Compare the dimensions before choosing a preset
Use the units written on the order, paper packaging, or frame specification. Multiplying inches by 25.4 gives millimetres exactly. A 4 inch short edge becomes 101.6 mm and a 6 inch long edge becomes 152.4 mm. The inch print is therefore 1.6 mm wider and 2.4 mm longer than the metric rectangle. Rotating either print changes its orientation, not these dimensions.
| Size | Exact dimensions | Shape |
|---|---|---|
| 4 × 6 inches | 101.6 × 152.4 mm | 2:3 |
| 10 × 15 centimetres | 100 × 150 mm | 2:3 |
| Difference on each dimension | 1.6 mm short edge; 2.4 mm long edge | Same proportions |
Choose pixels for the physical size you mean
At 300 PPI, 4 × 6 inches requires 1200 × 1800 pixels. An exact 10 × 15 cm rectangle requires about 1181.10 × 1771.65 pixels before rounding. Round each dimension up when asking for a minimum pixel count: 1182 × 1772 pixels. These numbers are a density calculation; they do not assess focus, compression, paper, or the printer.
A 1200 × 1800 pixel file printed at exactly 10 × 15 cm has 304.8 PPI in both directions. No extra pixels are necessary to make that smaller print. Conversely, changing a resolution label alone does not create detail. Use the image you will actually send to the lab, including any editing or crop.
The size difference does not create a new aspect-ratio crop
Both rectangles have the same 2:3 ratio. A 2:3 image can fill either one without a geometric crop, provided the lab does not apply extra borderless enlargement or trimming. Moving between these sizes is uniform scaling, not stretching. Reducing a 4 × 6 layout to 10 × 15 cm uses approximately 98.43% scale.
A 4:3 phone photo is a different problem. Filling either 2:3 rectangle removes part of that image; Fit keeps the full image with unused space. Inspect important edges and faces in the crop preview. The physical-size label cannot tell you which composition you prefer.
Use the frame or lab specification as the final input
A lab may group nearby inch and metric formats under one product label. Read its stated finished dimensions and trimming policy rather than assuming the two names are interchangeable. A frame may also have an opening smaller than its nominal print size. Measure the space that matters and keep essential content clear of the covered edge.
- Check the exact finished dimensions and units supplied by the lab or frame maker.
- Use the 4 × 6 photo preset for the inch size. For exact metric output, enter 10 × 15 with centimetres selected.
- Review Fill or Fit, then export a separate file and keep your original.
- For a home PDF layout, select matching paper and print at 100% or Actual size. Measure a test print before making the full batch.
Do not confuse 10 × 15 centimetres with 10 × 15 inches
The photo directory also includes a 10 × 15 inch size, which is a much larger rectangle: 254 × 381 mm. Its written unit is essential. Search labels and rounded names are useful for finding a format, but the physical dimensions are what determine paper fit and effective PPI. The calculator accepts inches, centimetres, and millimetres so you can keep that choice explicit.