The automatic calibration was like 10% smaller than the real size and I used the credit card method [1] It need to show the "crop lines, no bleed" lines to make the comparison easier https://toastdesignservices.co.uk/resources/what-crop-marks-...

I used something like this before to adjust the zoom in Word or Google Docs with the real size on screen. I prefer to overlap a A4 sheet on the screen. It's semitransparent, so it's easier to see the calibration. Also I'd like to use a transparent rules on the screen and overlap it over the virtual rule for calibration.

[1] For some stupid reason, it sound scary

Fascinating.

My setup: Firefox on X11 on a ThinkPad E15 Gen 2, with X11 misconfigured such that it is 96 DPI instead of the physically-correct 144 DPI.

On initial load, it has the green calibrated box saying "Detected: 24″ FHD Monitor (Auto-calibrated)", and my credit card measures as 131mm. This more-or-less the error that I'd expect given my misconfiguration: 131mm/85.6mm =~ 1.53 ; 144dpi/96dpi = 1.5. (Given those numbers, I figure it should be closer to 23" than 24", but whatever.)

But if I tell it "Standard Laptop", then my credit card measures as 97mm. (97mm/85.6mm =~ 1.13). I can't guess how that number is being arrived at. IME lots of X11 users will have their DPI misconfigured as either 96 DPI or (less likely) 192 DPI, but also lots of non-Apple laptops will have a pixel density of 40%-65% more than 96 DPI (on the rationale that lots misconfigured-as- or hardcoded-as- 96 DPI is common, that things designed to look good at 96 DPI assume a desktop monitor about 28" from your face (CSS3 defines device-independent pixels as such), but laptop use puts the screen 17"-20" from your face; and 28in/20in gives us a factor of 1.4 and 28in/17in gives us a factor of 1.65. But none of these numbers give me the factor of 1.13 that you show me. A good mystery!

The calibration with a credit card would I think be easier if one corner of the calibration rectangle was in a fixed position, so moving the slider would only change the other three corners. The fixed rectangle should probably be the one on the bottom left.

This seems really inaccurate to me. I had to manually enter my device (Galaxy A55 5G), and then I held a tape measure next to my phone. What the site claimed was 16cm was actually only 14.3cm.

FYI, the calibration modal is unreadable in dark mode.

96dpi seems like a bad default for mobile devices. The CSS spec says the reference pixel (https://www.w3.org/TR/css-values-3/#reference-pixel) should be 96dpi at ~28" viewing distance. But handheld devices are presumed to not be seen that far away and so are built with a DPI closer to 160 (after dividing by the device pixel ratio).

Cool. This is actually really useful - I'm building a document editor at the moment. I think having the option for real-world size would be a nice feature.

I've been busy measuring different document editors with a screenshot of the ruler and it seems they all have different interpretations of A4 width (210 mm) at 100% on a Macbook Pro 16in:

- Apple Pages shows about 118mm

- Google docs shows 160mm in Safari

- Linearity Curve gives about 200mm

Petition to change the foot to be the distance light travels in one nanosecond

Diagonal measurements aren't accurate with screens with rounded corners. I suggest adding a width-and-height calibration method. Calibration with a ruler would be good too; sit the ruler on the screen and zoom/pan(/rotate?) with multi-touch.

Cool idea and I can see myself using this.

My iPhone 14 Pro was incorrectly identified as an iPhone 11 though.

Maybe it doesn’t matter since both have the same screen size, but it can be confusing since the wrong model identified even if it’s the same size.

https://www.apple.com/au/iphone/compare/?modelList=iphone-11...

Is it not possible to use `window.devicePixelRatio` instead of calibration?

https://developer.mozilla.org/en-US/docs/Web/API/Window/devi...

The idea is great especially the popup scale.

Issues I faced: I use 27" Qhd monitor at 125% scale. The ruler showed 48cm width intially but when I shifted to 100% scale it became 60cm. If I shift to FHD, it goes to 36cm. I thought an actual size ruler will maintain width. Let me know if I am not using it correctly.

I save a screenshot for my future reference, is perfect. Maybe a suggestion to include in your website. The screenshot also shows the domain so a good way (for me) to find you again if needed

It's really clever, but the screen detection is overconfident; it gave me the green "calibrated" box while guessing a completely wrong display size for my laptop.

https://m.youtube.com/watch?v=JYqfVE-fykk&t=1m41s

It guessed my phone wrong. It thought iPhone 11 Pro Max but I have the 12 Pro Max (I just double checked in phone settings).

Don't devices/browsers report the display PPI somehow? If not, that's such a gap in the web standards.

Looks nice, but on a Google pixel 6 I get

> Unknown device. Using default 96 PPI. Calibrate for better accuracy.

Cool! Even though, my iPhone 15 Pro was misidentified as iPhone 16.

It didn't recognize my Lenovo IdeaPad running Fedora.

My iPhone 13 Pro is incorrectly identified as an iPhone 14

Nice app! My S10e was detected as a Galaxy Note 10.

haha it doesnt like a VM in a citrix session

picks up my macbook 14" doesn't detect framework 13

it detects the 13 pro as a 14, probably going off of Axx chip

Detected: 14″ FHD @ 125% DPI (Auto-calibrated)

Inches are 2.2" per inch and CM are also over 2 CM in length, almost an inch, just short.

But hey, even CMs are wrong and wonderfully explained in this short - enjoy https://www.youtube.com/shorts/Z2LLcVM2ih4

Tried a ruler and a measuring tape with my desktop LCD display it was off by 2/30 in both cases, in the same direction. Tried phone - was off by 8/20.

Nope, thanks.