Image File Size Calculator

How the Image File Size Calculator works

Every image is a grid of pixels, and every pixel needs a number of bits to store its colour. That fixes the raw size exactly. Compression then shrinks it by a factor that depends on the format, the quality setting and how much detail the picture holds.

Image file size calculator formula

The uncompressed formula is:

bytes = width × height × bits per pixel ÷ 8

Bits per pixel is the bit depth: 8 for greyscale, 24 for ordinary RGB colour, 32 when there is an alpha channel for transparency, and 48 for 16-bit-per-channel files from RAW editors and scanners. The calculator divides by 1,024 for KB and 1,048,576 for MB, the way Windows and most apps report file sizes.

Example 1. A 12-megapixel phone photo is 4032 × 3024 = 12,192,768 pixels. At 24 bits: 12,192,768 × 24 ÷ 8 = 36,578,304 bytes, which is 34.88 MB uncompressed, the size it takes in memory once opened in an editor. As a JPEG at quality 90 it is about 3.6 MB.

Example 2. A 1080 × 1080 Instagram square is 1,166,400 pixels. Uncompressed RGB is 3.34 MB; a JPEG at quality 75 is about 200 KB and a WebP at the same quality about 125 KB.

How compressed sizes are estimated

Compressed files do not have a fixed size, so the calculator uses typical bits per pixel measured on everyday photos:

Treat these as a middle estimate. A clear blue sky or a flat graphic compresses to a fraction of it; a noisy low-light shot or a picture of gravel or foliage can be twice as large. PNG is lossless, so for photos it stays large; for screenshots, logos and diagrams it is often smaller than JPEG.

Sizes of common images

The table below uses 24-bit colour; change the bit depth to rebuild it.

Uncompressed and typical JPEG size at 24 bits per pixel

How many photos fit in your storage

Enter a card or drive size in GB and the calculator divides it by the typical JPEG quality 90 size. A 64 GB card holds about 18,000 photos from a 12 MP phone, or about 4,500 from a 48 MP phone in full resolution mode. RAW files are far larger: roughly 1 to 1.5 bytes per pixel, so a 24 MP RAW is around 25 to 30 MB and the same card holds about 2,300. Leave 5 to 10% free for the file system.

Tips and pitfalls

Megapixels, not inches. File size depends only on pixels. A photo "at 300 DPI" is the same file as at 72 DPI; DPI only says how large it should print.

Upload limits. To hit a limit such as 2 MB or 500 KB, reduce the pixel count first, then the quality. Halving width and height cuts the file to about a quarter.

HEIC. iPhones save HEIC by default, which is about half the size of the equivalent JPEG, close to the WebP row.

Memory use. Editors work on the uncompressed size, often several copies of it for layers and undo. Five 48 MP layers need over 600 MB of memory.

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