PPI and DPI describe different layers
PPI means pixels per inch and is useful for describing how source image pixels are distributed across a physical print. DPI means dots per inch and is often used for printer dot placement. In everyday conversation people use DPI for both, but separating them avoids misleading calculations.
Actual Size Lab labels the calculated source-image figure as effective PPI. It does not convert that number into a printer hardware setting.
The print-size formula
A 3000-pixel-wide image printed at 300 PPI is 10 inches, or 254 mm, wide. The same image at 150 PPI is 20 inches wide but contains half as many source pixels per printed inch.
Why one quality threshold is not universal
Higher source PPI generally preserves finer detail at the same viewing distance, but perceived quality also depends on image sharpness, subject matter, printer, paper, viewing distance and eyesight. A poster viewed from across a room can work at a lower PPI than a small print examined closely.
The tool uses broad messages rather than a guarantee: below 150 PPI is flagged as low for close viewing; 150–299 PPI may be usable; around 300 PPI and above is a common close-view target. These are practical categories, not standards.
Printer DPI is not a required image PPI
A printer advertised at 1200 × 1200 DPI does not require a 1200 PPI photo. Printers may use multiple dots to reproduce one colour tone, and their driver performs its own rasterisation. Feed the print workflow a suitable image, choose the intended physical size and let the driver handle the device-specific dot pattern.
Why screen size is unreliable
CSS defines a fixed mathematical relationship among CSS units, including 96 CSS pixels per inch, but a CSS inch is not guaranteed to match a ruler on every screen. Device scaling and viewing conditions matter. That is why Actual Size Lab labels the intended output and uses an A4 PDF rather than claiming the browser preview is physically calibrated.
Technical reference: W3C CSS Values and Units.
Worked examples
| Source width | Effective PPI | Print width |
|---|---|---|
| 1200 px | 300 PPI | 4 in / 101.6 mm |
| 2400 px | 300 PPI | 8 in / 203.2 mm |
| 3000 px | 240 PPI | 12.5 in / 317.5 mm |
| 6000 px | 300 PPI | 20 in / 508 mm |