How to Resize a Photo for an Online Application Form
Application forms set two separate limits: the pixel size of the image and the size of the file in kilobytes. Resize the photo to the pixel dimensions first, then compress it to the KB limit — on a Mac both steps take about a minute and no upload.
Updated September 23, 20265 min read
By the File Acture team
The form wants a photo "600 × 800 pixels, JPEG, maximum 300 KB". Your camera produced a 4 MB, 4032 × 3024 file. The upload box rejects it without saying which rule you broke, and the site logs you out while you fix it.
Here is the order that works, what the numbers mean, and how to do it on a Mac without sending your photo to a website.
What the form is asking for
Read the requirements once and separate them into four groups. Each is a different setting, and a photo that passes three can still be rejected on the fourth.
- Dimensions in pixels. For example 600 × 800. This is the resolution of the image itself.
- File size in KB. For example "maximum 300 KB". This is compression, not resolution.
- Format. Almost always JPEG. A HEIC photo straight from an iPhone is rejected even at the right size — convert it first.
- Content rules. Plain background, face proportions, recent photo. No software fixes these; a photo booth or a careful retake does.
Pixels, DPI and the 35 × 45 mm confusion
Passport-style forms often describe a photo in millimetres — 35 × 45 mm at 300 DPI, for instance. A screen has no millimetres, so convert: 35 mm at 300 DPI is about 413 pixels, and 45 mm is about 531 pixels.
If the form gives both millimetres and pixels, follow the pixels. If it gives only millimetres, use the DPI it asks for to work out the pixel size, and ignore the DPI tag stored inside the file — no upload system measures your photo with a ruler.
Resize first, then compress
Order matters. Shrinking the pixel dimensions removes data the compressor would otherwise have to work around, so the same file size looks noticeably better.
In File Acture, open Resize Images and set Resize by to Longest edge, then type the larger of the two numbers the form asks for — 800 for a 600 × 800 photo. The aspect ratio is kept, and images are never enlarged, so a photo that is already small stays as it is.
Then open Compress Images, set Compress by to Target size and type the KB limit. The app tries JPEG qualities until the measured file fits, so you land just under 300 KB instead of guessing with a quality slider.
When the photo must be exactly 600 × 800
Resizing keeps the proportions of your photo, so a 4:3 image cannot become 3:4 without cropping. Crop to the right shape first in Preview (Tools > Rectangular Selection, then Tools > Crop), then resize by the longest edge.
The built-in way on a Mac
Preview can do the resize: open the photo, choose Tools > Adjust Size…, set width or height in pixels with "Scale proportionally" ticked, then File > Export… and drag the Quality slider down while watching the size that Preview shows under it.
It works, and for one photo it is fine. What it does not do is aim for a file size — you move the slider, read the number, move it again — and it handles one file at a time, which is the problem when a family of four needs the same document.
Doing it for several people or several forms
Drop all the photos in at once: every file gets the same longest edge and the same KB target, and the originals stay untouched in their folder. Renaming is left to you, because forms usually demand a specific file name.
Keep the full-size originals. The next form will ask for different numbers, and starting again from a 300 KB copy costs quality you cannot get back.
And do not upload the photo to a random site
A photo for an application is a photo of your face, often next to your ID. It belongs on your own machine. File Acture resizes and compresses locally, and nothing is uploaded; the free version covers 3 runs of up to 5 files on each Mac. See what Pro adds.
For a folder of photos rather than one, see resizing images in bulk and compressing them to a target size.
How to prepare an application photo with File Acture
- 1
Convert HEIC to JPEG if needed
iPhone photos are often HEIC. Use Convert Images with JPEG selected, or the form will refuse the file.
- 2
Crop to the right shape
If the form wants an exact ratio such as 600 × 800, crop in Preview first so resizing does not distort the photo.
- 3
Resize by longest edge
In Resize Images, set Resize by to Longest edge and type the larger number from the form (for example 800).
- 4
Compress to the KB limit
In Compress Images, set Compress by to Target size and enter the limit, such as 300 KB.
- 5
Check before uploading
Open the result and look at it at 100%: eyes and text should be sharp. Keep the original for the next form.
Hit the pixel and KB limits in one pass, on your Mac.
Free to try · runs on your device
| The form says | Meaning | What to set |
|---|---|---|
| 600 × 800 px | Image resolution | Longest edge 800, crop to 3:4 first |
| Max 300 KB | File size | Target size 300 KB |
| 35 × 45 mm, 300 DPI | Print size | About 413 × 531 px |
| JPEG only | Format | Convert Images → JPEG |
| Max 1 MB, any size | File size only | Target size 1 MB, no resize needed |
Questions
How do I reduce a photo to 300 KB?
Resize it to the pixel size the form asks for, then use Compress Images with Compress by set to Target size and 300 KB entered.
What is 35 × 45 mm in pixels?
At 300 DPI it is about 413 × 531 pixels; at 600 DPI about 827 × 1063. Use the DPI the form specifies.
Why is my photo rejected even though it is small enough?
Usually the format or the dimensions: a HEIC file instead of JPEG, or a width and height that do not match the required ratio.
Should I resize or compress first?
Resize first. Compressing a large photo down to a small file size wastes quality that resizing would have saved.
Can I prepare several photos at once?
Yes. Drop them all in and they get the same longest edge and the same target size. The free version allows 5 files per run.