Image File Size Calculator

How Image File Size Is Estimated

An uncompressed image stores a fixed number of bits for every pixel. Multiplying width by height gives the total pixel count, multiplying that by the bits-per-pixel color depth gives the total bits, and dividing by 8 converts bits to bytes — the uncompressed file size. Real-world image formats then apply compression, which this calculator approximates by dividing the uncompressed size by a typical compression ratio for the selected format.

$$\text{Size} = \frac{W \times H \times B}{8 \times R}$$

Worked Example

A 1920x1080 image at 24-bit RGB, uncompressed: 1920 times 1080 times 24, divided by 8, equals about 6,220,800 bytes, or roughly 5.93 megabytes. Saved as a typical PNG (compression ratio around 3x), the estimated size drops to about 1.98 megabytes; saved as a high-quality JPEG (ratio around 10x), it drops further to about 607.5 kilobytes.

Why This Is Only an Estimate

Actual compression depends heavily on image content, not just resolution and format. A photo with large areas of flat color or gentle gradients (a clear sky, a plain background) compresses far more than a busy, highly detailed photo (dense foliage, fine texture) at the same resolution and format — real files can land noticeably above or below this estimate depending on what's actually in the picture.

A Brief History of Image Compression

JPEG, still the most common photo format, was standardized in 1992 by the Joint Photographic Experts Group, the committee it's named after. It was designed specifically around how the human eye actually perceives images — it discards color and detail information people are less likely to notice while preserving the parts that matter most for perceived sharpness, which is what allows it to shrink a photo dramatically while still looking correct to the eye. That tradeoff, adjustable through a quality setting, is exactly the lossy compression described below.

PNG arrived a few years later, in 1996, for a different reason entirely: GIF, the dominant format for compressed graphics at the time, used a compression method covered by a patent that its owner, Unisys, began enforcing against software using it. PNG was built from the ground up as a free, patent-unencumbered replacement, using lossless compression so a saved PNG file decompresses back to pixel-for-pixel identical data, better suited to graphics, screenshots, and images with sharp edges or text than to photographs, where JPEG's lossy approach usually produces a much smaller file at a quality loss most viewers won't notice.

Common File Size Mistakes

Confusing bits and bytes is a frequent source of error — file sizes are conventionally reported in bytes (and KB/MB built from bytes), but color depth is specified in bits per pixel, so the division by 8 is essential and easy to accidentally skip.

Assuming JPEG and PNG compress by the same ratio is another — PNG's lossless compression typically achieves a much smaller reduction than JPEG's lossy compression, so swapping the compression option without changing the format assumption can produce a very misleading estimate.

Image File Terms You Should Know

Bit Depth (Bits per Pixel) — how many bits are used to store the color of a single pixel; higher bit depth allows more distinct colors but takes more space.

Lossless Compression — a compression method (like PNG) that shrinks a file without discarding any image data, so the decompressed image is identical to the original.

Lossy Compression — a compression method (like JPEG) that discards some image detail, usually detail less noticeable to the eye, to achieve a much smaller file size.