What can you do with the online ruler tool on onlinecompass.net?
The online ruler tool on onlinecompass.net lets you measure lengths and dimensions directly on your screen. It supports centimeters, millimeters, and inches, with inch subdivisions of 1/32, 1/16, 1/8, 1/4, and 1/2. You can calibrate the ruler with a standard-sized bank card.
The tool has four measurement modes: marking a single point, drawing a length from one side, drawing a length between two points, and drawing a rectangular area. It adapts to different screen sizes and orientations, and you can lock measurements, enter custom values, and download the result as an image. Calibrate the ruler at 100% browser zoom before you start measuring.
How to Calibrate the Ruler
To calibrate the ruler, place your bank card over the card shown on the screen. Move the scale slider until the two cards match, then click Save. If you skip this step, the ruler uses a default scale, and measurements may not match real sizes.
Note: The tool uses the ID-1 card size from the ISO/IEC 7810 standard, 85.60 × 53.98 mm, which most credit and debit cards follow. Many gift cards use it too, but not all, so a bank card is the safest reference. Set your browser zoom to 100% during calibration, and avoid zooming in or out afterward, as it will affect accuracy.
Supported Measurement Units
The ruler can display measurements in centimeters (cm), millimeters (mm), and inches. For inches, you can choose subdivisions of 1/32, 1/16, 1/8, 1/4, or 1/2 inch.
How to Use the Ruler
The ruler always starts from zero, so negative values aren't supported. It adjusts to your screen size, including when you rotate a mobile device to landscape mode. Click Clear to remove the current marking.
1. Marking a Single Point
Click anywhere on the ruler to see the coordinates (length and width) of that point. You can also enter length and width values in the bottom-right corner and click Set to mark that point in red.
You can switch to full-screen mode, lock the marked point so it doesn't move when you click or tap again, and download the ruler with the marked point using the Download Image button.
2. Drawing a Length from One Side
Drag the colored overlay from the left side of the ruler to the right to draw a length starting from zero. To enter a specific length, use the input fields in the bottom-right corner and click Set.
You can switch to full-screen mode, lock the drawn length so dragging doesn't change it, and download the ruler with the marked length using the Download Image button.
3. Drawing a Length between Two Points
Drag the colored area from any starting point to an endpoint to draw a custom length. You can also enter the start and end points and click Set.
You can switch to full-screen mode, lock the length, and download the result using the Download Image button.
4. Drawing a Rectangular Area (Length and Width)
In this mode, you can draw horizontally and vertically to form a square or rectangle from any of the four sides: left, right, top, or bottom. You can also enter the start and end values for length and width in the input fields and click Set to draw the colored overlay.
You can switch to full-screen mode, lock the shape so it doesn't change, and download it using the Download Image button.
Frequently Asked Questions
Why do I need to calibrate the ruler with a bank card?
Screen sizes and pixel densities (PPI/DPI) vary widely between devices, so an uncalibrated ruler may not match real-world measurements. A standard bank card (ISO/IEC 7810 ID-1 size: 85.60 × 53.98 mm) gives the tool a known physical size to scale the on-screen ruler to your display.
Does the ruler work accurately without calibration?
No. Without calibration, the ruler uses an assumed standard pixel density, so measurements can be off by anywhere from a few percent to around 40%, depending on your device.
Can I save my calibration for future visits?
Yes. Calibration is stored in your browser's local storage, so it stays saved on the same device and browser until you clear cookies and site data or recalibrate. In Safari, it can be deleted after seven days of browser use without interacting with this site.
Is the ruler accurate on high-DPI or Retina displays?
Yes, after calibration. Calibrating with your bank card sets the scale for your screen's pixel density, so the ruler shows real-size measurements on screens of any resolution. If you change your display resolution or scaling later, recalibrate.
Does the tool work on external monitors or multiple screens?
Yes, but calibration holds only for the screen you calibrated on, because browsers can't detect a monitor's physical pixel density. If you move the window to another monitor or change your display scaling, recalibrate on that screen.
Can I use this ruler on a touchscreen phone or tablet?
Yes. The ruler supports touch, and dragging with your finger works the same as dragging with a mouse.
Does the ruler work in portrait and landscape orientation?
Yes. The ruler adjusts to your device orientation automatically. Landscape mode provides a longer horizontal ruler.
Why does the ruler appear shorter on my phone than on a computer?
The ruler's maximum length depends on your physical screen size: roughly 6.5–7.5 cm on phones in portrait and 14–16 cm in landscape (slightly less in practice), about 20–25 cm on most tablets, and 30–60 cm or more on laptops and desktops.
Does the tool work in all browsers?
The tool works in Chrome, Firefox, Edge, and Safari. If your browser is set to block cookies and site data, it may not save your calibration, and private or incognito windows delete it when you close them.
Does the inch ruler show fractions or decimals?
The inch ruler shows fractional subdivisions (1/32, 1/16, 1/8, 1/4, and 1/2 inch). Decimal inch values aren't displayed but can be calculated from the fractions.
Can I measure diagonally or at angles?
No. The tool measures only horizontal and vertical distances (length and width). For diagonal measurements, use the rectangular mode and apply the Pythagorean theorem to the length and width values.
Can I measure curved or irregular shapes?
No. You can approximate a curve by measuring several straight segments and adding the lengths, or use the rectangular mode to measure the smallest rectangle around the shape.
Can I measure objects larger than my screen?
Yes. The ruler is limited to your screen size, so measure the object in sections and add the values, or move the object along the screen and note the cumulative marks.
Can I use the ruler for graphic design or sewing?
Yes, for quick checks such as pattern scaling or screen layouts. For work that needs exact measurements, verify against a physical ruler, as minor screen or calibration differences can occur.
Can I change the color of the overlay or markings?
No. The colored overlay and red marks use fixed colors.
Can I print the ruler for offline use?
Yes, you can download the ruler as an image and print it, but calibration applies only to your screen, so the printout may not be real size. After printing, check it against a physical ruler or your bank card (85.60 mm long) and adjust the print scale until they match.