Last updated: 2026-07-13

Color Depth and Bits Per Pixel

Color depth — also called bit depth or bits per pixel (bpp) — is the number of bits a format assigns to every pixel. It answers two questions at once: how many distinct colors an image can hold, and how much space one uncompressed frame takes.

Colors from bit depth

Each bit stores one binary digit, so a pixel with $b$ bits can take $2^b$ different values, and each value maps to a color:

Adding a single bit doubles the count. That is why the familiar depths grow so quickly:

A 32-bit image shows the same 16.7 million colors as 24-bit: the extra 8 bits carry transparency (an alpha channel), not additional color.

Uncompressed size

Every pixel occupies the same fixed number of bits, so the raw bitmap size is simply the pixel count times the bit depth, converted from bits to bytes:

where $W$ and $H$ are the width and height in pixels, $b$ is the bits per pixel, and dividing by 8 turns bits into bytes.

Worked example. A 1920 × 1080 frame at 24 bits per pixel:

This is the size a graphics card holds in its framebuffer, or what an uncompressed BMP would occupy. A 4K UHD frame (3840 × 2160) at the same depth is four times larger, about 24.9 MB.

Palette vs truecolor

Indexed (palette) formats store a short lookup table of colors and give each pixel an index into it. An 8-bit palette holds up to 256 entries, so each pixel needs only 8 bits regardless of how rich the individual colors are. This keeps flat graphics, logos, and line art small.

Truecolor formats instead store the red, green, and blue components directly in every pixel — usually 8 bits each for 24-bit color. No palette is involved, roughly 16.7 million colors are available, and smooth photographic gradients render without banding. The trade-off is size: a truecolor pixel is three bytes against a palette pixel's one.

Why saved files are smaller

The formula gives the uncompressed size. Real image files apply compression: PNG uses lossless compression that typically saves 20–50 %, while JPEG discards detail the eye is unlikely to notice and often shrinks a 24-bit photo to 10–20 % of its raw size. The uncompressed figure is still the right number for framebuffers, texture memory, and worst-case storage planning.

Related estimates

To fold resolution and bit depth into a single storage estimate for photographs, use the Image File Size Calculator. To convert a resolution into a megapixel rating, see the Megapixel & Print Size Calculator. And to compare a compressed result against its raw source, the Compression Ratio Calculator turns the two sizes into a ratio.