SPI flash chips are sized in megabits, BIOS files in bytes, and programmer software in whatever unit it likes. Mixing them up is one of the most common reasons a programmer refuses a file or a board fails to start after flashing. This calculator reads the capacity from the chip's marking and gives every form of its size, then checks a file against it.
How to use it
- Type the marking from the chip —
W25Q128JV,MX25L6406E,GD25B256— or a size such as64Mbitor8MB. - Optionally type the size of the BIOS file you want to write, in bytes, MB or hex.
- Read the capacity, the exact byte count, the last address, and whether the file fits.
How the size is encoded in the marking
In most 25-series SPI flash part numbers, the digits after the family letters give the capacity in megabits: W25Q64 is 64 Mbit, MX25L12835F is 128 Mbit, W25Q80 is 8 Mbit. The calculator recognises the common patterns. 24-series parts are I²C EEPROMs sized in kilobits — the number in 24C02 means 2 Kbit.
| Capacity | Bytes | Last address |
|---|---|---|
| 32 Mbit (4 MB) | 4,194,304 | 0x3FFFFF |
| 64 Mbit (8 MB) | 8,388,608 | 0x7FFFFF |
| 128 Mbit (16 MB) | 16,777,216 | 0xFFFFFF |
| 256 Mbit (32 MB) | 33,554,432 | 0x1FFFFFF |
The file check
- Same size as the chip — a full dump, ready to compare or write.
- Smaller — usually a single region (for example the BIOS region only) or a vendor update file, which must not be written raw to the chip.
- Larger — the file includes a header, a capsule wrapper, or images for more than one chip.
Vendor update files and full dumps are different things; that difference is explained in BIOS File Formats: .bin, .rom, .cap and .fd Explained. Some boards split the firmware across two chips, covered in Laptops with Two BIOS Chips: When the Dump Is Split.
Worked example
A board's BIOS chip reads 25Q64JVSIQ. That is 64 Mbit — 8 MB, 8,388,608 bytes, last address 0x7FFFFF. The file downloaded for the board is 16,777,216 bytes: twice the chip size. Either the file is for a different board revision with a 128 Mbit chip, or it covers two chips. Writing it would fail or be truncated. Checking the file's integrity is the next step, with File Checksum Calculator (SHA-256, SHA-1, CRC32).
FAQ
Why is my 128 Mbit chip shown as 16 MB?
Eight bits make a byte: 128 Mbit ÷ 8 = 16 MB (16 × 1,048,576 bytes).
Can I write a 4 MB file to an 8 MB chip?
Only if the board expects it at a specific offset, which is unusual. Normally the dump must match the chip size exactly.
What if the marking has no obvious size?
Look up the full part number in the manufacturer's datasheet, or let the programmer software identify the chip by its ID.