How can a screen ruler be actual size?

Match the calibration marker to a physical reference before measuring. CSS pixels do not report the physical size of your display, so the ruler shows pixel counts until you calibrate. The readout and tick spacing then use your measured scale.

What can I use to calibrate it?

Use a standard ID-1 bank card, 85.60 mm wide, an A4 short edge of 210 mm, or a US Letter short edge of 8.5 inches (215.9 mm). Choose the reference, match the marker to its far edge and press Use this measurement. Type a length accepts decimals and fractions such as 3 1/2 for another object of known length.

Which units can I use?

Inches have quarter-inch marks and a reading rounded to the nearest sixteenth. At a 1:1 drawing ratio, centimetres have a tick every millimetre, with centimetre numbers when space permits. Millimetres use the same physical ticks and nominal 10 mm numbers. Crowded screen numbers are thinned while every tick remains. A supplied drawing ratio changes the represented numbers; calibration and the printable ruler keep their physical lengths. Changing units keeps the calibration. Feet have inch ticks, and metres have centimetre ticks. A foot is 12 inches or 304.8 mm; a metre is 1,000 mm. These long targets need enough display width or suitable paper.

Can I use a six-inch, twelve-inch or 30 cm ruler?

Enter the desired length in Target length, choose its unit and press Place target on screen after calibrating. The target must fit the visible ruler: a phone may be too narrow for six inches, twelve inches or 30 cm. A target that will not fit gives an error rather than shrinking the scale. Printable output supports these lengths on paper large enough for the ruler and its margins.

How do I print a ruler at actual size?

Set Target length and download the printable SVG. Open the file and choose 100% or Actual size when printing; turn off Fit to page. The file uses physical millimetre dimensions independently of screen calibration. A 12-inch ruler is 304.8 mm long, so its file needs paper wider than 324.8 mm including margins. Check the printed marks against a physical ruler before measuring: printer and viewer settings can rescale the file.

Can I use this as a drawing scale or architectural ruler?

Set Scale ratio, calibrate against a physical reference and move the marker along the drawing displayed on this screen. The numbered marks and represented-length result apply your supplied ratio. The physical measurement and printable ruler stay at actual size. Check the drawing's displayed scale first: the page cannot infer it from an image or identify drawing dimensions.

Does the ruler work on an iPhone or another phone?

Use it in your phone browser, match a reference to the screen and drag the edge with a finger. The available length depends on the visible width. Changing phone orientation alters that width and clears the calibration, so calibrate in the orientation you will use.

Why did my calibration disappear?

A change in device pixel ratio or viewport width clears it. These changes can come from page zoom, resizing a window or moving to another display. The page reports the change it detected and asks you to match the reference again.

How precise is the measurement?

Precision depends on how closely you matched the reference edge. A one-pixel error in calibration changes the scale used for every later reading. Focus the marker and use arrow keys to move one pixel at a time, or Shift with an arrow to move ten. A wider reference can make the same pixel error a smaller proportion of its width.

Are measurements saved or uploaded?

Measurements stay in your browser. The main page remembers its unit and calibration on this device when storage is available. Preset pages can read a valid calibration but keep changes for that visit, preserving the main page settings.

Online Ruler

Use your screen as a ruler in inches or centimeters once you calibrate it against an ID-1 card or a sheet of paper. Every ruler drawn on a screen has to answer one question before it can be right: how large is a pixel on this display? No browser will say, so the usual answer is the CSS convention of 96 pixels to the inch, which is a rule about page layout rather than a fact about your screen. A 13.6-inch laptop reporting 1512 pixels across fits about 131 of them into a real inch, so its CSS inch is roughly three quarters of one, while a 24-inch monitor at 1920 fits about 92 and its CSS inch runs slightly long. This ruler refuses to pretend it knows. It is marked uncalibrated until you lay an ID-1 card, an A4 sheet or a Letter sheet against the rule and match the marker to its edge, and only then does it call anything a length. It prints the scale it worked out, so you can check the rule against a real one, and it clears the calibration when the page zoom, the display or the window width changes, saying what it saw change.

Hold a bank, credit or ID card against the rule and match the marker to its right edge.ISO/IEC 7810 ID-1, a standard bank card 85.60 mm wide

Not calibratedA zoom or a screen change clears it

Units
Calibrate against
Selected: 0 px, not a length yet

Target length (in)

Screen targets need calibration and enough visible width. The printable SVG has physical dimensions: open it, print at 100% or Actual size on paper large enough, and check its marks with a physical ruler. Printer scaling can change the size.

Drawing scale

Scale ratio (1:n)

At 1:100, 1 cm on a drawing represents 100 cm. After calibrating, drag the marker over a drawing displayed on this screen. Numbered marks and represented length use the chosen ratio; the physical reading, calibration references, targets and printable SVG stay at actual size. Confirm the drawing's displayed scale before measuring. Represented numbers are rounded to eight significant digits.

Calibrate before reading a represented drawing length.

Not calibrated yet, so these marks are not a measurement. Lay a bank card or a sheet of paper against the rule, match the marker to its edge, and the marks become real inches and centimetres.

Click or drag anywhere on the rule to move the edge. With the marker focused, the arrow keys move it one pixel at a time and shift with an arrow moves ten, so you can land on an exact figure. Nothing you do here leaves your browser: the calibration is kept on this device only.

An object of a known width, matcheda bank card is 85.60 mm under ISO/IEC 7810 ID-1
sets the scale for every mark on the rule
Nothing matched yetno reference given
marked uncalibrated, and no length is shown
The page zoom changesthe reference no longer holds
the calibration is cleared and the rule asks again

Exact to the reference you calibrate it against, and marked uncalibrated until you do. A browser cannot ask your display how large it is, so an uncalibrated screen ruler is a guess, and a page zoom or a screen change clears the calibration because the reference no longer holds.

Common questions

How can a screen ruler be actual size?: Match the calibration marker to a physical reference before measuring. CSS pixels do not report the physical size of your display, so the ruler shows pixel counts until you calibrate. The readout and tick spacing then use your measured scale.
What can I use to calibrate it?: Use a standard ID-1 bank card, 85.60 mm wide, an A4 short edge of 210 mm, or a US Letter short edge of 8.5 inches (215.9 mm). Choose the reference, match the marker to its far edge and press Use this measurement. Type a length accepts decimals and fractions such as 3 1/2 for another object of known length.
Which units can I use?: Inches have quarter-inch marks and a reading rounded to the nearest sixteenth. At a 1:1 drawing ratio, centimetres have a tick every millimetre, with centimetre numbers when space permits. Millimetres use the same physical ticks and nominal 10 mm numbers. Crowded screen numbers are thinned while every tick remains. A supplied drawing ratio changes the represented numbers; calibration and the printable ruler keep their physical lengths. Changing units keeps the calibration. Feet have inch ticks, and metres have centimetre ticks. A foot is 12 inches or 304.8 mm; a metre is 1,000 mm. These long targets need enough display width or suitable paper.
Can I use a six-inch, twelve-inch or 30 cm ruler?: Enter the desired length in Target length, choose its unit and press Place target on screen after calibrating. The target must fit the visible ruler: a phone may be too narrow for six inches, twelve inches or 30 cm. A target that will not fit gives an error rather than shrinking the scale. Printable output supports these lengths on paper large enough for the ruler and its margins.
How do I print a ruler at actual size?: Set Target length and download the printable SVG. Open the file and choose 100% or Actual size when printing; turn off Fit to page. The file uses physical millimetre dimensions independently of screen calibration. A 12-inch ruler is 304.8 mm long, so its file needs paper wider than 324.8 mm including margins. Check the printed marks against a physical ruler before measuring: printer and viewer settings can rescale the file.
Can I use this as a drawing scale or architectural ruler?: Set Scale ratio, calibrate against a physical reference and move the marker along the drawing displayed on this screen. The numbered marks and represented-length result apply your supplied ratio. The physical measurement and printable ruler stay at actual size. Check the drawing's displayed scale first: the page cannot infer it from an image or identify drawing dimensions.
Does the ruler work on an iPhone or another phone?: Use it in your phone browser, match a reference to the screen and drag the edge with a finger. The available length depends on the visible width. Changing phone orientation alters that width and clears the calibration, so calibrate in the orientation you will use.
Why did my calibration disappear?: A change in device pixel ratio or viewport width clears it. These changes can come from page zoom, resizing a window or moving to another display. The page reports the change it detected and asks you to match the reference again.
How precise is the measurement?: Precision depends on how closely you matched the reference edge. A one-pixel error in calibration changes the scale used for every later reading. Focus the marker and use arrow keys to move one pixel at a time, or Shift with an arrow to move ten. A wider reference can make the same pixel error a smaller proportion of its width.
Are measurements saved or uploaded?: Measurements stay in your browser. The main page remembers its unit and calibration on this device when storage is available. Preset pages can read a valid calibration but keep changes for that visit, preserving the main page settings.