Scale ruler online
Pick your screen once, then read plan distances directly off the display. Six graduated faces, from 1:20 to 1:125.
Choose your device from the list so the ruler is drawn at true size. If yours is not there, pick "My screen" and enter its diagonal and resolution.
Enter the panel diagonal in inches and the full pixel resolution. Both are in your display settings, or on the manufacturer's spec sheet.
Enter a diagonal between 3 and 120 inches and a resolution between 240 and 16000 pixels.
Pick the scales you are working in. Each one is drawn as a separate graduated rule.
Your scale ruler
Select at least one scale to draw the ruler.
Scale converter
Prefer to type the numbers? Convert between real dimensions and drawing dimensions, or work out the ratio from two known lengths.
Mode: calculating lengths from scale
Result
The scale determines the ratio between the length on a map or model and the actual length. A scale of 1:20 means that each unit on the map corresponds to 20 units in reality.
For example, if a segment 2.5 meters long on the map corresponds to 50 meters in reality, then the map’s scale is 1:20 (or 0.05 as a scale factor).
A scale ruler reads distances off a drawing without any arithmetic. Set it to the scale of your plan, lay it along the line you want to measure, and read the real dimension directly. This one works the same way, except the rule is drawn on your screen. Once you have calibrated the display, a centimetre on the page is a real centimetre, so you can measure a PDF or an image on screen exactly as you would measure a printed sheet with a physical architect's scale.
Why calibration matters
Browsers do not know how large your screen physically is. They work in CSS pixels, and the standard assumption of 96 pixels per inch is wrong on most modern displays. A laptop at 1920×1080 on a 13.3 inch panel packs about 166 pixels into every inch; the same resolution on a 24 inch monitor packs about 92. Draw a ruler without accounting for that and it will be off by up to 40 percent.
Two numbers fix it: the diagonal of your panel and its pixel resolution. Together they give the pixel density, and from there the exact width of a centimetre on your display. Pick your device from the list and the page does that arithmetic for you. If your model is not listed, choose "My screen" and type the two values in; your entry is stored in the browser, so you only do it once per device.
How to use the ruler
1. Set your screen
Choose the closest match from the list, or enter your own diagonal and resolution. Both numbers are in your operating system's display settings. If you skip this step the ruler falls back to 96 pixels per inch, which is close enough for rough work but not for measuring.
2. Choose your scales
Tick the ratios you need. Each one is drawn as its own graduated face, marked in metres, the same arrangement a triangular scale rule uses to carry six scales on one tool. Showing two or three at once makes it easy to compare how the same drawing reads at different ratios.
3. Measure
Hold your drawing against the screen, or place a window with your PDF beside the ruler, and read the value at the end of the line. The minor divisions subdivide each labelled interval into ten, five or two parts depending on how much room the scale allows.
For a longer rule, open it full screen. Rotating the rule 90 degrees runs it along the other axis of the display, which on a phone held upright is the longer one.
Which scale should you use
Architectural floor plans are usually drawn at 1:50 or 1:100. Site plans and larger buildings move to 1:200 or 1:500. Detail drawings, joinery and sections tighten to 1:20 or 1:25, where a centimetre on paper stands for 20 or 25 centimetres of real building. Engineering and survey work in some countries uses 1:75 and 1:125, which is why both appear here even though they are less common in construction.
If you are not sure which scale a drawing uses, look for the ratio printed in the title block, or find a dimension that is already labelled. Measure that line with the ruler at 1:100, then divide the labelled value by what you read: the result is the ratio you need. The scale converter below does the same job from two typed lengths.
Accuracy and limits
After calibration the ruler is accurate to roughly half a millimetre on screen, which at 1:100 means about 5 centimetres of real distance. That is fine for checking a dimension or estimating a run of wall. It is not a substitute for the dimensions written on the drawing, and you should never scale off a drawing marked "do not scale".
Two things will break the calibration: changing the browser zoom level, and moving the window to a second monitor with a different pixel density. In both cases, calibrate again. If your screen has a scaling setting in the operating system, the ruler follows whatever the browser reports, so calibrate on the display you actually measure on.
Frequently asked questions
A browser cannot tell how physically large your screen is, only how many pixels it has. Pixel density varies enormously between devices, so a ruler drawn without that information can be wrong by 40 percent. Picking your device, or entering its diagonal and resolution, gives the page the one fact it is missing.
Choose "My screen" and fill in two values: the panel diagonal in inches and the full pixel resolution, width by height. On Windows both are under Settings, System, Display; on macOS under System Settings, Displays; on Android and iOS the model specification lists them. Enter the native resolution rather than any scaled figure the system reports.
The faces are 1:20, 1:25, 1:50, 1:75, 1:100 and 1:125, the six ratios carried on a standard triangular scale rule. You can display any combination of them at once. For ratios outside that range, such as 1:200, 1:500 or 1:25000, use the scale converter on this page, which handles any ratio you type.
Once calibrated it reads to about half a millimetre on screen. At 1:50 that is roughly 2.5 centimetres of real distance, and at 1:100 roughly 5 centimetres. Use it to check and estimate, not to replace the figured dimensions printed on a drawing, which are always the authority.
Yes. Calibrate on each device separately, because pixel density differs between them. On a narrow screen the ruler shows fewer graduations, since it only draws intervals that have room to be labelled legibly. Turning the phone to landscape gives you a longer rule to measure against.
You can, provided the drawing is displayed at 100 percent zoom. Any zoom applied by your PDF viewer or image viewer changes the size on screen and invalidates the reading. With a photo, be aware that camera perspective distorts distances unless the shot was taken square on to the sheet.