Image File Size Calculator: How to Estimate KB & MB

This calculator estimates the file size of an uncompressed raster image (like a BMP or TIFF) based on its pixel dimensions, bit depth, and optional compression ratio. You enter the width and height in pixels, the bits per pixel (color depth), and a compression factor, and it returns the estimated file size in bytes, kilobytes, and megabytes.

Formula

The core formula for an uncompressed bitmap image is:

File Size (bytes) = (Width × Height × Bits per Pixel) / 8

Where:

• Width – number of pixels horizontally (unit: pixels)
• Height – number of pixels vertically (unit: pixels)
• Bits per Pixel – color depth (unit: bits). Common values: 1 (black & white), 8 (grayscale or 256 colors), 24 (true color), 32 (true color + alpha).
• 8 – converts bits to bytes (8 bits = 1 byte).

If you apply compression (e.g., JPEG, PNG), divide the uncompressed result by the compression ratio. A ratio of 10 means the compressed file is roughly 10 times smaller.

Worked Example

Using the default values: width = 1920 px, height = 1080 px, bits per pixel = 24, compression ratio = 1 (no compression).

1. Total pixels = 1920 × 1080 = 2,073,600 pixels
2. Total bits = 2,073,600 × 24 = 49,766,400 bits
3. Bytes = 49,766,400 / 8 = 6,220,800 bytes
4. KB = 6,220,800 / 1024 = 6,075 KB (rounded to 2 decimals: 6,075.00 KB)
5. MB = 6,075 / 1024 = 5.93 MB (rounded to 2 decimals)

The calculator shows 6,220,800 bytes, 6,075.00 KB, and 5.93 MB – exactly matching this calculation.

Why Image File Size Matters

File size affects storage space, upload/download speed, and printing quality. A 12-megapixel photo at 24-bit color is about 36 MB uncompressed – that’s why cameras and phones use JPEG compression to shrink files to a few MB. Understanding the raw size helps you plan storage, choose compression settings, or estimate bandwidth needs.

How to Use This Calculator

• Enter the pixel dimensions of your image (you can find these in image properties or editing software).
• Select the bits per pixel – if unsure, 24 is standard for color photos, 8 for grayscale.
• Set the compression ratio to 1 for uncompressed formats like BMP. For JPEG, use a typical ratio of 10–20; for PNG with lossless compression, use 2–3.
• Read the estimated file size in bytes, KB, and MB.

For example, a 4000×3000 photo (12 MP) at 24-bit, with JPEG compression ratio of 10, gives: (4000×3000×24/8)/10 = 3,600,000 bytes ≈ 3.43 MB – a realistic size for a high-quality JPEG.

Common Mistakes and Misconceptions

• Mixing up KB and MB: 1 MB = 1024 KB, not 1000 KB. Many storage devices use decimal (1 MB = 1000 KB), but image files are usually measured in binary (1024). This calculator uses binary.
• Ignoring compression: JPEG and PNG are compressed, so the formula without division overestimates size massively. Always apply a compression ratio.
• Confusing bits with bytes: 24 bits per pixel is 3 bytes per pixel, not 24 bytes. The division by 8 is essential.
• Assuming all JPEGs have the same size: Compression ratio varies with image content – a photo of a blue sky compresses far more than a noisy night scene.

Limitations of the Formula

This formula gives an estimate for uncompressed or roughly compressed images. Actual file sizes for compressed formats depend on the compression algorithm, image complexity, and quality settings. For example, a JPEG at quality 90 might have a ratio of 8, while quality 50 might give 20. The calculator is a planning tool, not a precise predictor.

Also, file headers and metadata (EXIF, color profiles) add a small overhead, usually a few KB, which is negligible for large images but can be significant for tiny icons.

Alternatives and Related Tools

If you need the exact file size of an existing image, simply right-click the file and select Properties (Windows) or Get Info (Mac). For resizing images, use photo editing software or online compressors. This calculator is most useful when you are planning ahead – e.g., estimating how many images fit on a memory card, or what resolution to use for a web page.

Frequently Asked Questions

What is a typical bits per pixel value?

1 for black-and-white, 8 for grayscale, 24 for color photos, and 32 for color with transparency. Most JPEGs use 24-bit color internally.

How do I find the pixel dimensions of an image?

On Windows, right-click the file → Properties → Details tab. On Mac, right-click → Get Info. In photo editors, look under Image Size or Canvas Size.

Does this calculator work for PNG files?

Yes, but PNG uses lossless compression, so the compression ratio is typically between 1.5 and 3, depending on the image. Use a ratio of 2 as a rough estimate.

Why is my JPEG smaller than this calculator says?

Because JPEG compression is very effective for photos. A ratio of 10–20 is common, so a 5.93 MB uncompressed image might be only 300–600 KB as a JPEG.

What is the difference between KB and KiB?

KB (kilobyte) sometimes means 1000 bytes, but in image files it usually means 1024 bytes (KiB). This calculator uses 1024, which matches most operating systems.

Can I use this for video?

No, video compression is far more complex. This formula only applies to single still images.

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in w | Width | pixels | 1920 | 1
in h | Height | pixels | 1080 | 1
in b | Bits per pixel | bits | 24 | 1
in c | Compression ratio | - | 1 | 0.1
out size_b | File size (bytes) | bytes | (w * h * b / 8) / c | 0
out size_kb | File size (KB) | KB | ((w * h * b / 8) / c) / 1024 | 2
out size_mb | File size (MB) | MB | (((w * h * b / 8) / c) / 1024) / 1024 | 2