How to open large CSV files online without uploading them
Drop the file onto the box. The viewer indexes it in one pass, showing progress and a cancel button, then displays the table. Everything after that — scrolling, jumping to a row, filtering, sorting — works on the file in place.
How it opens a huge file without loading it
A browser tab has a limited amount of memory, so anything that reads a whole large file into it — the way most online CSV tools work — fails or freezes past a few tens of megabytes. This toolkit never loads the file. It reads it in eight-megabyte blocks, scans each block for where every row begins, and keeps only that list of positions.
When you scroll to a row, it reads just the bytes of the rows on screen and parses those. The table is virtualised, so only a screenful of rows exists at any moment, whether the file has a thousand rows or a hundred million.
What you can do with the file
- Scroll and jump to any row, with column types shown and column widths you can drag.
- Filter rows with conditions such as contains, equals, greater than, regex or is empty, and download the matching rows.
- Sort by any column with an external merge sort that does not need the file to fit in memory.
- Remove duplicate rows, split the file, or convert it to Excel or JSON Lines.
Messy files, handled
- Quoted fields that contain commas, doubled quotes and line breaks are read correctly, so a note with a newline in it does not split into two rows.
- Comma, semicolon, tab and pipe delimiters are detected from the start of the file, including the semicolons European exports use.
- A byte-order mark, and Windows (CRLF) or Unix (LF) line endings, are handled.
- Rows with more or fewer fields than the header are counted and shown, not silently dropped.
- Files with no header row are supported with one checkbox.
- UTF-8 files are supported. A UTF-16 file needs saving as UTF-8 first, and the tool tells you so.
Honest limits
The largest file tested is 1 GB. Duplicate detection compares 53-bit hashes of rows rather than storing them, so it is exact for practical purposes below a few million distinct rows and has a very small chance of a false match above that. Filtered, sorted and deduplicated results are written to a new file you can download or open again in the viewer.
How to use it
- Drop your CSV file onto the box.
- Wait for the index to finish — the row count appears in the status bar.
- Scroll, jump to a row, or use the tools below the table to filter, sort or dedupe.
- Download the result, or open it again in the viewer.
Frequently asked questions
How do I open a CSV file that is too big for Excel?
Open it here. The file is read in pieces instead of loaded whole, so its size does not matter to the page. You can then filter it down to what you need and download that as a smaller file, or split it into parts Excel can open.
How large a CSV file can it handle?
The largest file it has been run on is 1 GB, opened and scrolled in a real browser. It streams the file instead of loading it, so memory grows with the number of rows (about 8 bytes each for the index), not with the size of the file. Very large row counts, roughly above a hundred million, may exhaust a browser tab’s memory.
Is my file uploaded anywhere?
No. The file is read in pieces by code running in your browser and is never sent to a server. The site records only an anonymous counter with the operation and rough size buckets, such as “100–500 MB” and “1M–10M rows” — never file names, column names or any data.
Why is the first row treated as a header?
Most CSV files start with column names, so it is assumed by default when that row contains no numbers. Untick “First row is a header” if yours has none.
Can I edit the data?
Not in place. This is a viewer and a set of file operations: filter, sort, dedupe, split, merge and convert each produce a new file and leave the original untouched.
Tested on files up to 1 GB. Results depend on your browser and available memory.