Why calibration comes first
A browser knows how many CSS pixels it can draw, but it cannot reliably tell how large each pixel appears on your physical display. Screen density, operating-system scaling and browser zoom all affect that relationship. This tool starts with an uncalibrated preview and waits for you to match a known object.
The reference rectangle uses the 3.370 × 2.125-inch card dimensions specified for this tool. Hold a standard payment card against the display and adjust the outline until the outer edges match. The approximate metric size is 85.60 × 53.98 mm. Cards with an unusual size are not suitable references. You do not need to enter, upload or photograph any card information.
How the size is calculated
After you match the card, the drawn card width divided by 3.370 gives the number of CSS pixels per inch at that display setting. Each item dimension is multiplied by that value. For example, at a matched setting of 100 pixels per inch, a 6 × 4-inch front outline is drawn 600 × 400 CSS pixels. The side view substitutes the 3-inch depth for the 6-inch width.
The default 6 × 4 × 3-inch item is an example, not a product measurement. Enter the outside dimensions of your own object. The reference card appears alongside it at the same scale. The tool shows a rectangular envelope, so rounded corners, handles and irregular surfaces are not represented.
Keep the display setting unchanged
Move the browser to another monitor or change its zoom, and the previous calibration may no longer apply. This tool removes the calibrated status when it detects a change in device pixel ratio, viewport scale or window dimensions. Detection is not perfect on every system. Recheck the card yourself after changing any display setting, even if no notice appears.
Calibration stays in this page’s memory. Reloading the page starts over. The outline can extend beyond your screen, especially for appliances or monitors. Scroll to inspect it. Shrinking it to fit would stop it from representing the same physical scale.
Common questions
Is it accurate enough to buy or install something?
Use it to understand scale, then verify the actual product and space with a tape measure. The result depends on the card matching, the reference card’s dimensions and unchanged display settings. It cannot measure manufacturing tolerances, clearance, mounting holes or a three-dimensional fit.
Can I use my phone or MacBook?
Yes, when the browser can show the outline and you can compare it with a standard card. A small display may need scrolling. Keep the device and browser zoom unchanged, and do not pinch to shrink the item after calibration.
Why does the outline go beyond the page?
The physical item may be wider or taller than the visible area of your screen. The scrollable preview preserves the chosen scale. For a large item’s fit inside a room, use the room planner instead.
Does a screenshot keep the same size?
Not reliably. Image viewers, messaging apps and print dialogs may resize screenshots. Calibrate and inspect the live page on the display you intend to use.