Print-on-demand image size: pixels, DPI, and the requirement that wins

The product template beats a universal DPI rule

Search results often collapse print preparation into one sentence: use 300 DPI. That is incomplete. A provider may accept a lower effective resolution for a large-format product, require a fixed canvas for a particular garment, or publish different rules for each print area.

The reliable order is product first, template second, pixels third. Open the provider's current file-guideline panel or official template, note the required canvas and format, and prepare the file for that target. Recheck the requirement when you change products because a shirt front, mug wrap, blanket, and phone case do not share one print area.

Calculate effective resolution from real pixels

For a raster file, divide pixel width by the intended printed width in inches. A 4500-pixel-wide file printed 15 inches wide supplies 300 pixels per inch. The same file stretched to 30 inches supplies 150 pixels per inch. No metadata edit is involved; the pixel count and physical size do the work.

This calculation also exposes fake fixes. A 1200-pixel image labeled 300 DPI still contains only 1200 pixels. At 12 inches wide it supplies 100 pixels per inch. Resaving it with a different DPI label may change the size an editor proposes, but it does not restore missing edges, lettering, or texture.

• 4500 px ÷ 15 in = 300 effective pixels per inch
• 4500 px ÷ 30 in = 150 effective pixels per inch
• 1200 px ÷ 12 in = 100 effective pixels per inch

Official provider examples show why the answer varies

Amazon Merch on Demand's official product templates specify a 4500 × 5400 pixel PNG with transparency and a file size below 25 MB for the referenced apparel template. That is a concrete canvas requirement, not a generic suggestion.

Printify recommends 300 DPI for JPEG and PNG files in general, while noting that some large products use 120–150 DPI. Printful's current print-file guide describes 150 DPI as a common target and a 150–300 DPI range for larger prints. The apparent disagreement is the lesson: use the current requirement for the product and provider in front of you.

A practical preflight before upload

Check the image at 100% zoom, then preview it at the final product size. Pay special attention to small type, thin strokes, hard diagonals, and soft transparency because these expose weak files faster than a smooth photograph does.

Use the provider's mockup as a placement check, not as proof of print quality. A small browser preview can hide halos and interpolation artifacts. Keep the original source, export a target-specific copy, and order a sample when the product or artwork matters enough that a screen preview is not sufficient evidence.

• Confirm the exact print-area template and accepted format.
• Calculate effective pixels per inch at the intended size.
• Inspect edges and lettering at 100% zoom.
• Preview every placement and color variant you intend to sell.
• Keep a source file separate from provider-specific exports.