For a custom-size poster, start with the finished width and height, then make the photograph fit those proportions without stretching it. Check the crop, the available detail and any border before you export the file.
That order matters. Changing the measurements at the end can remove part of a carefully arranged photograph or leave white space you did not expect.
Our Poster Creator uses centimetres for custom dimensions. If your measurements are in millimetres, divide by ten: 450 mm is 45 cm, not 450 cm.
Separate the paper size from the picture inside it
The size you order describes the finished print. If your artwork includes a white border, that border occupies part of those dimensions; it is not added outside them.
For example, a 30 × 40 cm print with a 2 cm border on each side leaves an image area of 26 × 36 cm. This smaller rectangle has its own proportions. A photograph that neatly fits the full sheet may need a different crop to fit the bordered version.
If the poster is going into an existing frame, establish the required paper size before preparing the file. The frame's outside dimensions are not the print size, and the opening in a mount may be smaller than the paper it holds. Measure those separately rather than trying to compensate later in the image editor.
Our frame-measuring guide explains how to establish those measurements.
Match the proportions before changing the resolution
Aspect ratio is simply the relationship between width and height. A 3000 × 4500 pixel portrait photograph has a 2:3 ratio. It fits a 30 × 45 cm poster without a shape change. It does not exactly fit 30 × 40 cm, which has a 3:4 ratio.
To place that photograph in 30 × 40 cm, you have three honest options:
- Crop some of the picture so it fills the sheet.
- Keep the whole picture and include the intended white space in your prepared artwork.
- Choose a print size closer to the photograph's original proportions.
Stretching the photograph is not a fourth equivalent option. It changes the shape of people and objects. A face may become wider or narrower even if the distortion is subtle on a small screen.
A worked crop example: 30 × 45 cm into 30 × 40 cm
Take that 3000 × 4500 pixel portrait photograph. To fill a 30 × 40 cm poster, it needs to become 3000 × 4000 pixels.
That means removing 500 pixels from its height: about 11% of the original height. A centred crop would remove 250 pixels from the top and 250 from the bottom. You do not have to centre it; moving the crop can protect a head, a hand or another important detail.
If you keep the whole image instead, scaling it to 40 cm high makes it about 26.67 cm wide. Centred on a 30 cm sheet, it leaves roughly 1.67 cm of white space on each side, with none at the top or bottom. That is a fit-to-sheet result, not an even border all the way round.
This is why “keep the whole photograph” and “make all four borders equal” sometimes require a more deliberate layout.
Check resolution after the final crop
For a detailed poster viewed closely, 300 ppi is a useful preparation target when the original contains enough pixels. It is not a promise that every photograph needs resampling to that number, nor a guarantee that a blurred original will look sharp.
To calculate the pixels required, divide centimetres by 2.54 and multiply by the chosen ppi. The following figures are rounded up:
| Finished size | Pixels at 200 ppi | Pixels at 300 ppi |
|---|---|---|
| 30 × 40 cm | 2363 × 3150 | 3544 × 4725 |
| 30 × 45 cm | 2363 × 3544 | 3544 × 5315 |
| 50 × 70 cm | 3938 × 5512 | 5906 × 8268 |
These are planning figures, not upload minimums. Consider the image itself and how it will be viewed. Small writing, detailed illustration and a close-up portrait can reveal softness differently.
Do not repeatedly export, reopen and save a JPEG while experimenting. Keep the original untouched, work from a separate editable copy and export the finished version when the composition is settled.
Keep lettering and important details away from the edge
If you are adding a date, caption or quotation, give it breathing room inside the intended visible area. A frame lip or mount can cover part of the sheet, and a design with lettering touching the edge leaves no room for that.
Use the actual frame or mount measurements when planning the visible area. There is no single safe margin that suits every frame. A fine printed border can also make a small mismatch more noticeable than an image that naturally continues towards the edge.
Do not add crop marks, printer registration marks or an extra bleed area unless we have specifically requested them. Those belong to particular production workflows and should not accidentally become part of your uploaded poster artwork.
Check the final file in the creator
Upload the finished image, select the custom dimensions and confirm the orientation. Inspect the whole composition, then the edges: is the border intentional, is the text inside the visible area, and has anything important been cropped?
Our posters are printed on 230gsm matte paper. A backlit screen will not look identical to a paper print, so avoid using maximum screen brightness as your only judgement of shadow detail. Do not make a series of strong brightness changes just to chase the screen's glow.
Use the custom poster calculator to check the price, then continue to the creator with your chosen dimensions. For a finished print, correct proportions and a carefully checked file are more useful than an impressive-looking file size.