When a plan is marked with a notation such as "NTS" (not to scale), "approximate," or "diagrammatic," it means the drawing gives approximate positions and sizes only, so you cannot scale distances or dimensions from the sheet. The mark is a communication contract: it tells you the geometry shown is illustrative, and the controlling dimensions live in written dimensions, schedules, or referenced documents rather than in measured line lengths.
The marking matters because construction, fabrication, and permitting all depend on which numbers are authoritative. A drawing set that mixes scaled and non-scaled views is normal, but a single view that is both marked approximate and used as a measuring surface creates a predictable failure mode. The sections below explain the mechanism behind scale, how approximate notations propagate through a drawing set, where the practice breaks down, and what a reader should do when they encounter the marking.
What the marking changes about the drawing
A technical drawing encodes geometry two ways. One is the drawn line itself, whose length on paper corresponds to a real length through a stated ratio. The other is the written dimension, a numeral placed near the geometry with extension lines and arrowheads. When a view is marked approximate or not to scale, the drawn line stops being evidence. The written dimension, or a referenced schedule, becomes the only reliable statement of size.
The mechanism is straightforward once you separate the two channels. Paper shrinks and stretches with humidity and printing, and a scan or photocopy introduces its own distortion. Even a born-digital file displayed at the wrong zoom changes apparent lengths. A stated scale such as 1/4 inch equals 1 foot survives mild paper movement because the tolerance is absorbed at the job site, but a view explicitly marked as approximate removes that assumption entirely.
So the notation is a claim about authority, not about precision of drafting. The drafter is telling you the geometry is a diagram of relationships, not a measurable record. Positions and sizes are shown to convey intent and topology, and the numbers elsewhere govern.
Why scale and measurement diverge
The counterintuitive detail is that a drawing can be perfectly accurate and still not be measurable. Accuracy describes whether the drawn geometry matches the design intent. Measurability describes whether you may convert paper distance into real distance. A view can be accurate, correctly drafted, and still marked approximate because the drafter knows the view will be reproduced at unknown sizes or used only to show arrangement.
This is why "do not scale drawing" appears on sheet borders even when a scale bar is printed. The scale bar is a convenience for rough visualization, not a license to measure. Most professional drafting standards treat scaled measurement as a check or a rough estimate, never as a substitute for a written dimension. When the author goes further and marks the view approximate, the drawing is signaling that even the rough check is unreliable for that view.
A professional caveat follows. On many projects the general notes say not to scale the drawings while individual views carry stated scales, and the two instructions coexist because they govern different readers. The contractor may scale for layout planning, but fabrication dimensions come from the dimension strings and schedules. Reconciling the two instructions means reading the sheet notes as a hierarchy: written dimensions override scaled measurement, and an approximate marking overrides both for that view.
Where approximate views appear in a set
Approximate or diagrammatic views cluster in predictable places. They appear in schematic diagrams for plumbing, electrical, and controls, where the point is connectivity rather than geometry. They appear in site plans and vicinity maps where a small-scale reproduction makes measurement meaningless. They appear in detail bubbles and key plans that only locate a detail, not size it.
They also appear in as-built and record drawings more often than teams expect. A record drawing may show a revised condition without resurveying the space, so the revision is diagrammatic and the original dimensions are retained elsewhere. The same logic applies to coordination views exported from a building model at a reduced scale for a meeting.
| View type | Typical marking | What governs size | Common mistake |
|---|---|---|---|
| Schematic (plumbing, electrical, controls) | Diagrammatic, not to scale | Equipment schedules, riser diagrams | Scaling pipe or conduit runs for material takeoff |
| Site plan at small scale | Approximate, vicinity map | Survey data, dimension strings | Measuring setbacks from a reduced plot |
| Detail key plan | Not to scale, approximate | The referenced detail sheet | Sizing a component from the key plan |
| Coordination or meeting view | Not to scale | The model or dimensioned sheets | Treating an exported view as a record |
| Record or as-built sheet | Approximate where revised | Original dimensions plus field survey | Assuming revisions were resurveyed |
| Marketing or permit illustration | Approximate | The stamped drawing set | Relying on an unstamped illustration |
The table is a reading aid, not a code citation. Local drafting standards and project specifications vary, and the sheet notes are the authority for any given project.
Misconceptions that cause errors
The first misconception is that "not to scale" means the drawing is wrong. It does not. It means the drawing is not a measuring instrument for that view. The geometry can be entirely correct while the scale is unstated, variable, or deliberately suppressed.
The second is that a printed scale bar restores measurability. A scale bar only supports approximate checking, and only if the reproduction is faithful. A scan that compressed the image, or a print at reduced size, invalidates the bar along with the drawing.
The third is that approximate views are low-priority. In practice they often carry the coordination intent for an entire system, and misreading them as measurable invites takeoff errors that surface late. The fourth is that digital files are immune. A PDF exported at "fit to page" has no stable scale at all, and on-screen measurement tools inherit whatever distortion the export introduced.
Practical actions for readers and reviewers
Treat the notation as a routing instruction: find where the authoritative number lives before you use the geometry for anything.
Locate the written dimension, schedule, or referenced detail that controls each size before measuring anything. Check the sheet notes for a general "do not scale" instruction and apply it to every view that lacks a stated, reproducible scale. Verify the scale bar against a known written dimension if you must check a view, and treat any mismatch as a stop signal. Confirm the file was exported at full size; a "fit to page" setting removes scale entirely. Flag any approximate view used for material takeoff, fabrication, or permit submission for review by the responsible professional. Red flag: a dimension string that conflicts with a scaled measurement on the same view, because the conflict means one of them is stale.
The ordering matters. Written dimensions and schedules outrank scaled measurement, and an approximate marking outranks both for that view. If the conflict cannot be resolved from the sheet, the escalation is a request for information to the designer, not an assumption.
Limits of what the drawing can tell you
Scale and notation cannot substitute for verification of the built condition. Existing conditions shown on a plan are frequently approximate because the survey may predate renovations, and the drawing may not have been updated. A dimension on a drawing describes design intent at a point in time, not the field condition at the moment of construction.
The same limit applies to interpretation. Drafting conventions differ between disciplines and between offices within a discipline, and the marking that means approximate on one sheet may be used loosely on another. The only reliable reading comes from the project's own notes and specifications.
Beyond that, the drawing set cannot resolve questions it does not address. If a view is marked approximate and no written dimension or schedule exists for a given element, the correct conclusion is that the size is undetermined by that document. Assuming a measured value in that situation is inventing information the author did not provide.
[This is general educational information about reading technical drawings, not a professional engineering, architectural, or legal opinion. Drawing conventions, codes, and project specifications vary, so consult a qualified local design professional or the authority having jurisdiction for decisions about a specific project. For immediate safety concerns on a job site, contact local emergency services.]