BMP Size Calculator
Estimate bitmap file size instantly using image width, height, and bit depth. This calculator factors in BMP row padding, pixel data, headers, and palette size for indexed color modes.
What this tool calculates: uncompressed BMP file size for common Windows BMP files using a 14 byte file header, 40 byte DIB header, optional color table, and 4 byte row alignment.
How a BMP size calculator works
A BMP size calculator estimates the storage required for a bitmap image saved in the BMP format. BMP files are simple, widely documented raster image files that store image data in a relatively direct way. Because of that simplicity, BMP is often used in graphics education, low level image processing, legacy Windows workflows, and file format demonstrations. The tradeoff is that BMP files can become very large compared with compressed formats like JPEG, WebP, or PNG.
To calculate a BMP file size accurately, you need more than width multiplied by height. A valid BMP file usually contains several parts: a file header, a DIB header, an optional color table, and the pixel array. The pixel array itself is also subject to row padding. Each row of pixel data must be aligned to a 4 byte boundary, so the actual storage per row is often slightly larger than the raw pixel bits alone would suggest. This padding is why a proper BMP size calculator gives more accurate results than a simple image resolution calculator.
The calculator above follows the standard formula used for common uncompressed Windows BMP files with the 40 byte BITMAPINFOHEADER. It also automatically adds a palette for indexed images, such as 1 bit, 4 bit, and 8 bit BMPs. For 24 bit and 32 bit BMP files, there is no palette in the basic model because pixel values are stored directly.
Core formula: total BMP size = 14 byte file header + 40 byte DIB header + palette size + padded pixel array size.
The exact BMP size formula
For an uncompressed BMP, the practical calculation is:
- Calculate raw bits per row: width × bit depth
- Convert row bits to bytes and pad to a multiple of 4 bytes
- Multiply padded row size by image height
- Add the 14 byte BMP header and 40 byte DIB header
- Add palette size for indexed color modes
The row size is commonly expressed as:
row size = floor((bit depth × width + 31) / 32) × 4
This formula works because it rounds each row up to the next 32 bit boundary, which is equivalent to 4 bytes. Then:
pixel array size = row size × height
Finally:
total file size = 54 + palette size + pixel array size
For palette sizes in standard indexed BMP files:
- 1 bit: 2 colors × 4 bytes each = 8 bytes
- 4 bit: 16 colors × 4 bytes each = 64 bytes
- 8 bit: 256 colors × 4 bytes each = 1024 bytes
- 16, 24, 32 bit: usually 0 palette bytes in the basic uncompressed model
Example: 1920 × 1080 at 24 bit
A 24 bit BMP uses 3 bytes per pixel in raw terms, but row padding still matters.
- Raw bytes per row: 1920 × 3 = 5760
- 5760 is already divisible by 4, so padded row size is 5760 bytes
- Pixel array size: 5760 × 1080 = 6,220,800 bytes
- Headers: 54 bytes
- Total BMP size: 6,220,854 bytes
That is about 5.93 MiB or 6.22 MB depending on which unit convention you use. This is why BMP files are considered storage heavy. A compressed image at the same visual dimensions may be dramatically smaller.
Why row padding matters so much
Many users are surprised when BMP file size does not match width × height × bytes per pixel exactly. The reason is scanline alignment. In a BMP file, each row must end on a 4 byte boundary. If the image width does not naturally produce a multiple of 4 bytes per row, extra padding bytes are inserted. Those bytes do not hold visible image content, but they still increase file size.
For example, imagine an image that is 101 pixels wide at 24 bit color. The raw row size is 101 × 3 = 303 bytes. Since 303 is not divisible by 4, the row is padded to 304 bytes. That means 1 extra byte per row. Across a tall image, padding can add up quickly. If the image were 5000 pixels high, the padding alone would total 5000 bytes.
This is a major reason a BMP size calculator is valuable. It handles the alignment automatically and avoids underestimating storage. If you are planning memory buffers, software exports, or archival capacity, these details matter.
BMP size by common resolutions and bit depths
The table below shows approximate total sizes for standard uncompressed BMP files using the 54 byte base header. Indexed modes include their standard palettes. These figures are practical examples rather than abstract theory, and they illustrate how quickly file size scales with both resolution and color depth.
| Resolution | Bit depth | Padded row size | Pixel array size | Total BMP size |
|---|---|---|---|---|
| 800 × 600 | 8-bit | 800 bytes | 480,000 bytes | 481,078 bytes |
| 800 × 600 | 24-bit | 2,400 bytes | 1,440,000 bytes | 1,440,054 bytes |
| 1280 × 720 | 24-bit | 3,840 bytes | 2,764,800 bytes | 2,764,854 bytes |
| 1920 × 1080 | 24-bit | 5,760 bytes | 6,220,800 bytes | 6,220,854 bytes |
| 3840 × 2160 | 24-bit | 11,520 bytes | 24,883,200 bytes | 24,883,254 bytes |
| 3840 × 2160 | 32-bit | 15,360 bytes | 33,177,600 bytes | 33,177,654 bytes |
These examples make the storage impact very clear. Moving from Full HD to 4K multiplies the total pixel count by four. Moving from 24 bit to 32 bit further increases storage by one third. For raw workflows, that can have a major effect on disk usage, file transfer time, and memory consumption.
Comparison of BMP bit depths
Not every BMP stores color the same way. Bit depth determines how much color information is available for each pixel, whether a palette is needed, and how large the file becomes. Here is a useful comparison:
| Bit depth | Colors available | Palette required | Typical use case | Storage impact |
|---|---|---|---|---|
| 1-bit | 2 colors | Yes, 8 bytes | Monochrome masks, simple icons | Very low |
| 4-bit | 16 colors | Yes, 64 bytes | Legacy graphics, small indexed assets | Low |
| 8-bit | 256 colors | Yes, 1024 bytes | Paletted images, older software pipelines | Moderate |
| 16-bit | 65,536 colors | No | Older display formats, embedded use | Medium |
| 24-bit | 16.7 million colors | No | Standard true color bitmap | High |
| 32-bit | 16.7 million colors plus 8 extra bits | No | Graphics buffers, software rendering pipelines | Very high |
When to use a BMP size calculator
A BMP file size tool is especially useful when you need predictable, uncompressed storage estimates. That includes software development, computer graphics coursework, reverse engineering old image assets, preparing test files, and estimating memory buffers before loading an image into an application.
Common scenarios
- Planning how much disk space a folder of exported bitmaps will use
- Estimating RAM needed for a pixel buffer in a desktop application
- Comparing indexed color versus true color file sizes
- Teaching image encoding fundamentals in schools or technical training
- Checking whether an image format choice is practical for email, download, or archival transfer
If your workflow is web focused, BMP is usually not the best output format because it lacks the efficient compression common on the modern web. Still, it remains extremely useful for understanding how raster images work at the byte level. That transparency makes BMP ideal for learning and testing.
BMP versus compressed formats
One of the biggest lessons from using a BMP size calculator is how inefficient raw bitmap storage can be compared with compressed formats. A 1920 × 1080 24 bit BMP is roughly 6.22 MB. The same photograph saved as a compressed JPEG can often be well under 1 MB, while a PNG of a flat design graphic may also be much smaller depending on image content.
That does not mean BMP is obsolete. Uncompressed formats are useful because they are straightforward and fast to decode conceptually. They can be better for certain internal workflows, forensic analysis, educational demonstrations, and interoperability with older systems. But when storage or bandwidth matters, knowing the BMP size in advance helps you make smarter decisions.
Expert tips for more accurate BMP size planning
- Check whether the BMP is truly uncompressed. Some BMP variants support RLE compression, especially for indexed color images. This calculator assumes no compression.
- Know the DIB header type. The common 40 byte BITMAPINFOHEADER is used here. Other DIB headers can be larger and add overhead.
- Remember row padding. This is often the biggest source of miscalculation.
- Understand unit differences. MB may mean 1,000,000 bytes, while MiB means 1,048,576 bytes.
- Consider batch totals. A single image may seem manageable, but thousands of BMPs can consume enormous storage quickly.
Useful references and authoritative resources
If you want to validate BMP structure, raster imaging concepts, or digital preservation guidance, these sources are useful:
- Library of Congress: BMP File Format Summary
- Library of Congress: Raster Image Format Overview
- University of Michigan Library: Image File Format Basics
Frequently asked questions about BMP file size
Is BMP file size just width × height × bytes per pixel?
No. That gives you only the raw image data estimate. A BMP also includes headers, optional palette data, and row padding to a 4 byte boundary.
Why does 32-bit BMP use more storage than 24-bit BMP?
A 32 bit image stores 4 bytes per pixel instead of 3. That increases the pixel array by about 33 percent at the same resolution. It can also simplify row alignment because rows are naturally multiples of 4 bytes more often.
Does BMP support compression?
Some BMP variants do, but many common BMP files are uncompressed. This calculator is designed for the uncompressed case because it is the most straightforward and predictable for size estimation.
Why do indexed images need a palette?
At 1 bit, 4 bit, and 8 bit depths, each pixel stores an index rather than direct full color values. The palette maps those indexes to actual colors. That palette adds file overhead but can still save significant space compared with full true color storage.
Should I use BMP on websites?
Usually no. Web formats like JPEG, PNG, AVIF, and WebP are typically much more efficient. BMP is better suited to specific desktop, educational, archival review, or technical use cases.
Final takeaway
A BMP size calculator is one of the simplest but most useful tools for anyone who works with raster graphics. It turns image dimensions and bit depth into an accurate storage estimate by accounting for the real structure of the BMP format. That includes the file header, DIB header, palette where applicable, and 4 byte row alignment.
If you need a fast estimate for a legacy graphics project, a programming exercise, or a batch export workflow, the calculator above provides a reliable result. Simply enter the width, height, and bit depth, then review the detailed breakdown and chart. You will immediately see how much of the file comes from actual pixel data and how much comes from structural overhead. That insight can help you choose a more efficient format, optimize your image pipeline, or better understand how bitmaps are stored under the hood.