Is converting JSON 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 JSON 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 JSON 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 a .json file

A .json file can be almost any shape, and only a few of those shapes are a table. What the reader accepts, and what it does with the rest, is the thing worth knowing before you upload one.

→A wrapper is unwrapped only when the object has exactly one key and that key holds a list of objects. {"meta": {...}, "data": [...]} has two keys, so it is read as a single row instead.
→An array of strings, or an array of arrays, is refused with "Expected a JSON array of objects" - there are no column names in it to build a table from.
→Columns are the union of the keys seen in the first 10,000 records, in the order they first appear. A key that shows up only after that is not part of the table.

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 JSON

→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 JSON 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 JSON 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.