Resolution, DPI, cm and Pixels Explained — "How Many Pixels Is 3×4cm at 300dpi?" Conversion Table
How pixels, DPI (PPI) and cm relate, the conversion formula, print vs. screen, pixel tables for ID photos, prints and A4, and how big a phone photo can print.
You often see instructions like "prepare a 3×4cm photo at 300dpi" or "72dpi is not enough for printing," but once you understand exactly how pixels and DPI relate, most spec problems can be solved with a single calculation. This article explains the concepts and provides ready-to-use conversion tables.
Three units
- Pixels (px): the number of dots that make up a digital image. A 4032×3024 photo is made of 4,032 dots across and 3,024 down. The amount of information a file actually holds is determined by its pixel count.
- cm and inches: the physical size when printed on paper. One inch is exactly 2.54cm.
- DPI / PPI: how many dots or pixels are placed within one inch. Strictly speaking, PPI is the pixel density of the image and DPI is the density of ink dots a printer lays down, but photo spec guidelines use the two interchangeably.
The key formula
pixels = cm ÷ 2.54 × dpi · cm = pixels ÷ dpi × 2.54
For example, converting 3×4cm at 300dpi gives 3 ÷ 2.54 × 300 ≈ 354 wide and 4 ÷ 2.54 × 300 ≈ 472 tall, so 354×472 pixels. Rounding is the usual practice, but some institutions truncate instead (e.g., 3.5cm × 100dpi = 137.8 → the Korean national civil service application uses 137 pixels).
Conversely, printing a 4032×3024 pixel photo at 300dpi means you can print it sharply up to 4032 ÷ 300 × 2.54 ≈ 34.1cm by 3024 ÷ 300 × 2.54 ≈ 25.6cm.
Pixel conversion table by size
| Print size | Use | 150dpi | 300dpi | 600dpi |
|---|---|---|---|---|
| 2.5×3.5cm | Korean standard ID photo | 148×207 | 295×413 | 591×827 |
| 3×4cm | Korean half-size ID photo | 177×236 | 354×472 | 709×945 |
| 3.5×4.5cm | Passport and ID cards | 207×266 | 413×531 | 827×1063 |
| 5.08×5.08cm (2×2 inches) | US passport and visa | 300×300 | 600×600 | 1200×1200 |
| 10.2×15.2cm (4×6 inches) | Standard photo print | 600×900 | 1200×1800 | 2400×3600 |
| 21×29.7cm | A4 | 1240×1754 | 2480×3508 | 4961×7016 |
The 413×531 pixels obtained by converting a 3.5×4.5cm passport photo at 300dpi exactly matches the size the Korean Ministry of Foreign Affairs recommends for online passport applications. The Ministry also recommends 300DPI for printed photos.
Print and screen work differently
Print: DPI determines size
A printer places pixels on paper according to the DPI value recorded in the file (or the size specified in the print dialog). The same 1200×1800 pixels become 4×6 inches at 300dpi and 8×12 inches at 150dpi. The larger you print, the bigger each pixel becomes and the rougher it looks.
Common guidelines are:
- 300dpi: the standard for photo prints viewed up close, ID photos and printed materials
- 150–200dpi: posters viewed from a little distance, internal documents
- 100dpi or less: banners and large-format prints viewed from far away
Screen: only the pixel count matters
On screen, the file's DPI value is ignored and only the pixel count matters. On web pages, CSS defines 1in as 96px (W3C CSS Values), but this is just a convention for on-screen display and doesn't match a real inch on the monitor. The same image looks different depending on the monitor's size, resolution and scaling settings.
So the saying "72dpi images are for the web" is only half right. On the web, whether it's 72dpi or 300dpi, images with the same pixel count look exactly the same.
How large can my photo print sharply?
Divide the pixel count by the dpi and multiply by 2.54 to get the maximum print size.
| Image size (px) | Typical example | Print at 300dpi | Print at 200dpi |
|---|---|---|---|
| 1080×1350 | Instagram portrait post | about 9.1×11.4cm | about 13.7×17.1cm |
| 1920×1080 | Full HD screenshot | about 16.3×9.1cm | about 24.4×13.7cm |
| 4032×3024 | 12MP smartphone | about 34.1×25.6cm | about 51.2×38.4cm |
| 8064×6048 | 48MP smartphone | about 68.3×51.2cm | about 102.4×76.8cm |
Photos downloaded from social media have already been reduced to around 1080 pixels, so if you want to print large for an album or frame, it's better to find and use the original file.
Enlarging (upscaling) is not a solution
When you enlarge a photo that lacks pixels, the program fills in by calculating in-between values from surrounding pixels, so edges become blurry. AI upscaling adds "plausible" detail, but it's guessing at information that was never there, so it isn't suitable where authenticity matters, as with ID photos. Korean passport photo rules also don't allow edited or AI-composited photos. Starting with an original larger than the size you need is the only reliable method.
What changing only the DPI value means
When you change the "resolution" from 72 to 300 in a photo editor with the resampling (recalculating pixels) option turned off, the pixel count stays the same and only the print size shrinks. Image quality doesn't change at all. With resampling on, new pixels are created, but since that means guessing at missing information, the image doesn't get sharper — only the file gets bigger.
JPG, PNG and BMP files have a place to store a DPI value (such as the resolution fields in the BMP header). Files re-saved through Canvas in a web tool may have this value reset to a default or dropped, but as long as the pixel count is right, it's no problem for online submissions. For files going to a print shop, it's more reliable to calculate the print size in pixels and hand it over.
Screen pixel density and web image size
Today's phone and laptop screens use high-density (Retina) displays that draw one CSS pixel with 2–3 physical pixels. So if you make an image for a 400px-wide area on a web page exactly 400 pixels wide, it looks slightly blurry on high-density screens, while preparing it at about 800 pixels makes it look sharp. On the other hand, uploading a 4000-pixel original as-is makes no visible difference on screen but increases the amount of data transferred. It's common to prepare web images at about 2× the display size.
Steps when you're given a spec
- If the spec gives pixels, those pixels take priority. Treat cm and dpi as reference only.
- If it only gives cm and dpi, calculate the pixels with the formula above.
- First match the aspect ratio (width:height) by cutting it with Crop, then resize to the calculated pixels. If you only match the size while the ratio is different, the face gets stretched or squashed.
- If there's a file size (KB) limit, meet it with Compress Image. Application and ID specs are collected in ID photo size guide.
Frequently asked questions
If I change a file's DPI from 72 to 300, does the quality improve?
If the pixel count stays the same, the quality doesn't change. The DPI value is just a note saying 'how many pixels to place per inch when printing,' so changing it only makes the print size smaller. For sharper prints, you need to capture more pixels in the first place.
The upload spec only gives a DPI, not pixels. What should I do?
If a size in cm is also given, calculate the pixels with cm ÷ 2.54 × dpi. Online submissions are ultimately judged by pixel size and file size in most cases.
Is viewing at 100% on my monitor the same as the print size?
Usually not. Screens display pixels according to the monitor's physical pixel density and scaling settings, so the same file appears at a different real-world size on each monitor.
Try it now
Sources
- W3C — CSS Values and Units Module Level 4, Absolute lengths
- Korean Ministry of Foreign Affairs passport portal — Passport photo rules (300DPI recommended)
- Korean Ministry of Foreign Affairs passport portal — Online passport photo check (413×531 pixels)
- Microsoft Learn — Bitmap Storage (resolution fields)