Concise answer

Multiply the physical size in inches by the target PPI. If the size is in centimetres, divide by 2.54 first. If it is in millimetres, divide by 25.4 first. Calculate width and height separately, then round to whole pixels using a clearly stated rule.

The basic formula

For each axis:

pixels = inches × PPI

Example: 8 inches at 300 PPI.

8 × 300 = 2400 pixels

For an 8 × 10 image:

  • width = 8 × 300 = 2400
  • height = 10 × 300 = 3000

Result: 2400 × 3000 pixels.

Why the formula works

PPI literally means pixels per inch.

If each inch contains 300 pixels, then 8 inches contain eight groups of 300.

The same logic works for any target:

  • 8 inches at 150 PPI = 1200 pixels
  • 8 inches at 200 PPI = 1600 pixels
  • 8 inches at 240 PPI = 1920 pixels
  • 8 inches at 600 PPI = 4800 pixels

The arithmetic is linear on each axis.

Convert centimetres to pixels

One inch equals exactly 2.54 centimetres.

Formula:

pixels = (centimetres ÷ 2.54) × PPI

Example: 10 cm at 300 PPI.

10 ÷ 2.54 × 300 ≈ 1181.10 pixels

For a practical raster dimension, round according to the use case.

If the result is a minimum requirement, round up to 1182 pixels.

Convert millimetres to pixels

One inch equals exactly 25.4 millimetres.

Formula:

pixels = (millimetres ÷ 25.4) × PPI

Example: 35 mm at 300 PPI.

35 ÷ 25.4 × 300 ≈ 413.39 pixels

A practical rounded result is around 413 or 414 pixels depending on the tool’s rounding rule.

For minimum requirements, use the ceiling.

Example: 35 × 45 mm at 300 PPI

Width:

35 ÷ 25.4 × 300 ≈ 413.39

Height:

45 ÷ 25.4 × 300 ≈ 531.50

A minimum-oriented calculator could return 414 × 532 pixels.

A nearest-whole-pixel calculator may return 413 × 531.

Neither changes the fact that the exact physical-to-pixel conversion lands between whole-pixel values.

This is why official systems that require exact digital dimensions should be followed directly instead of deriving a pixel size from a physical standard unless the authority tells you to do so.

Example: A4 at 300 PPI

A4: 210 × 297 mm.

Width:

210 ÷ 25.4 × 300 ≈ 2480.31

Height:

297 ÷ 25.4 × 300 ≈ 3507.87

Common rounded result:

2480 × 3508 pixels

At 150 PPI:

1240 × 1754 pixels approximately.

At 600 PPI:

4961 × 7016 pixels approximately.

Example: 4 × 6 photo

At 300 PPI:

  • 4 × 300 = 1200
  • 6 × 300 = 1800

At 240 PPI:

  • 960 × 1440

At 200 PPI:

  • 800 × 1200

At 150 PPI:

  • 600 × 900

The correct target depends on the print requirement and expected viewing conditions.

Calculate megapixels

After finding width and height:

megapixels = width × height ÷ 1,000,000

An 8 × 10 at 300 PPI is:

2400 × 3000 = 7,200,000

or:

7.2 MP

This is the minimum raster count for that exact crop at that exact sampling density. A camera file may need more pixels before crop.

Aspect ratio can change the requirement

Suppose you need an 8 × 10 print, which is 4:5.

Your camera image is 3:2.

The calculator says 2400 × 3000 pixels are needed at 300 PPI, but a 3:2 source cannot become 4:5 without cropping or borders.

If you want a full-bleed 8 × 10, start with enough pixels that after crop at least 2400 × 3000 remain.

This is why megapixels alone can be misleading.

Bleed and trim

Commercial print jobs can include bleed outside the final trim size.

If the printer asks for 0.125 inch bleed on each side, an 8 × 10 design becomes 8.25 × 10.25 inches in total raster area.

At 300 PPI:

  • 8.25 × 300 = 2475 pixels
  • 10.25 × 300 = 3075 pixels

Always calculate from the full production size when bleed is part of the image.

What target PPI should you use?

Use the print provider’s current requirement first.

If there is no specified value, 300 PPI is a common high-detail target for close-viewed photographic print work. Lower values may be appropriate for large formats viewed farther away. Higher values can make sense in specialized reproduction and scanning workflows.

The formula does not tell you which target is correct. It only tells you how many pixels that target requires.

Rounding rules a good calculator should expose

Three useful options are:

  • nearest whole pixel
  • round up for minimum requirement
  • round down to fit within a maximum dimension

For print-preparation planning, “round up” is often the safest when the target is a minimum.

The interface should never silently output a rounded value without telling the user.

Common mistakes

Do not multiply millimetres directly by PPI. Convert to inches first.

Do not calculate only one axis if aspect ratio can change.

Do not forget crop and bleed.

Do not assume the camera’s total megapixels remain available after editing.

Do not change DPI metadata and assume you have reached the calculated pixel requirement.

Related entities and semantic terms

DPI to pixels, PPI formula, inches, centimetres, millimetres, 25.4 mm per inch, 2.54 cm per inch, megapixels, print dimensions, bleed, aspect ratio.

FAQs

How many pixels is 1 inch at 300 PPI?

Exactly 300 pixels by definition of the target density.

How do I convert 50 mm to pixels at 300 PPI?

Use 50 ÷ 25.4 × 300, which is about 590.55 pixels. Round according to the requirement.

Why do metric print sizes produce fractional pixel results?

Because the metric dimension converted to inches often does not multiply into a whole number at the chosen PPI. Digital rasters require whole pixels, so rounding is unavoidable.

Do I calculate with the trimmed size or the size including bleed?

Use the full raster area required by the printer. If bleed extends beyond trim, include it in the calculation.

Closing section

Pixel requirements are simple once units are controlled. Convert physical size to inches, multiply by the target PPI, account for crop and bleed, and round transparently. The DPI Calculators page can automate the arithmetic while still showing the formula.

Source and verification notes

Formulas are unit arithmetic based on the exact definitions of inch, centimetre and millimetre. Print-planning context is consistent with current Adobe resolution guidance.