POD Print File Checker

Reading the result, check by check

Up to nine rows come back for every file. Five can stop an upload, one can only ever nag you, and three never judge anything. Before any of them the image is decoded onto a canvas, which browsers cap at 16,384 px a side and 268,435,456 px of area — past either ceiling the tool refuses the file rather than reporting on the blank pixels a clamped canvas returns.

File format

Read from the first bytes of the file, never from the extension or the type the browser reports: 89 50 4E 47 is PNG, FF D8 FF is JPEG, and GIF and WebP are recognised only so the failure can name them. Amazon Merch on Demand passes PNG alone; the other three pass PNG or JPEG. A fail here means re-exporting — renaming a .jpg to .png changes nothing, because the check never looks at the name. Printful and Printify also accept SVG, which this tool cannot inspect, so an SVG is out of scope rather than rejected.

Pixel dimensions

Amazon fails anything under 486 × 486 px (the documentation says "greater than 485 × 485") and warns — not fails — when the long edge is under 4,500 px. Printful fails over 20,000 px on either side, Redbubble over 13,500 px. Printify has no platform-wide cap here at all. Read a pass on Printful, Printify or Redbubble narrowly: it means you are inside the platform's outer limits, not that you have met the minimum the specific product demands, which those platforms set per print area.

Template match

Amazon only, and purely informational. Your dimensions are compared for exact equality against six sizes: 4500 × 5400 (t-shirt and all products), 4500 × 4050 (hoodie), 1800 × 3200 (phone case), 1200 × 1200 (polo and backpack), 1500 × 675 (trucker and baseball hat) and 485 × 485 (PopSocket). One pixel off and the row reads "Not a standard template size", which is not a fail and never becomes one. It tells you which template you happen to be sitting on; it does not push you onto one.

File size

Compared strictly below the cap, in binary megabytes: 26,214,400 bytes for Amazon's 25 MB, 209,715,200 for Printful's 200 MB, 104,857,600 for Printify's 100 MB and 314,572,800 for Redbubble's 300 MB. This is the failure that has nothing to do with your design and everything to do with your export settings, and it is the one the Auto-fix panel can actually solve on its own.

Alpha channel

A single walk over the decoded pixels looking for any alpha below 255. The walk uses a stride of ceil(sqrt(pixels / 4,000,000)), so files up to four megapixels are read pixel by pixel and larger ones are sampled on a grid — a 4500 × 5400 print file gets a stride of 3 (2.7 million of its 24.3 million pixels), a 13,500 px square gets a stride of 7. A JPEG has no alpha channel to find, which fails on Amazon and warns elsewhere. A PNG in which every pixel is opaque is the export that was flattened onto a white layer on the way out of your editor; the fix is in the editor, not here.

Background

Separate from the alpha check, and deliberately gentler. The tool averages alpha over an 8 × 8 patch in each of the four corners (the whole image, if it is smaller than 8 px). All four averaging exactly zero passes. A highest corner average below 16 of 255 warns about a stray low-opacity layer — faint enough to look like nothing on screen, dense enough to print as a haze on a dark garment. Anything at 16 or above warns too, but never fails: a design that genuinely is a full-bleed rectangle also has opaque corners, and only you know which of the two you made.

Trimming

Amazon and PNG only, and the one row that can warn but never fail. It takes the bounding box of every pixel with alpha above zero and warns when the canvas is more than 2 px wider or taller than that box, reporting the empty share as a percentage. One caveat worth knowing: when the alpha walk used a stride above 1, the box is deliberately grown outward by (stride − 1) on each side, so on a large file the reported artwork box is a slight over-estimate. A few extra transparent pixels are harmless; an auto-trim that clips your design is not.

DPI metadata

Reported, never judged. For a PNG the tool walks the chunks to pHYs and only trusts it when the unit byte is 1 (pixels per metre), converting with round(ppuX × 0.0254); a unit byte of 0 means the file declared an aspect ratio rather than a density, and nothing is reported. For a JPEG it reads the JFIF APP0 density: units 1 is dpi as written, units 2 is dots per centimetre multiplied by 2.54, units 0 gives nothing. "Not declared" is the normal, harmless answer for most exporters.

Colour profile

Always "CAN'T CHECK", on every file, on every platform. The canvas hands JavaScript pixels the browser has already colour-managed, so an untagged file and an sRGB-tagged file arrive identical. The tool cannot see the difference and will not pretend it can.

What Auto-fix actually changes

The three fixes run in a fixed order, and the order matters. Trimming happens first, cropping the canvas down to the alpha bounding box found during the check. Then "Fit onto a canvas" centres that result on the exact template you picked (or a custom size you type as 4500x5400), scaled by min(target width ÷ width, target height ÷ height, 1) and padded with transparency. Last, the file is re-encoded as PNG; if it still misses the size limit, the tool loops — multiplying the scale by max(0.6, sqrt(limit ÷ current size) × 0.92) each round and giving up at a tenth of the original — until the blob fits. The finished file is then put back through exactly the same inspection routine as your original, so the report you see afterwards describes the fixed file as measured, not as intended. It downloads as yourfilename-platform-fixed.png.

What it leaves alone is the more useful half. It performs no colour conversion, so an untagged or CMYK-authored file comes out as untagged as it went in. It drops DPI metadata entirely — canvas output carries no pHYs chunk — so a file that declared 300 DPI before a fix reads "Not declared" after one; harmless for POD, but surprising. It cannot invent transparency: a design flattened onto white goes in opaque and comes out opaque, only now in a differently sized frame. And it never enlarges anything, which is where sellers most often mistake a fix for a repair. Fitting a 1,200 px design onto Amazon's 4500 × 5400 canvas pads it out to 4500 × 5400 and the dimensions row turns green — but the artwork is still 1,200 px of real detail in the middle of a large transparent rectangle, and it prints at whatever size 1,200 px supports. Passing the uploader's arithmetic and having enough resolution to print well are two different things, and only one of them can be fixed after the fact.

Related tools