How to Scale a PDF Drawing and Measure It

A PDF of a floor plan or site plan is drawn to scale, but that scale only holds while the page stays at its original size. Exports, prints and screenshots change the size of the drawing without changing the words on it. Here's how to set the scale so it's right, check it, and then measure lengths and areas.

1. 1

   Find the scale note and a printed dimension

   The note is usually in the title block or under each drawing's title. Pick the longest printed dimension you can find, such as an overall building dimension.
2. 2

   Set the scale from the dimension

   Measure that dimension on the PDF and divide its real length by the measured length. That ratio is the true scale of the page you have, whatever the note says.
3. 3

   Check it on a second dimension

   Measure another printed dimension, ideally long and running the other way. Within about 1% means the scale is good.
4. 4

   Measure distances and areas

   Multiply drawing lengths by the scale. For areas, work in real lengths, or let a measuring tool apply the scale as you trace.

How does scale work on a PDF?

A drawing scale is a ratio between the drawing and the building. 1/4" = 1'-0" means a quarter inch on the sheet stands for one foot, so the building is 48 times larger than the drawing (1:48). A PDF page has a fixed physical size, measured in points (72 to the inch), so a PDF saved at the original sheet size keeps its scale: a 20-foot wall on a 1/4-inch-scale sheet is 5 inches long on the page. Zooming in on screen doesn't change that, because measuring tools work in page units, not screen pixels.

Watch the near-misses. 1/4-inch scale is 1:48, not 1:50, and 1" = 20' is 1:240, not 1:20. The 1:48 versus 1:50 mix-up is only about 4% in length, but area goes with the square of the scale, so it becomes about 8.5% in square footage. Every scale has its own page under drawing scales, and the scale converter turns inches on paper into feet in either direction.

Why does "fit to page" break the scale?

Because it resizes the drawing and leaves the scale note alone. The US Fire Administration's guide to using engineer and architect scales warns that "during blueprint reproduction, the image size may be adjusted to fit the paper, so it may not precisely represent the scale the designer intended to use," and that written dimensions "always take precedence over scaled measurements."

The same thing happens in a PDF viewer. Adobe's page sizing options include Fit, which shrinks or enlarges each page to fit the paper, and Actual size, which prints without scaling. Fit is a common default. It also happens when someone re-saves a large sheet to a Letter-size PDF, takes a screenshot, or pastes the plan into another document. Here's what fitting a 24 × 36 in sheet onto 11 × 17 in paper does:

The fit factor is 11 ÷ 24 = 0.458 (the short sides set the limit), and printer margins usually shrink it a little more. A half-size set, a 24 × 36 sheet printed on 12 × 18 paper, turns 1/4-inch scale into 1/8-inch scale exactly. To print a drawing that you can measure with an architect's scale, choose actual size on paper large enough for the whole sheet.

How do you set the scale from a known dimension?

Divide the real length by the length on the drawing, in the same units. Using the reduced print above, where a 24'-0" dimension measures 2-3/4 inches:

Use the longest dimension you can, with ends you can find exactly. Your reading error stays about the same size whatever you measure, so it's a smaller share of a long line: misreading by 1/32 inch is 1.1% of a 2-3/4-inch line but 0.5% of a 6-inch one. Overall dimensions along the outside of the building are usually the best choice. If the plan has no printed dimensions, use something of known size, such as a door (interior doors are commonly 30–36 in wide; a 3'0" door is 36 in) or a wall you've measured on site. More on reading them in how to read floor plan dimensions.

How do you check the scale is right?

Measure a second printed dimension and compare. Pick one that's long and runs in the other direction: a scan or fax can be stretched more one way than the other, and a check in the same direction won't catch it. The error is (measured − printed) ÷ printed.

On a printout, within about 1% is as good as a ruler gets. On a vector PDF measured on screen, a correct scale should agree with every dimension to within an inch or so. If one dimension disagrees and the others agree, it may be measured to a different point (a wall's centerline instead of its face), or it was edited without the drawing being changed. Trust the majority.

Measuring distance and area once it's scaled

A distance is the length on the drawing times the scale. Area scales with the square of the scale: one square inch at 1/4-inch scale is 4 ft × 4 ft = 16 sq ft, and at 1/8-inch scale it's 64 sq ft. That's also why scale errors grow in area: a 2% error in the scale is about a 4% error in square footage.

For a room, convert each side to feet first, then multiply. A room that measures 3 × 3-1/2 inches at 1/4-inch scale is 12 × 14 ft, or 168 sq ft. Break irregular shapes into rectangles and triangles, as shown in L-shaped and irregular rooms, or trace the outline with a tool that adds it up for you. For a whole house, see how to calculate square footage from a floor plan.

Free ways to scale and measure a PDF

• By hand. Print at actual size, or work out the true scale from a dimension as above, and measure with a ruler or an architect's scale. The scale converter does the conversion.
• Adobe Acrobat. Acrobat has a measuring tool. Adobe's help pages cover which versions include it and how to set its scale.
• FloorFigures. Free, in your browser, and the PDF never leaves your device. Details below.

Scaling a PDF in FloorFigures

FloorFigures reads the printed dimensions on a PDF (12'-6", 3600, 3.6 m) and its scale notes (1/4" = 1'-0", 1" = 20', 1:100) and sets the scale automatically. When the note disagrees with the dimensions, which is what a resized PDF looks like, it warns you and goes with the dimensions. It keeps the drawing's vector lines, so measurements snap to the actual wall lines and corners instead of to a blurry image.

1. Open the PDF. Each page becomes a sheet. Nothing is uploaded.
2. Check the scale it found, or set your own with Set scale (S): click both ends of a known dimension and type its length. Stretched scans can have separate horizontal and vertical scales.
3. Check it with Check scale (K): measure another dimension and see the error.
4. Measure: Distance (D) for lengths, a click inside a room (M) for its area, or A to find every room at once. The scale and its check are printed in the PDF report.