Scale ruler online
Read plan distances straight off your screen. Pick your device or calibrate with a bank card, choose the scales, and measure the way you would with a triangular scale rule.
Calibrate your screen
An automatic estimate is set for your device. Pick your model from the list, or calibrate with a bank card for an exact size.
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.
Calibrate with a card
Hold a bank card, ID card or driving licence flat against the screen. They all share one standard size. Move the slider until the dashed outline matches the card edge to edge, then save.
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.
Result
At 1:20, 250 cm on the map or plan is 50 m in reality.
- Scale Ratio
- 1:20
- Scale Factor
- 0.05
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 arithmetic. Set it to the scale of the plan, lay it along a line and read the real dimension. This one is drawn on your screen. Once the screen is calibrated, a centimetre on the display is a real centimetre, so you can measure a PDF or an image the same way you would measure a printed sheet with an architect's scale.
Why the ruler needs calibrating
A browser knows how many pixels your screen has, but not how large it is. Websites assume 96 pixels per inch, which is close on a desktop monitor and far off on phones and laptops: on a typical phone a ruler drawn that way comes out about 40 percent too small. The page therefore starts from an estimate for your type of device and offers two ways to make it exact.
Two ways to calibrate
Choose your device
Pick your phone, tablet, laptop or monitor from the calibration list. The page calculates the pixel density from the screen size and resolution. If your model is missing, choose My screen and enter the diagonal in inches and the resolution in pixels.
Calibrate with a bank card
Every bank card has the same size, 85.60 × 53.98 mm. Open Calibrate with a card, hold the card flat against the screen with one corner on the corner of the dashed outline, and move the slider until the opposite edge matches. This works on any screen, including models that are not in the list. The result is saved in your browser, so you only do it once per device.
How to measure
- Calibrate the screen, or accept the estimate for a quick check.
- Tick the scales you work in. Each one is drawn as its own graduated face, like the six faces of a triangular scale rule.
- Place your drawing next to the ruler at 100 percent zoom and read the value at the end of the line.
For a longer rule, open it full screen. Rotating it 90 degrees runs it along the longer side of a phone held upright.
Which scale to use
Floor plans are usually drawn at 1:50 or 1:100. Details, joinery and sections use 1:20 or 1:25, and some engineering and survey drawings use 1:75 or 1:125. If a drawing does not state its scale, measure a dimension that is written on it, then use the scale converter on this page to work out the ratio.
Accuracy and limits
Calibrated with a card, the ruler reads to about half a millimetre on screen, which is 5 cm of real distance at 1:100. That is enough to check a dimension, not to replace the figured dimensions on a drawing. Changing the browser zoom or moving the window to another monitor changes the size on screen, so calibrate again in those cases.
Frequently asked questions
A browser can count the pixels on your screen but cannot tell how large the screen is. Without that, a ruler on a phone comes out about 40 percent too small. Choosing your device, or matching a bank card, gives the page the missing size.
All bank cards share one standard size, 85.60 × 53.98 mm. Hold the card against the screen with a corner on the corner of the dashed outline and move the slider until the opposite edge matches. The page then knows exactly how many pixels make a centimetre on your screen.
Calibrate with a bank card, which works on any screen. Alternatively, choose My screen and enter the diagonal in inches and the resolution in pixels from your display settings or the manufacturer's specification.
It draws 1:20, 1:25, 1:50, 1:75, 1:100 and 1:125, the six scales of a standard triangular scale rule, in any combination. For other ratios such as 1:200 or 1:500, use the scale converter on this page.
After calibration it reads to about half a millimetre on screen: about 2.5 cm of real distance at 1:50 and 5 cm at 1:100. Use it to check and estimate; the dimensions written on a drawing always take priority.
Yes, as long as the drawing is shown at 100 percent zoom. Any zoom in the PDF viewer changes the size on screen. With a photo, the sheet must have been shot straight on, or the perspective distorts distances.