Identify paper size from dimensions in mm or inches

Enter dimensions in millimetres or inches, and it compares them with ISO A, B, and C formats plus widely used North American office sizes. Portrait and landscape measurements are both accepted. A small default tolerance accounts for ordinary measuring and trimming differences, while an optional custom tolerance gives automated workflows explicit control. If no standard has both edges within that limit, the request returns an error instead of presenting a misleading guess.

Measure the sheet and choose the correct unit

Measure the two straight edges of the paper, enter them as width and height, and select either millimetres or inches. The order does not prevent a match: a 297 by 210 mm landscape sheet and a 210 by 297 mm portrait sheet are both compared with the same standard dimensions. Use the actual trimmed sheet rather than the printable area shown by an application or printer driver. Printable areas exclude margins and therefore do not describe the physical paper. The default tolerance is 2 mm for millimetre input and 0.08 inches for inch input, enough to absorb ordinary ruler precision and modest cutting variation without making unrelated formats look interchangeable. Width and height must be positive finite numbers. Decimal values are accepted, which is useful when a ruler, scanner, or production system reports fractional inches. The result preserves the submitted orientation while reporting the matched standard in its conventional portrait dimensions, making the distinction clear for downstream users and systems.

Understand how the closest match is selected

Every submitted pair is converted internally to millimetres and compared against a fixed table of ISO A0 through A10, ISO B0 through B10, ISO C0 through C10, and common North American formats such as Letter, Legal, Tabloid, Executive, Statement, Folio, Junior Legal, and Government Letter. The comparison ignores orientation by sorting the short and long edges. A candidate qualifies only when the absolute error on each edge is no greater than the tolerance. Among qualifying candidates, the algorithm first minimizes the larger edge error and then the combined error, producing a deterministic closest result. The response includes the paper name, family, standard, detected orientation, conventional standard dimensions, edge differences, and effective tolerance. These details make the decision auditable rather than returning only a label. The table and arithmetic are local and fixed: there is no network request, probabilistic classification, changing external catalog, or hidden metadata lookup involved in deciding the answer.

Set tolerance carefully and handle rejected measurements

Tolerance represents the maximum allowed discrepancy for each individual edge, not a percentage and not a combined allowance. A tolerance of 1 mm means both the short-edge difference and long-edge difference must independently be 1 mm or less. Use a tighter value when dimensions originate from reliable design metadata or calibrated production equipment. Use a slightly wider value for hand measurements, scans with uncertain cropping, or paper that may have been trimmed, but keep it small enough to preserve the meaning of a standard-size identification. If no candidate qualifies, the capability returns an invalid-input error stating that no standard lies within the selected tolerance. That failure is intentional: it distinguishes genuinely custom stock, an incorrect unit, a measurement mistake, or excessive trimming from a recognized format. Automated callers should treat the error as a prompt to verify the measurements or route the item as custom paper, rather than silently assigning the numerically nearest standard regardless of distance.