A digital photo or scan is a grid of pixels, and every pixel stores a colour using a fixed number of bits. Multiply the number of pixels by the bits per pixel and you have the raw amount of data – the size of a BMP, an uncompressed TIFF or the image in your computer’s memory once a JPEG has been opened. That number is the starting point for planning storage, choosing scan settings or understanding why 16-bit editing files are so large.
Bit depth matters as much as resolution. A 1-bit fax-style image needs one bit per pixel; standard 24-bit colour needs three bytes; a 48-bit RGB file from a raw converter needs six. Doubling the bit depth doubles the file, while doubling the width and height quadruples it.
Real files are often smaller because of compression. PNG and lossless TIFF typically save 30–60% on photos, and JPEG at high quality usually reaches around a tenth of the raw size. The estimates shown are typical ranges, not guarantees – noisy or detailed images compress less.
How to use the Image File Size Calculator
- Image width (pixels)
- Image height (pixels)
- Bit depth
- Resolution (for print size)
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the pixel dimensions, choose the bit depth and, if you want the print size, the resolution in pixels per inch. The calculator multiplies width, height and bits per pixel, divides by 8 to get bytes and shows the result in the unit you pick (1 kB = 1,000 bytes; KiB, MiB and GiB use 1,024).
Worked example
A 4,000 × 3,000 photo has 12,000,000 pixels (12 MP). At 24 bits per pixel it needs 12,000,000 × 24 ÷ 8 = 36,000,000 bytes = 36 MB (34.33 MiB) uncompressed. Saved as a lossless PNG it would typically take about 18 MB, and as a high-quality JPEG about 3.6 MB. The same file in 48-bit for editing doubles to 72 MB. At 300 PPI it prints at 13.33 × 10 in (33.87 × 25.4 cm).
Uncompressed size of common resolutions (24-bit)
| Resolution | Pixels | Size (MB) |
|---|---|---|
| 640 × 480 (VGA) | 307,200 | 0.92 |
| 1280 × 720 (HD) | 921,600 | 2.76 |
| 1920 × 1080 (Full HD) | 2,073,600 | 6.22 |
| 3840 × 2160 (4K UHD) | 8,294,400 | 24.88 |
| 6000 × 4000 (24 MP camera) | 24,000,000 | 72.00 |
Inputs explained
| Field | What to enter |
|---|---|
| Image width (pixels) | A number |
| Image height (pixels) | A number |
| Bit depth | Choose one: 1-bit (black and white), 8-bit greyscale, 8-bit indexed (256 colours), 16-bit high colour (65,536 colours) … |
| Resolution (for print size) | In PPI |
What each result means
- Uncompressed file size = [Image width (pixels)]×[Image height (pixels)]×[Bits per pixel]÷8
- Total pixels = [Image width (pixels)]×[Image height (pixels)]
- Megapixels = [Image width (pixels)]×[Image height (pixels)]÷1e6
- Typical lossless (PNG/LZW TIFF, ≈ 50%) = [Uncompressed file size]×0.5
- Typical high-quality JPEG (≈ 1/10) = [Image width (pixels)]×[Image height (pixels)]×min([Bits per pixel], 24)÷8÷10
- Print width at this resolution = [Image width (pixels)]÷[Resolution (for print size)]×0.0254
- Print height at this resolution = [Image height (pixels)]÷[Resolution (for print size)]×0.0254
Formula
Uncompressed size (bytes) = width × height × bit depth ÷ 8 · Megapixels = width × height ÷ 1,000,000 · Colours = 2^(bits per colour sample × channels) · Print size = pixels ÷ PPI
Worked example
Inputs — Image width (pixels): 4,000; Image height (pixels): 3,000; Bit depth: 24-bit RGB true colour; Resolution (for print size): 300 PPI.
Results — Bits per pixel = 24; Uncompressed file size = 36 MB; Total pixels = 12,000,000; Megapixels = 12 MP; Number of colours = 16,777,216; Typical lossless (PNG/LZW TIFF, ≈ 50%) = 18 MB; Typical high-quality JPEG (≈ 1/10) = 3.6 MB; Print width at this resolution = 13.33 in; Print height at this resolution = 10 in.
Key terms
- Bit depth
- Number of bits used to store each pixel's colour (for example 24 = 8 bits each for red, green and blue).
- Raster image
- An image made of a grid of pixels, as opposed to a vector image made of shapes.
- Megapixel
- One million pixels.
- Lossless compression
- Compression (PNG, LZW, ZIP) that rebuilds the image exactly.
- Lossy compression
- Compression (JPEG, WebP, HEIC) that discards some detail to save much more space.
Frequently asked questions
How big is a 24-bit 1920 × 1080 image?
1920 × 1080 × 3 bytes = 6,220,800 bytes, about 6.22 MB (5.93 MiB) uncompressed.
How do I calculate image file size?
Multiply width × height × bit depth and divide by 8 for bytes, then by 1,000,000 for MB.
Why is my JPEG much smaller than this?
JPEG discards detail the eye barely notices; at high quality a photo is typically 5–15 times smaller than its uncompressed size.
Does PPI or DPI change the file size?
No. The PPI value is only a print instruction; the file size depends on the pixel count and bit depth.
What bit depth should I scan at?
24-bit colour (or 8-bit greyscale) is enough for viewing; use 48-bit when you plan heavy editing of tones and colours.
How many colours does 24-bit give?
2²⁴ = 16,777,216 colours. 32-bit RGBA has the same colours plus an 8-bit transparency channel.
What is the difference between MB and MiB?
1 MB = 1,000,000 bytes; 1 MiB = 1,048,576 bytes. Operating systems such as Windows show MiB but label them "MB".