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Drawing Scale Calculator (Plan to Real Size and Back)

Plan to real size and back at metric, architects' or engineers' scales, and the scale a print is really at from one dimension it states.

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Real size from the drawing, drawn length from the real size, or a check of the print's own scale.

The first two are the same arithmetic in opposite directions: multiply by the scale's number to go from paper to site, divide to go from site to paper. The third works the scale out from a dimension the drawing states, a room width or a grid spacing, measured on the copy in hand, which tells you whether that copy was printed at the size its title block assumes.

The scale in the drawing's title block or under the view's title, or leave it on typed below and give the scale as two lengths.

Metric drawings write a scale as a ratio, 1:50; US architectural drawings as so much of an inch to the foot, 1/4 in = 1 ft; US civil and site drawings as so many feet to the inch, 1 in = 20 ft. The list carries the standard ones of all three. A scale not in it, such as 1:25, 1:75 or 1/16 in = 1 ft, is entered as the two lengths below. In the third option this is the scale the print claims to be at, which the dimension you measure is checked against.

The drawn side of the scale as the drawing writes it: the 1 of 1:50, or the 1/4 in (6.35 mm) of a quarter-inch scale.

A ratio scale puts the same unit on both sides, so 1:50 is 1 mm standing for 50 mm, or 1 in for 50 in; either pair gives the same answer. A US scale names two units: 1/4 in = 1 ft is a quarter of an inch on the drawing standing for a foot on site, and 1 in = 20 ft is an inch standing for twenty feet. While the box is in inches, a fraction can be typed as it is written, 1/4 or 3/32; while it is in millimetres, give the fraction its inch mark, 1/4" or 3/32", because a bare 1/4 there is a quarter of a millimetre.

The real length that drawn length stands for: 50 mm (1.97 in) at 1:50, 1 ft (305 mm) at a quarter-inch scale.

Typed in any length unit the box offers, so 1:1250 can go in as 1 mm and 1250 mm, and 1 in = 20 ft as 1 in and 20 ft. A length in another unit can be typed with its unit, 20' or 1250mm, and is restated in the box's own. The page divides this by the drawn side to get the scale's number, and prints that number in the answer so the pair can be checked.

The length taken off the print with a rule, measured straight between the two points.

Measure with an ordinary rule and let the page apply the scale, or with a scale rule and type what it reads at 1:1. Measure the whole length in one go rather than adding short pieces together, because every reading carries its own error and the scale multiplies each of them. A reading in inches can be typed as a fraction, 3 1/4; while the box is in millimetres, give it the inch mark, 3 1/4".

Tools needed: Rule or scale rule

Answer at this scale

13 ft

High confidence

The length measured on the drawing times the scale's number. The tape reading beside it is the nearest sixteenth of an inch in feet and inches; in metric the real size is exact to the reading it came from.

Scale used
48 (1:48)
Real size, as a tape reading
156 in (13 ft 0 in)
Measured on the drawing
3.25 in
Then change the inputs to see how far the answer moves.

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How this was calculated

Formula source(s)

  • ISO 5455:1979 Technical drawings — Scales (BS EN ISO 5455:1995 in the UK), read in IS 10713:1983, Scales for use on technical drawings, which states it is identical with ISO 5455-1979: recommended scales for enlargement 50:1, 20:1, 10:1, 5:1 and 2:1, full size 1:1, and reduction 1:2, 1:5, 1:10, 1:20, 1:50, 1:100, 1:200, 1:500, 1:1000, 1:2000, 1:5000 and 1:10000; designated SCALE X : 1 for an enlargement and SCALE 1 : X for a reduction; and "The scale of a print may be different from that of the original drawing"
  • Planning Practice Guidance, Making an application, paragraph 024 (Reference ID 14-024-20140306), on location plans: "The scale should typically be 1:1250 or 1:2500, but wherever possible the plan should be scaled to fit onto A4 or A3 size paper."
  • Alvin, Triangular Scale Architect 12" Plastic (model 110P), maker's specification: "Architect scales have the following graduations: 3/32, 3/16, 1/8, 1/4, 1/2, 1, 3/8, 3/4, 1.5, 3 inches to the foot; one edge to 16ths"
  • Alvin, Triangular Scale Engineer 12" Plastic (model 111P), maker's specification: "Engineer Triangular Scale made with 10, 20, 30, 40, 50, 60 parts to the inch"
  • Thomas E. French, A Manual of Engineering Drawing for Students and Draftsmen, 2nd edition (McGraw-Hill, 1918), on scales: for plotting and map drawing, the civil engineers' scale of decimal parts, 10, 20, 30, 40, 50, 60, 80 and 100 to the inch
  • NIST Handbook 44 (2026 edition), Appendix C, for the arithmetic only: 12 inches = 1 foot, so a scale of so much of an inch to the foot is 1 to twelve divided by that much (1/4 in = 1 ft is 1:48), and 1 in = 20 ft is 1:240

Inputs used

What to Work Out
Real size from the drawing
Scale
Typed below as two lengths
Scale: Length on the Drawing
0.25 in
Scale: Real Length It Stands For
1 ft
Length Measured on the Drawing
3.25 in
Real Size
14 ft
A Dimension the Drawing States
12 ft
The Same Dimension Measured on This Print
1.5 in

Intermediate steps

Scale used
48 (1:48)
Real size, as a tape reading
156 in (13 ft 0 in)
Measured on the drawing
3.25 in
Final result13 ft

Confidence note: The length measured on the drawing times the scale's number. The tape reading beside it is the nearest sixteenth of an inch in feet and inches; in metric the real size is exact to the reading it came from.

What this calculation does not cover

  • A length taken off a drawing is only as good as the print it was taken from. Prints and PDFs are often scaled to fit the paper or the screen, so the scale in the title block can be wrong for the copy in hand; check a dimension the drawing states with the third option before taking any other measurement off it.
  • Every reading error is multiplied by the scale. At 1:100 each millimetre misread is 100 mm (3.94 in) on site, and at 1/4 in = 1 ft each 1/16 in misread is 3 in (76 mm). A dimension written on the drawing governs over one measured from it, which is why many title blocks say not to scale from the drawing at all.
  • A scale bar printed on the sheet is enlarged or reduced with the drawing, so measuring against the bar survives a copy at the wrong size where a written scale does not. Paper also moves a little with the weather and the printer, and a long measurement across a sheet carries that too.
  • The scale applies to plans, sections and elevations drawn square-on. A perspective view, a detail marked not to scale, or a detail with its own scale in its title is not measured at the sheet's scale, and a long section drawn with a different horizontal and vertical scale is worked one direction at a time.
3.25 in
Schematic, drawn to the proportions you entered — not to scale on screen.

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-10-05 · v1.0.0

Regulatory standards & verification citations6
  1. ISO 5455:1979 Technical drawings — Scales (BS EN ISO 5455:1995 in the UK), read in IS 10713:1983, Scales for use on technical drawings, which states it is identical with ISO 5455-1979: recommended scales for enlargement 50:1, 20:1, 10:1, 5:1 and 2:1, full size 1:1, and reduction 1:2, 1:5, 1:10, 1:20, 1:50, 1:100, 1:200, 1:500, 1:1000, 1:2000, 1:5000 and 1:10000; designated SCALE X : 1 for an enlargement and SCALE 1 : X for a reduction; and "The scale of a print may be different from that of the original drawing"
  2. Planning Practice Guidance, Making an application, paragraph 024 (Reference ID 14-024-20140306), on location plans: "The scale should typically be 1:1250 or 1:2500, but wherever possible the plan should be scaled to fit onto A4 or A3 size paper."
  3. Alvin, Triangular Scale Architect 12" Plastic (model 110P), maker's specification: "Architect scales have the following graduations: 3/32, 3/16, 1/8, 1/4, 1/2, 1, 3/8, 3/4, 1.5, 3 inches to the foot; one edge to 16ths"
  4. Alvin, Triangular Scale Engineer 12" Plastic (model 111P), maker's specification: "Engineer Triangular Scale made with 10, 20, 30, 40, 50, 60 parts to the inch"
  5. Thomas E. French, A Manual of Engineering Drawing for Students and Draftsmen, 2nd edition (McGraw-Hill, 1918), on scales: for plotting and map drawing, the civil engineers' scale of decimal parts, 10, 20, 30, 40, 50, 60, 80 and 100 to the inch
  6. NIST Handbook 44 (2026 edition), Appendix C, for the arithmetic only: 12 inches = 1 foot, so a scale of so much of an inch to the foot is 1 to twelve divided by that much (1/4 in = 1 ft is 1:48), and 1 in = 20 ft is 1:240

Which documents these citations point at

Standards referenced: ISO 5455, ISO 5455-1979 (International Organization for Standardization, International).

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Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

How to calculate drawing scale (plan to real size and back) in 9 steps

  1. What to Work OutReal size from the drawing, drawn length from the real size, or a check of the print's own scale.
  2. ScaleThe scale in the drawing's title block or under the view's title, or leave it on typed below and give the scale as two lengths.
  3. Scale: Length on the DrawingThe drawn side of the scale as the drawing writes it: the 1 of 1:50, or the 1/4 in (6.35 mm) of a quarter-inch scale.
  4. Scale: Real Length It Stands ForThe real length that drawn length stands for: 50 mm (1.97 in) at 1:50, 1 ft (305 mm) at a quarter-inch scale.
  5. Length Measured on the DrawingThe length taken off the print with a rule, measured straight between the two points.
  6. Real SizeThe real dimension to be drawn, as built or as designed.
  7. A Dimension the Drawing StatesA length written on the drawing in figures: a room width, a grid spacing, an overall dimension.
  8. The Same Dimension Measured on This PrintThat stated dimension, measured end to end on the copy in hand with an ordinary rule.
  9. Answer at this scaleThe tool computes the answer at this scale from those figures and shows the formula, its sources, and a confidence rating alongside it.

Frequently asked questions

What does a scale of 1:50 mean?
One unit on the drawing stands for fifty of the same unit in reality, whatever the unit: 1 mm for 50 mm, 1 in for 50 in. So a wall drawn 85 mm long is 85 × 50 = 4250 mm, 4.25 m, on site. Going the other way, a 4.2 m wall is drawn 4200 ÷ 50 = 84 mm long.
How is 1/4 in = 1 ft written as a ratio?
A foot is twelve inches, so a quarter of an inch standing for a foot is a quarter standing for twelve, which is 1:48. Every architects' scale converts the same way, twelve divided by the fraction: 1/8 in = 1 ft is 1:96, 1/2 in = 1 ft is 1:24, 3 in = 1 ft is 1:4. The ratio form is why 1/4 in = 1 ft is close to, but not the same as, 1:50.
What are the engineers' scales such as 1 in = 20 ft?
They are read off a civil engineer's scale, whose edges are divided into 10, 20, 30, 40, 50 and 60 parts to the inch, and they are used on site plans, roads and surveys. As a ratio, an inch standing for twenty feet is an inch standing for 240 inches, 1:240; 1 in = 10 ft is 1:120 and 1 in = 100 ft is 1:1200.
Which metric scales are standard?
ISO 5455, adopted in the UK as BS EN ISO 5455, recommends 1:2, 1:5, 1:10, 1:20, 1:50, 1:100, 1:200, 1:500, 1:1000, 1:2000, 1:5000 and 1:10000 for reductions. It gives 1:1 for full size and 2:1 up to 50:1 for enlargements; the larger scales carry details and the smaller ones sites. English planning guidance asks for a location plan typically at 1:1250 or 1:2500, which is why both are on the list although neither is in ISO 5455's.
My drawing says 1:50 at A1, but I have an A3 copy. What scale is it?
1:100. Each A size is half the area of the one before it, so its sides are smaller by the square root of two, and two steps down, A1 to A3, halves every length. A 1:50 drawing printed on A3 measures at 1:100, provided it was printed at the full sheet size and not fitted to the page. The third option on this page confirms it from any dimension the drawing states.
Why do drawings say not to scale them?
Because a measured length carries the error of the print, the paper and the rule, multiplied by the scale, while a written dimension carries none of that. Where a figure is written, it governs. Scaling is for the lengths nobody dimensioned, a quick check, or a take-off, and the measurement should be checked against a stated dimension or the scale bar first.
Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.