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This page is an editorial and informational resource about Readable on a Curve: Printing UPC Barcodes and GS1 Data Matrix Codes on Small Round Packaging.
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When to use UPC-A, UPC-E or GS1 DataMatrix on packs with very little flat area.
How to calculate the arc a symbol subtends and the minimum diameter it needs.
Contrast, glare and print gain on tinplate, aluminum, molded plastics and labels.
Grading to ISO/IEC 15416 and 15415 and scan testing formed packs before release.
A barcode that verifies at grade A on a flat proof can fail at the register when it is printed on a tin lid 65 mm across. The geometry is unforgiving: small round packs offer little flat area, the surfaces are often metallic or glossy, and the curvature foreshortens the bars at the edge of the symbol. This guide works through the decisions in the order a prepress team meets them: which symbol to use, how large it must be, where it can sit on a lid, base or side wall, which print process and substrate will hold it, how to manage contrast and print gain, what to put inside a GS1 DataMatrix, and how to verify the result before a full run. The figures are taken from GS1 and ISO/IEC practice, and the curvature numbers are working rules of thumb rather than formal limits.
Most barcode guidance assumes a carton face or a flat label. Small round packaging breaks that assumption in three ways at once. First, the usable area is small: a shallow tin of 60 to 75 mm diameter may have a side wall only 15 to 25 mm tall, and its lid usually carries an embossed ring, a rolled rim and a brand panel competing for the same space. Second, every surface except the center of the lid is curved, and some lids are slightly domed as well. Third, the materials common at this scale, such as tinplate, bare or lacquered aluminum, and molded polypropylene, are either reflective or translucent, which works against the dark-on-light contrast a scanner needs.
Retail scanners read a linear symbol by measuring the relative widths of bars and spaces along a scan line. Anything that changes those widths between the left and right edge of the symbol, such as curvature, ink spread or glare, eats into the decode margin. A 2D imager reading a GS1 DataMatrix is more tolerant of orientation but still needs a clean module grid, a clear finder pattern and a quiet zone that is not interrupted by a seam or an emboss.
The practical consequence is that the barcode cannot be the last element placed on the artwork. On small round packs it has to be planned with the structure: which face, which orientation, which process and which magnification are fixed early, and the decorative design is built around them.
For goods sold through US retail point of sale, the default symbol is UPC-A, which carries a 12-digit GTIN. Its main drawback on small packs is width: at 100% magnification the full symbol, including quiet zones, is 37.29 mm wide and the bars are 22.85 mm tall.
UPC-E is the zero-suppressed version. It encodes a GTIN-12 that contains enough zeros to be compressed into six data digits, and at 100% it is about 22.11 mm wide including quiet zones. It exists precisely for small packages, but only certain number ranges can be zero-suppressed, so the GTIN has to be allocated with UPC-E in mind. A company prefix that does not allow suppression cannot produce a UPC-E, however small the pack.
GS1 DataMatrix is a 2D matrix symbol that starts with the FNC1 character and carries GS1 Application Identifier data. A GTIN-only DataMatrix can be under 10 mm square. Its advantages on curved surfaces are significant: it is read by imagers in any orientation, it carries Reed-Solomon error correction, and its compact footprint means it subtends a smaller arc on a cylinder. The constraint is the scanner base. GS1 US and retailers are moving point of sale toward 2D under the program often called Sunrise 2027, but many lanes still rely on linear symbols, so a common transitional approach is to carry both a UPC and a GS1 DataMatrix, or a UPC alongside a GS1 Digital Link QR code.
The X-dimension is the nominal width of the narrowest bar or space, or of one module in a matrix symbol. For UPC-A and UPC-E the X-dimension at 100% magnification is 0.33 mm (0.013 in). GS1 permits a magnification range of 80% to 200% for retail point of sale, which corresponds to an X-dimension of 0.264 mm to 0.660 mm. Below 80%, scan performance falls off quickly because the bars approach the resolution of the scanner and the variation of the print process.
Bar height is part of the specification too. Truncating the bars, that is, shortening their height below the nominal value for the chosen magnification, reduces the ability of omnidirectional scanners to find the symbol from different angles. GS1 discourages truncation for symbols scanned at retail point of sale, and on small round packs the temptation to truncate is high, so the question deserves an explicit decision rather than a silent edit in the artwork file.
For GS1 DataMatrix scanned at general retail point of sale, the GS1 General Specifications set a minimum, target and maximum X-dimension; in recent editions the minimum is in the region of 0.375 mm and the target around 0.625 mm, but the tables are revised and should be checked against the current edition before artwork is frozen. Each additional row and column of the matrix adds one X-dimension to the overall size, so the data content directly drives the footprint.
Quiet zones are not optional margins. UPC-A requires 9X on both sides. UPC-E requires 9X on the left and 7X on the right. GS1 DataMatrix requires a quiet zone of 1X on all four sides, and many printers use 2X to 3X on metal to absorb registration drift. A quiet zone that runs into an emboss shoulder or a lid rim is a quiet zone that has effectively been lost.
When a linear symbol wraps around a cylinder, the scanner sees the bars near the edges at an angle. A bar that sits at angle φ from the line of sight appears narrower by a factor of cos φ. If the full symbol, including quiet zones, subtends an arc of θ, the outermost elements sit at θ/2 from the center. At a subtended arc of 30 degrees the edge elements look about 3.4% narrower; at 45 degrees about 7.6%; at 60 degrees about 13.4%. Because a UPC is decoded from ratios of widths, the error builds toward both edges, where the guard bars and the first data characters sit.
The arc angle in radians is the symbol width divided by the radius. Rearranged, the smallest diameter that keeps a given symbol within an arc of θ is D = 2W/θ. With θ set to 30 degrees, the minimum diameter is about 3.82 times the symbol width; at 60 degrees it is about 1.91 times. The table below applies that rule to common symbols. Treat 30 degrees as a comfortable working target and 60 degrees as a ceiling that requires test scans on the real part.
| Symbol and size | X-dimension | Width incl. quiet zones | Min. diameter, 30° arc | Min. diameter, 60° arc |
|---|---|---|---|---|
| UPC-A at 100% | 0.330 mm | 37.29 mm | 142 mm | 71 mm |
| UPC-A at 80% | 0.264 mm | 29.83 mm | 114 mm | 57 mm |
| UPC-E at 100% | 0.330 mm | 22.11 mm | 85 mm | 42 mm |
| UPC-E at 80% | 0.264 mm | 17.69 mm | 68 mm | 34 mm |
| GS1 DataMatrix 16x16 | 0.500 mm | 9.0 mm | 34 mm | 17 mm |
| GS1 DataMatrix 20x20 | 0.500 mm | 11.0 mm | 42 mm | 21 mm |
The numbers make the core problem plain. A UPC-A wrapped around the side of a 65 mm tin with bars parallel to the axis is beyond even the generous ceiling. A GS1 DataMatrix at 0.5 mm, by contrast, sits comfortably on almost any small container. For a 2D symbol the concern is less foreshortening than module distortion: on tight radii, modules at the edge of the matrix print and image as rectangles, which the imager tolerates up to a point, so keeping the symbol near the top of the curve and lighting it diffusely helps.
Orientation on a cylinder is described with the same terms used for flat packs. In picket fence orientation the bars run parallel to the axis of the cylinder, so the width of the symbol wraps around the curve. In ladder orientation the bars run around the circumference and the symbol width runs along the axis, which keeps the scan direction flat. On small diameters ladder orientation is almost always the better choice for the side wall, because the curvature now affects only bar length, which the scanner does not measure.
The limitation of ladder orientation on shallow round packs is wall height. A UPC-E at 80% needs about 17.7 mm of straight wall along the axis, plus clearance from the rolled rim or thread and any bead. On a tin with a 20 mm wall and a 1.5 mm curl, that is a very tight fit, and register tolerance on a curved metal body can consume the remainder.
The flat center of a lid is usually the best-behaved surface on a small round pack. Before placing a symbol there, measure the true flat zone, not the outer diameter: a 70 mm lid with a 4 mm rim shoulder and a 3 mm embossed ring may offer a flat disc only 50 to 55 mm across. Domed lids, common on aluminum, can have a rise of 0.5 to 1.5 mm across the panel, enough to tilt the outer bars of a UPC-A. Center the symbol on the axis of the dome where possible.
Bases are attractive because they are hidden from the brand panel, but they often carry a recessed panel, a stacking bead or a molded gate mark on plastic parts. The quiet zones must stay clear of all three. Bases also take scuffing when packs slide across shelves and conveyors, so ink rub resistance matters more here than anywhere else on the pack.
A printed band or label around the side wall changes the substrate but not the geometry: the band is still curved. It does, however, allow a white or matte background on a metallic body and moves printing to flexographic or digital processes with well-understood bar width control.
The same symbol will behave differently depending on the material and the process. Metal decorating by sheet-fed offset lithography prints flat sheets that are later formed, so the artwork must be distorted in prepress to compensate for draw and stretch on drawn lids and bodies. Dry offset is used on preformed two-piece containers and transfers all colors from a single blanket, which limits trapping and fine registration. Pad printing reaches curved and recessed areas but has modest resolution. Digital inkjet on preformed parts is increasingly common for variable data such as lot and date, and laser marking on anodized or coated aluminum produces permanent marks without ink.
| Substrate and process | Typical risk to the symbol | Mitigation |
|---|---|---|
| Tinplate, sheet-fed offset, oven-baked inks | Distortion from forming; gloss varnish glare | Distortion-corrected artwork; white base coat; matte overprint in the symbol area |
| Aluminum, dry offset on formed body | Specular metal showing through spaces; limited register | Opaque white under the symbol; single-color black bars; generous quiet zones |
| Molded polypropylene, in-mold label | Label shift during molding; slight shrink | Place symbol away from the gate; allow 1 to 2 mm extra margin |
| Molded polypropylene, pad print | Ink spread on low-energy surface; soft edges | Flame or corona treatment; increase X-dimension; prefer DataMatrix |
| Paper or film label, flexography | Print gain across the web; plate swell | Bar width reduction tuned to press fingerprint; bars in machine direction |
| Coated aluminum, laser marking | Low contrast on light coatings; heat discoloration | Dark coating ablated to a light base, or tuned fiber laser parameters; verify contrast |
| Any substrate, thermal inkjet for variable data | Dot gain, satellites, uneven drying on metal | Primer or receptive coating; DataMatrix with dot-peen style module spacing |
On flexographic labels, run the bars in the machine direction of the web when possible. Bars that run across the web are more prone to slur and gain because each bar edge meets the impression nip along its full length. On offset metal, ask the decorator for a bar width reduction value measured on their press with their inks, not a generic figure.
Point of sale laser scanners and most imagers illuminate at or near 660 nm, in the red part of the spectrum. That single fact explains most color failures. Red, orange and warm pink bars reflect strongly at 660 nm and therefore look almost white to the scanner. Dark blue, dark green, black and dark brown bars absorb red light and print well. Backgrounds can be white, light yellow, light orange or light pink, because they all reflect red light; light blue and light green backgrounds are risky because they absorb part of it.
ISO/IEC 15416 grades a linear symbol on parameters including symbol contrast (the difference between the highest and lowest reflectance in the scan profile), minimum edge contrast, modulation (how consistent the contrast is across narrow and wide elements) and defects such as voids and specks. On metal, the danger is the space color. Bare aluminum or tinplate is specular: at some angles it bounces the light straight back into the scanner and looks brilliantly white, at others it reflects away and looks black. A space that changes from white to black as the pack tilts will fail modulation and edge contrast. The standard fix is not exotic: an opaque white ground printed under the entire symbol and its quiet zones.
Finish matters as much as color. High-gloss varnish produces hot spots, especially under the point light of a handheld imager. A matte or satin overprint in a window around the symbol reduces specular reflection without changing the bar colors. Where the decorative design calls for a full metallic look, a rectangular white panel with matte overprint is the usual compromise. Process color builds for the bars, made of overlapping halftone dots, should be avoided; print bars in a single solid ink so the edges are sharp and the contrast is consistent.
Every print process spreads ink to some degree. On flexography and dry offset the spread can add 0.03 to 0.08 mm to each bar; on pad printing and inkjet on low-energy plastics it can be more. Because spaces shrink by the same amount bars grow, uncorrected gain shifts the bar and space widths in opposite directions and the symbol drifts out of tolerance.
The correction is bar width reduction (BWR): each bar in the artwork is made narrower by a fixed amount so that after gain it prints at its nominal width. BWR is applied as an absolute value, not a percentage, because gain is a property of the press, plate and ink rather than of the symbol size. A value of 0.05 mm that is harmless on a 100% UPC-A becomes a large fraction of the X-dimension on an 80% symbol, which is one more reason not to shrink symbols below what the press can hold.
Determining BWR properly involves printing a fingerprint target, a set of test symbols at known magnifications, on the production substrate under production conditions, then measuring the printed bar widths with a verifier. The difference between nominal and printed widths gives the gain, and the BWR is set to match it. For 2D symbols the equivalent idea is module size compensation: inkjet and laser systems often allow dot size or spot size adjustment so that adjacent dark modules do not merge.
Barcode generation software and prepress RIPs handle BWR automatically when told the value, but the setting is easy to lose when artwork moves between agencies, decorators and label converters. Record the BWR on the artwork specification sheet alongside the magnification, orientation and quiet zones, and check it on every reprint.
A GS1 DataMatrix encodes element strings: an Application Identifier followed by its data. The most common on consumer goods are (01) for the GTIN-14, (10) for batch or lot, (17) for expiry date in YYMMDD form, (15) for a best-before date and (21) for a serial number. The symbol begins with FNC1 to signal GS1 syntax, and variable-length fields such as batch must be terminated with a separator, FNC1 in data, unless they are the last element string in the symbol. A practical habit is to place fixed-length fields first and the variable-length fields last.
Content drives size. Digits are packed two per codeword, so (01) plus 14 digits needs eight codewords, plus one for the leading FNC1. A 14x14 matrix holds eight data codewords and a 16x16 holds twelve, so a GTIN-only symbol typically lands at 16x16. Adding a six-character alphanumeric batch and an expiry date commonly pushes the symbol to 18x18 or 20x20. At an X-dimension of 0.5 mm, that is the difference between a 9 mm and an 11 mm footprint including quiet zones, which is small on a lid but may matter on a narrow side wall.
Rectangular formats, including the extended rectangular sizes sometimes labeled DMRE, can fit long thin bands such as 8x32 or 12x26 modules. Before using them, confirm that the scanners at the receiving end support the specific rectangular sizes chosen.
When batch and date change during a run, the DataMatrix is usually applied as variable data by inkjet or laser, while the UPC is printed with the static decoration. In that case the two processes have different contrast and gain behavior, and each symbol needs its own verification. Keep the GTIN in both symbols identical; a mismatch between the linear and the 2D GTIN is one of the faults retailers flag most often.
Scanning a symbol with a phone or a single handheld scanner shows that it can be read once; it does not show that it will read reliably across thousands of scanners. Verification against ISO/IEC 15416 for linear symbols and ISO/IEC 15415 for 2D symbols measures print quality with a calibrated instrument and produces a grade from 4.0 (A) down to 0 (F), reported with the measuring aperture and light wavelength. For UPC at retail point of sale, GS1 specifies a minimum grade of 1.5/06/660: overall grade 1.5 (C), aperture 0.150 mm (6 mil) and 660 nm light. GS1 DataMatrix grades are specified in a similar form, with the aperture tied to the X-dimension.
Both ISO methods assume a flat symbol. A verifier pressed against a curved lid or wall will report degraded results that reflect the geometry rather than the print. The established practice is therefore twofold:
Record the verifier report for each artwork version and each production site. Typical failure codes point to causes directly: low symbol contrast suggests the background or bar color; poor modulation suggests gain or glare; decode failures at the edges suggest curvature; quiet zone errors suggest emboss or rim interference. A verification habit built at the first proof stage is far cheaper than a retailer chargeback for unscannable stock.
Use this list before artwork is released and again at the press check. Each item is something that routinely goes wrong on small round packs.
Most scan problems on small round packs fall into a handful of repeating patterns, and recognizing them saves time at the press check.
The symbol scans when the pack faces the scanner squarely, but fails when it is rotated slightly. This is the signature of curvature on a picket fence symbol: as the pack turns, one edge moves further around the curve and the outer characters fall out of tolerance. Rotate to ladder orientation, move to a flatter face or reduce symbol width.
The symbol verifies well on the flat sheet but grades poorly on the formed part even in the flat zone. Forming has stretched the print. Drawn lids pull the artwork outward from the center, and if the distortion compensation was built for a different tool, bars and spaces change width unevenly. Request a formed sample from the actual tooling before approving.
The symbol reads under some lights and not others. Glare from gloss varnish or bare metal is the usual cause. Handheld imagers with a narrow, bright illuminator are most affected. A matte window and a white ground resolve most cases.
The DataMatrix reads on one scanner and not another. Check whether the scanner supports the chosen rectangular size, and whether the FNC1 and separators are correctly placed; a symbol that is a valid DataMatrix but not valid GS1 syntax may be rejected by retail systems that expect parsed Application Identifiers.
Grades drift down over a long run. Plates wear, blankets swell and inkjet heads clog. Build sampling into the run, for example one verified sample at start, middle and end of each lot, so that drift is caught before the whole batch is packed.
Handled early, the curve is a design constraint like any other. Size the symbol to the diameter, put it where the surface is flattest, give it a clean white ground, and measure the result with an instrument rather than a single successful scan.
Who publishes this resource, why it exists, and how to reach the editor with a correction or a question about the guide.
Snuio publishes technical notes on the printing and decoration of small consumer packaging, written for prepress staff, decorators and packaging engineers. The guides favor measurable criteria, standard terms and worked numbers over opinion, and they point readers to the current GS1 and ISO/IEC documents as the final authority.