Split and Join Large Files

About splitting a large file

Some limit is always in the way. Email stops at 25 MB. A form refuses anything over 100 MB. A USB stick formatted as FAT32 cannot hold a single file over 4 GB, no matter how much space is free.

The classic answer on Windows is copy /b — which can join files, but cannot split them at all. So people reach for an archiver, or give up and use a cloud link.

This does both halves. Split a file into numbered parts, send or carry them, then join them back into a file that is byte for byte the original.

How to use it

To split:

• Drop the big file on the page, or click to choose it
• Set the size of each part, or how many parts you want — or pick a known limit from the list
• Press Save the parts into a folder, or take a download link for each part

To join:

• Switch to Join the parts back
• Select every part at once — the order is worked out for you
• Check the list, then press Join them

Part names, and why they look like that

Parts are named backup.zip.001, backup.zip.002, and so on — the convention every splitter has used for thirty years.

Two things come out of that. The original name and extension survive the round trip, so joining knows what the file was called. And the three-digit numbering means a plain alphabetical sort — in Explorer, in copy /b, anywhere — still puts part 2 before part 10.

Joining is the half people get wrong

copy /b a+b+c whole.zip works, but only if the parts are listed in the right order, and Windows sorts part10 before part2 whenever the numbering has no leading zeros. The result is a file of exactly the right size that is silently corrupt.

Here the parts are ordered by comparing the digits as numbers, the order is shown before anything is written, and a gap in the sequence is called out. If part 4 is missing, you are told, rather than handed a broken file.

Size, or number of parts

• Size of each part — the usual choice when something else set the limit. The list of known limits fills it in for you: email attachment, Gmail, a 100 MB upload cap, a CD, a FAT32 stick, a DVD
• Number of parts — when you want three roughly equal pieces rather than a specific size

Saving the parts

Chrome, Edge and Opera can write the parts straight into a folder you choose, streaming each one to disk. Memory stays flat, so the size of the file genuinely does not matter — a 40 GB file works the same as a 40 MB one.

Every browser can instead take a download link for each part. They are links you click one at a time on purpose: a browser blocks repeated downloads a page starts by itself, so handing over twelve at once would quietly fail after the first.

What people use it for

• Sending a file too big for email in several messages
• Getting a large file past an upload size limit on a form or portal
• Putting a big video or disk image onto a FAT32 USB stick
• Splitting a backup across several DVDs or discs
• Uploading over a shaky connection, where a failed 8 GB upload has to start again but a failed 500 MB part does not
• Putting back together parts someone else sent you

Good to know

• Nothing is uploaded. The file never leaves your device, whatever its size
• The original file is never changed — splitting only reads it
• Joining is exact: put every part back in order and you get the original bytes, so a ZIP or installer works as it did
• Keep all the parts together. Every one is needed
• Up to 999 parts, which is what the three-digit naming allows
• Parts are plain files with no header of their own, so copy /b and cat can join them too
• Works in every browser, and on phones and tablets for single files
• This is not compression — the parts add up to the size of the original. Zip it first if you want it smaller

Related tools

• Zip and Unzip Files — compress it before splitting
• Compress Video to a Size — often better than splitting a video
• Merge Text and CSV Files — joining, but for text with headers
• Compress PDF — get a PDF under the limit instead
• Folder Sync and Backup — move whole folders rather than one big file