Is converting NDJSON to DuckDB free?

Yes. Files up to 10MB convert free with no account. There is a limit of five conversions per day per IP address.

Will my NDJSON data keep its types in DuckDB?

Yes — that is the point. Whole numbers, decimals, dates and booleans are mapped to real DuckDB types rather than everything becoming text. The mapping table above shows exactly what you get.

Do you store my file?

No. The file is written to a temporary directory, converted, and the directory is deleted as soon as your download has been sent.

Convert NDJSON to DuckDB

Upload a .json or .ndjson file and get back a DuckDB file. Types are preserved rather than flattened to text. Free, no registration, up to 10MB.

1Upload your file

Click to upload or drag and drop

CSV, Excel, SQLite, SQL, DBF (max 10 MB)

2Choose target format

How your data is typed in DuckDB

Generated from the converter's own DuckDB type mapping — this is what it actually emits, not an example.

Your data	DuckDB type
Whole number	BIGINT
Decimal (money)	DECIMAL(10,2)
Floating point	DOUBLE
True / false	BOOLEAN
Date	DATE
Date and time	TIMESTAMP
Short text	VARCHAR
Long text	VARCHAR
Binary data	BLOB

Working from an .ndjson file

Newline-delimited JSON is one complete object per line with no enclosing array, which is exactly what makes it appendable: a log or an export can keep growing without anything having to rewrite the end of the file.

→Each line is parsed on its own and blank lines are ignored, but a single malformed or truncated line fails the whole file - the usual outcome of copying a log that was still being written.
→Lines need not share their keys. The columns are what the records collectively use, and a record that omits one gets NULL in that column.
→A file that is one line long and wraps its array, {"rows": [...]}, is unwrapped into those rows rather than read as a single record.

Decimals and text in the .duckdb file

DuckDB takes the source's declared types more nearly unchanged than any other target here, which makes the two places where it cannot the things worth reading: the width of a decimal, and the length of a string.

→DECIMAL tops out at 38 digits, so a wider source column is clamped rather than refused: an Oracle NUMBER(50,10) is created here as DECIMAL(38,10), and the scale is never allowed to grow past the precision holding it.
→A decimal that declared no scale gets six places for its fraction rather than none, inside that same rule: a source precision of 10 becomes DECIMAL(10,6), one of 5 becomes DECIMAL(5,5), and one that declared no precision either becomes DECIMAL(18,6).
→Strings become VARCHAR with no length attached to them. DuckDB's VARCHAR is unbounded, so no column has to be sized in advance and no row can be turned away for being longer than the column it goes in.

Reading NDJSON

→JSON types are trusted: 1 and "1" stay different things.
→Numbers with a decimal point are read as exact decimals rather than floats.
→Both an array of objects and newline-delimited JSON work, and a wrapped array like {"data": [...]} is unwrapped automatically.
→Nested objects are kept as JSON rather than flattened into invented column names.

Writing DuckDB

→A real .duckdb file, not a script — open it straight away with the DuckDB CLI.
→DuckDB has genuine DECIMAL and BOOLEAN types, so nothing is flattened on the way in.

What you get

A ZIP containing database.duckdb. If anything about your data needed a judgement call, a _warnings.txt explains it.

Questions

Is converting NDJSON to DuckDB free?: Yes. Files up to 10MB convert free with no account. There is a limit of five conversions per day per IP address.
Will my NDJSON data keep its types in DuckDB?: Yes — that is the point. Whole numbers, decimals, dates and booleans are mapped to real DuckDB types rather than everything becoming text. The mapping table above shows exactly what you get.
Do you store my file?: No. The file is written to a temporary directory, converted, and the directory is deleted as soon as your download has been sent.