the computer is sending a Full HD picture and the monitor is enlarging it. The page sees the signal, never the panel. Four panel pixels fit each signal pixel exactly, yet most monitors smooth lower resolutions, so one-pixel lines look soft.
DCI 4K, 4096 by 2160The cinema form of 4K, set by Digital Cinema Initiatives. It is 256 pixels wider than the 3840 by 2160 of monitors and TVs, 8,847,360 pixels in all, with an aspect ratio near 1.90 to 1. Its main use is digital cinema: projectors and production monitors.
More than 3840 by 2160 at a pixel ratio of 2Real pixels such as 5120 by 2880 on a 4K screen usually mean the computer draws a larger picture and shrinks it. macOS scales by a fixed factor of 2 and makes its in-between sizes this way. The numbers describe that drawing, not the signal. Expect soft one-pixel patterns.
What you see and what it means 7
Every line separate and the field evenWith the numbers reading 3840 by 2160 once the pixel ratio is applied, and the one-pixel grids clean in both directions, the picture is reaching the panel pixel for pixel. Finish with the border and the red and blue lines to check the edges and the colour.
Wide bands or ripples across the linesA moiré pattern: two grids of slightly different pitch laid over each other, here the drawn lines and the panel's own pixels. It means the picture is being resized by an uneven amount somewhere: in the browser zoom, the operating system's resolution setting or the screen itself.
Smooth grey with no lines, even up closeEither the pixels are being blended by a smoothing scaler, or they are simply too small for your eyes. On a 27 inch 4K monitor a pixel is about 0.16 mm wide. Use a magnifying glass or a phone camera held close before you decide which it is.
Only the two-pixel block is cleanThe picture has half the detail of the panel in each direction. The usual cause is a Full HD signal on a 4K screen, which the numbers at the top confirm as 1920 by 1080 with a ratio of 1. Raise the resolution in your display settings.
Red and blue smear while black and white stay sharpColour is arriving at lower resolution than brightness. With 4:2:0 subsampling a 4K picture carries colour at only 1920 by 1080, which suits video but blurs fine coloured detail. HDMI 2.0 added 4:2:0. HDMI 1.4 does not officially allow it, though driver updates gave it to some graphics cards.
Faint colour fringes on small browser textExpected. Subpixel rendering sharpens text by lighting the red, green and blue parts of each pixel separately, which leaves faint coloured edges when you look closely. It is used mainly on standard-density screens. Judge resolution by the line grids, not by the letters.
One side of the border is missingThe screen is showing a cropped, enlarged picture, a TV habit known as overscan. Every pixel is then resampled, so no fine pattern can come through clean. Look in the TV's picture size or aspect settings for the option that shows the whole image unscaled.
If the picture is not pixel for pixel 6
Reset the browser zoom to 100 percentPage zoom changes the pixel ratio the browser reports: zoom to 110 percent on a plain screen and it reads 1.1. The page cannot tell zoom from operating system scaling, so reset zoom first, usually Ctrl and 0, or Command and 0. Pinch zoom does not change the ratio.
Choose the native resolution in display settingsA flat panel has one fixed grid of pixels and looks its best only when the signal matches it. Open the operating system's display settings, pick this screen and choose 3840 by 2160. Then reload this page and read the numbers again.
Pick a scaling level on purposeScaling is a choice, not a fault. It keeps all the pixels and makes text and buttons larger. 200 percent maps each layout pixel to a whole block of four. In-between levels such as 150 percent cannot, so very thin lines in ordinary pages may look slightly soft.
Use a port and cable that carry 4K at 60 HzIf 3840 by 2160 is missing from the list, or appears only at 30 Hz, suspect the link. HDMI 1.4 carries 4K at 30 Hz in full colour, and HDMI 2.0 raised that to 60 Hz. Devices need not support their version's maximum, so check both ends and the cable.
On a TV, ask for every pixel and full colourTVs are set up for video. Turn off overscan in the picture size menu so the border shows on all four sides. Then look for an input or colour format setting that gives RGB or 4:4:4, so the red and blue lines are as sharp as the black and white.
Two screens: test each one where it sitsThe numbers describe the screen the browser window is on. Drag the window to another monitor and the pixel ratio can change, because each display has its own density and scaling. Move the window fully onto the screen you care about, then reload the page.
How this list was compiled: Written for this page. Every pixel count, scaled size, line count, ruler tick and pixel pitch was computed: 3840 times 2160 is 8,294,400, the size a browser reports is the panel size divided by the pixel ratio, and a pixel on a 27 inch 3840 by 2160 screen is 25.4 mm divided by 163 pixels an inch. That 3840 by 2160 is the dominant consumer 4K resolution with four times the pixels of 1080p, and that 4096 by 2160 is the Digital Cinema Initiatives container used mainly in digital cinema, are from https://en.wikipedia.org/wiki/4K_resolution. That devicePixelRatio is the ratio of physical pixels to CSS pixels, that page zoom changes it while pinch zoom does not, that 2 is expected on HiDPI screens, and that it can change when a window is dragged to another display, are from https://developer.mozilla.org/en-US/docs/Web/API/Window/devicePixelRatio. Screen.width in CSS pixels is from https://developer.mozilla.org/en-US/docs/Web/API/Screen/width. A flat panel's single fixed resolution, and most monitors smoothing every lower resolution, are from https://en.wikipedia.org/wiki/Native_resolution. The pixels-per-inch method and the 163 figure for a 27 inch screen are from https://en.wikipedia.org/wiki/Pixel_density, subpixel rendering of text from https://en.wikipedia.org/wiki/Subpixel_rendering, 4:2:0 sampling colour at half resolution both ways from https://en.wikipedia.org/wiki/Chroma_subsampling, moiré from https://en.wikipedia.org/wiki/Moir%C3%A9_pattern, overscan from https://en.wikipedia.org/wiki/Overscan, and the HDMI 1.4 and 2.0 limits for 4K, with 4:2:0 listed as an HDMI 2.0 addition and the note that HDMI 1.4 does not officially allow it although some graphics cards added it through driver updates, from https://en.wikipedia.org/wiki/HDMI. That macOS HiDPI mode uses a fixed scaling factor of 2, and that its other sizes are rendered at a higher or lower resolution than the screen's own and then scaled to it, are from https://en.wikipedia.org/wiki/Resolution_independence. All checked 2026-09-19. A web page sees the signal the computer sends, never the panel itself. Licence: own work
How it works
Run a 4k monitor test in two steps. First the tool reads the screen size and pixel ratio your browser is given, and shows the real pixel count. Then tap or click Go full screen and step through one-pixel grids, dot-matrix text and an edge border. They only look clean at native resolution.
Read the full guide
Why a 4K screen can read 2560 by 1440
Browsers measure in CSS pixels, not screen pixels. Scaling at 150 percent shrinks the reported size and raises the pixel ratio to match. Real pixels multiplies the two, and on a 4K screen it should read 3840 by 2160. Browser zoom changes the ratio too, so reset it to 100 percent first.
What the one-pixel patterns prove
The numbers cannot see the panel. The grids can. In full screen, every black and white line should be separate, with the field even from edge to edge. The right arrow, a click or a tap shows the next pattern. Wide bands or ripples mean the picture is being resized. What to look for names seven signs and six fixes, from zoom to TV overscan.
Check both directions. A picture can be clean on vertical lines and soft on horizontal ones.
On a TV, show the one-pixel border first. A missing side means overscan, which resizes everything.
Filing a return or a support ticket? Copy my numbers copies all four figures as plain text.
Questions and answers
Why does it say my 4K monitor is 1920 by 1080?
Either scaling is at 200 percent or the computer is sending Full HD. The pixel ratio tells you which. At 2 it is scaling, with all 3840 by 2160 pixels in use. At 1 it is Full HD, so choose 3840 by 2160 in your display settings. If that is missing, suspect the port: over HDMI, 4K at 60 Hz in full colour needs version 2.0 or later.
Can a web page detect my monitor's true resolution?
Not directly. A page is told the screen size in CSS pixels and a pixel ratio, and multiplies them. It cannot see the panel's own grid, or tell operating system scaling from browser zoom, because both change the same ratio. This page pairs the numbers with patterns you judge by eye.
Is scaling bad for a 4K monitor?
No. Scaling keeps every pixel and makes text and controls larger. At 125 or 150 percent a thin line cannot always land on whole pixels, so some hairlines look soft. These patterns are drawn in device pixels, so on Windows they should stay exact. A Mac in a scaled mode draws a larger picture and shrinks it, so there even these lines can look soft.