KOREvery quantity in a takeoff goes through the scale. A length that is 4.2% too long gives an area that is 8.5% too big, and the error repeats across the whole sheet. Here's how to read the common scales, convert a measurement taken on paper, spot a plan printed smaller than the original, and check the scale against a dimension before you measure.
What a scale means
A scale tells you what real length a length on paper stands for.
- 1/4" = 1'-0": 1/4" on paper equals 1 ft on site. So 1" on paper equals 4 ft.
- 1:50: 1 unit on paper equals 50 units in reality, whatever the unit. 1 mm equals 50 mm, 1 cm equals 50 cm.
To compare an imperial scale with a metric one, turn it into a ratio:
For an engineering scale such as 1" = 20', the ratio is 20 × 12 = 240, so 1:240.
Common scales
| Scale | Ratio | 1" on paper = | Often used for |
|---|---|---|---|
| 1/16" = 1'-0" | 1:192 | 16'-0" | overall plans |
| 3/32" = 1'-0" | 1:128 | 10'-8" | overall plans |
| 1/8" = 1'-0" | 1:96 | 8'-0" | site plans, elevations |
| 3/16" = 1'-0" | 1:64 | 5'-4" | plans, elevations |
| 1/4" = 1'-0" | 1:48 | 4'-0" | residential floor plans |
| 1/2" = 1'-0" | 1:24 | 2'-0" | wall sections, stairs |
| 1 1/2" = 1'-0" | 1:8 | 8" | details |
| 1" = 20' | 1:240 | 20'-0" | site plan (engineering scale) |
In metric, the math is direct:
| Scale | 1 cm on paper = | Often used for |
|---|---|---|
| 1:20 | 20 cm | details |
| 1:50 | 50 cm | floor plans, sections |
| 1:100 | 1 m | plans, elevations |
| 1:200 | 2 m | site plans |
These uses vary from one office to another. The scale that counts is always the one written on the view.
Converting a measurement taken on paper
- A wall that measures 10" on a 1/4" sheet: 10 × 4 ft = 40'-0".
- 3 5/8" on a 1/8" sheet: 3.625 × 8 ft = 29'-0".
- 136 mm on a 1:50 sheet: 136 × 50 = 6,800 mm = 6.8 m (about 22'-3 3/4").
To go from one system to the other, two exact values are enough: 1 ft = 0.3048 m and 1 in = 25.4 mm. A 40'-0" house is therefore 12.192 m long.
The 1:48 versus 1:50 trap
1/4" = 1'-0" isn't 1:50: it's 1:48. Likewise, 1/8" = 1'-0" is 1:96, not 1:100. If you measure a 1/4" plan with the 1:50 side of a metric scale ruler, every length comes out 4.2% too long: a 40'-0" wall becomes 41'-8". On areas, the gap grows to 8.5%, because it counts twice (length and width).
When the plan is printed smaller
The written scale only holds on the original paper size. The most common sizes:
| Size | Dimensions |
|---|---|
| ARCH D | 24 x 36 in |
| ARCH C | 18 x 24 in |
| Tabloid | 11 x 17 in |
| Letter | 8.5 x 11 in |
| A1 | 594 x 841 mm |
| A3 | 297 x 420 mm |
Take a set drawn on 24 x 36 in, printed or exported to PDF "fit to page" on 11 x 17. The two sides don't shrink the same way: 17 ÷ 36 = 0.472 and 11 ÷ 24 = 0.458. For the whole sheet to fit, the smaller one wins: the drawing is reduced to 45.8%, sometimes a little less if the printer keeps a margin.
What a 40'-0" wall gives on that print:
| Measured at… | Result | Gap |
|---|---|---|
| 1/4" (the written scale) | 18'-4" | −54% |
| 1/8" ("11 x 17 is half size") | 36'-8" | −8.3% |
The second case is the more dangerous one, because the result looks plausible. The true half size of a 24 x 36 is 12 x 18 (ARCH B), reduced to exactly 50%. On 11 x 17, the drawing ends up at about 1:105, which matches no common scale.
In metric, A sizes keep the same proportions: an A1 printed on A3 is reduced to about 50% (420 ÷ 841 = 0.499), and a 1:50 plan becomes roughly a 1:100 plan, as long as the print didn't add margins.
How to spot a reduced print:
- The graphic scale (the small graduated bar) shrinks with the print; the written scale doesn't. At 1/4", the 8' mark should measure 2": if it measures less, the sheet was reduced.
- The PDF page size, shown in the document properties of your PDF reader.
- A written dimension, measured end to end (see below).
Traps on the sheet itself
- Several scales on one sheet. A 1/4" plan, a 1/2" section and 1 1/2" details can share the same sheet. The scale that counts is the one written under the title of the view you're measuring.
- "AS NOTED" in the title block. Each view has its own scale, written under its title.
- "NOT TO SCALE" (N.T.S.). Don't measure that view: use only the written dimensions.
- A detail number right next to the scale. Read as 2 1/8" = 1'-0", it isn't a common scale: it's often detail 2, at 1/8".
- 3/16" and 3/32". They look alike, but one is twice the other (1:64 and 1:128). Reading one for the other doubles or halves every length.
- Scanned plans. A scan can be slightly reduced or distorted, and not always the same way in both directions.
- PDFs exported "fit to page". The PDF itself is no longer at the original size, even if it was never printed.
Many sets also carry a note such as "Do not scale drawings". Measuring to scale is still the usual way to estimate, but a written dimension always wins over a measurement.
Checking the scale against a dimension
Before you measure a sheet, take one check measurement:
- Pick the longest written dimension in the view, for example the overall length of the house.
- Measure it end to end with the scale you think is right.
- Work out the gap: (measured − written) ÷ written.
Why the longest one: a reading error of 1/16" on paper is worth 3" at 1/4". On a 3'-0" door, that's an 8.3% gap; on a 40'-0" dimension, only 0.6%.
Examples on a written dimension of 40'-0":
| Measured | Gap | What it means |
|---|---|---|
| 40'-1" | +0.2% | normal: reading precision |
| 39'-6" | −1.25% | measure again; if the gap stays, recalibrate |
| 38'-0" | −5% | typical of a PDF reduced to 95% |
| 18'-4" | −54% | a 24 x 36 printed on 11 x 17 |
| 80'-0" | +100% | 1/8" scale used on a 1/4" plan |
As a rule of thumb: a few tenths of a percent is the precision of the click or the ruler. Beyond about 1%, measure again zoomed in. If the gap stays, the scale or the print is wrong. Don't fix the quantities by hand with a percentage: recalibrate the sheet on the dimension, then measure.
On a scanned plan, check both directions: one horizontal and one vertical dimension. If they don't give the same gap, the scan is distorted: calibrate on the direction you measure most, or ask for the original PDF.
An unchecked scale is one of the common takeoff mistakes, along with walls measured twice and waste counted twice.
In KOR
In Plans & takeoff, when you import a PDF plan set, KOR reads each sheet's number and title from the title block, and the scale written in the PDF text (1/4" = 1'-0", 1" = 20', 1:50…), among common scales. That scale is always shown "to confirm": you are the one who confirms it. If several scales are written on the sheet, KOR lists them and you pick the one for the view you're measuring. The scale window also shows the sheet's paper size.
To calibrate, click both ends of a known dimension and type its real length (12'-6", 3.81 m or 3810 mm). If the sheet already has a scale, KOR also shows what that dimension measures with it, so the check happens in the same step.
For sheets the text reading couldn't read (missing number, title or scale), or on demand with "Reread with AI", an AI reading also reads one written dimension on the sheet and compares it with the scale. If they don't agree, the scale turns red with "Check manually: measure a known dimension to confirm the scale". The Tape tool measures a distance on the plan without saving anything, handy for checking a dimension. Measurements display in feet-inches or metric, as you prefer.
Summary
- Read the scale under the view's title, not only in the title block.
- Convert with the ratio: 1/4" = 1:48 and 1/8" = 1:96, not 1:50 or 1:100.
- Check the page size: a 24 x 36 printed on 11 x 17 is no longer at the written scale.
- Never measure a view marked "not to scale".
- Measure the longest written dimension; beyond about 1% off, recalibrate.
- On a scanned plan, check both directions.