Step 1: Find the scale
Most plans print the scale in or near the title block. It is written either as a ratio, such as 1:100, or as an architectural note, such as 1/4" = 1'-0". Some plans also draw a scale bar, which is a short ruler printed on the sheet.
- Look at the title block first, then under each drawing's title.
- Look for a scale bar. It is the most reliable of the three because it shrinks and grows with the drawing.
- Check the dimension strings along the walls. Printed dimensions are the best way to test any scale.
- Watch for more than one scale on a sheet. Enlarged details often use a different scale from the main plan.
A printed scale note is only true at the paper size the plan was drawn for. If the PDF was resized when it was exported or printed, for example with a "fit to page" setting, the note no longer matches the drawing. A scale bar stays correct after resizing because it was resized along with everything else.
Step 2: Check the scale against something you know
Pick a long dimension printed on the plan, such as the overall length of the building, and measure the same distance on screen using the scale. The two should match. A long dimension is a better test than a short one, because a small error in picking the ends matters less over a long distance.
Then test the other direction too. A scan can be stretched in one direction and still look right in the other. Doors give you a second, rougher check: interior doors are often about 3 ft (900 mm) wide, but they vary, so use a door to spot a scale that is badly wrong, not to set one.
Step 3: Work out what the PDF page stands for
A PDF page has a size on paper. If a sheet is 36 by 24 inches and the scale is 1/4" = 1'-0", each inch of paper stands for 4 ft, so the sheet shows an area 144 ft by 96 ft. On a metric 1:100 drawing, 1 mm on the paper stands for 100 mm in the building, so 10 mm on paper is 1 m. If the page size and the printed building size do not fit together, the scale or the page size has changed.
Step 4: Get the units right
- Convert feet and inches to decimal feet before you do any arithmetic: 6'-8" is 6.667 ft, because 8 inches is 8/12 of a foot.
- Keep one unit all the way through and convert only at the end. 1 ft is 0.3048 m, and 1 sq ft is about 0.0929 sq m.
- Write the unit next to every number. A bare number is how a metre becomes a foot.
Step 5: Say which measurement you took
A room has more than one size. Measured to the inside faces of the walls, it gives the area you can paint or floor. Measured to the centre of the walls, it gives a larger gross area. Neither is wrong, but mixing them is. Pick one for each purpose and note which one you used.
A checklist to run before you trust a number
- The scale matches a long printed dimension, in both directions.
- A door or other standard element has a believable size.
- Room dimensions add up to the overall dimension, allowing for the thickness of the walls.
- The rooms' areas add up to roughly the building's area, and the difference is explained by walls.
- The PDF has not been resized since you checked the scale.
- You have written down the scale, where it came from, and whether you confirmed it.
Do the arithmetic once on a real room
Take a room 12 ft by 10 ft with 8 ft ceilings. The perimeter is 44 ft, so the wall area is 44 × 8 = 352 sq ft before doors and windows. Our free calculators assume 20 sq ft for a door and 15 sq ft for a window, which brings one door and one window down to 317 sq ft of wall. The floor is 120 sq ft. You can try your own room in the drywall calculator, the paint calculator or the baseboard calculator.
How QWIKIT Plans handles the scale
QWIKIT Plans works out the scale from the printed scale note or from the dimension labels, and checks it against door widths. You can set the scale yourself, and every result shows how sure the software is of the scale. The software is not released yet, and nothing in this guide depends on it: the steps above work with any plan and a calculator.