convert CSV to Excel (.xlsx) without Excel destroying your data

Open a CSV in Excel and it guesses at every cell. 0001234 becomes 1234. A ZIP code of 07030 becomes 7030. 1/2/2024 becomes whichever date your locale prefers. A 16-digit account number is rounded. A cell beginning with = is run as a formula. None of that is a bug in your data - it is what happens when a file format has no way to say what a value IS.

A real workbook does. Every cell here carries its own type, so nothing has to be guessed at.

how to use it

Drop a .csv - or a TSV, a semicolon or pipe delimited file, a pasted range from Excel or Google Sheets. The delimiter detects itself.
Check the column types in the schema panel. They are read from your actual data, and they decide what each cell becomes in the workbook.
Clean it if you like: drop columns, rename, filter, dedupe, scrub PII. Every change is a visible step.
Pick xlsx beside the export button and choose export clean file. The whole file streams through your steps into a real workbook.

what stays intact

Leading zeros: 0001234 and 07030 are written as text, so they arrive as themselves. The rule is that a column's type only PROPOSES what a cell becomes - the literal text gets a veto, and any value that would not survive the round trip through a number is written as text instead.
Long identifiers: Excel keeps 15 significant digits and quietly rounds the rest, which is a lower ceiling than most tools check for. Anything past it is written as text, so a 16-digit NPI or account number arrives whole rather than ending in a zero it never had.
Real dates: unambiguous ISO dates become genuine date cells that Excel can sort and filter, not text that merely looks like a date.
Ambiguous dates are left alone: 1/2/2024 means two different days on two different continents, so it is written exactly as your source wrote it rather than silently picking a side.
No formula injection: a value starting with =, +, - or @ is text, not a formula - and not because it was sanitized or prefixed with a quote. Text lives in one part of the format and formulas in another, so the file simply contains no formulas.
No encoding question at all: accented characters and CJK just work. There is no BOM to remember and no delimiter to pick, which is the entire reason the "excel-safe" CSV workaround exists in the first place.

honest about the edges

A trailing zero on a decimal is display rather than value, so 1.50 is written as the number 1.5. If you need the string, keep the column as text.
The .xlsx format stops at 1,048,576 rows. Past that, export CSV instead - that path has no limit and streams at any size.
One sheet per export, named after your dataset.

Coming the other way? Excel to CSV reads workbooks with the same care - every sheet, leading zeros intact, dates readable.

every tool in the box

data workbench - profile, clean, transform, and export CSV, JSON, and SQLite - the full suite
JSON flattener - nested JSON or NDJSON to flat columns, arrays and objects unrolled
CSV PII scrubber - find emails, phones, SSNs, and card numbers - mask, hash, or drop them
JSON to CSV - convert JSON or NDJSON to CSV with full column control
CSV to JSON - convert CSV to JSON or NDJSON, streamed at any size
Excel to CSV - read .xlsx workbooks and export CSV, sheets and all
CSV to Excel - write a real .xlsx where leading zeros and dates survive (this page)
SQL on CSV - real SQL over your files: joins, GROUP BY, a read-only console
CSV to SQLite - drop several files, get one .sqlite database with discovered foreign keys
CSV to SQL - CREATE TABLE and INSERT statements for Postgres, MySQL, SQL Server, SQLite
JSON to TypeScript - TypeScript interfaces and JSON Schema derived from your real data
large CSV viewer - open files past Excel's 1,048,576-row limit without loading them into RAM
regex painter - highlight example matches, get a verified regular expression back
log parser - read log files as events: layouts detected, stack traces kept whole, timestamps normalized, and a volume-over-time strip drawn in SQL across every row
visualize - chart any table you have open - bar, line, area or point - drag across it to filter, and export svg, png, or the vega-lite spec that drew it
theme builder - design, test, and export custom color themes with live previews, palette derivation, and contrast diagnostics

Like every unfckr tool, this runs 100% in your browser: no upload, no servers, no signup, no analytics. A Content-Security-Policy header blocks every programmatic network channel, so not even malicious code could phone home - and you can watch it happen: the privacy page runs six live escape attempts and shows the browser refusing each one.

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