CSV Market Data to Parquet: Faster, Smaller Files

A folder of price CSV files gets slow to load. Parquet keeps the same data in a smaller, typed file that research tools read quickly.

Open the CSV to Parquet converter

Open the CSV to Parquet converter, drop the file and turn on type detection so prices and volumes become numbers and timestamps become dates. Download the Parquet file. For many files, use the batch option.

Check the types before you trust the file. Symbols should stay text. Dates should be real dates in one time zone, and prices should not contain text such as N/A. The Data Profiler finds blanks, and Data Transformation can fix a column type before you export.

To check the result, convert the Parquet back to CSV and compare the first and last rows and the row count with the original. Files over about 500 MB are best handled in large file mode.

Everything runs in your browser and nothing is uploaded.

Open the CSV to Parquet converter

Related tool pages

  • JSON, CSV, Parquet and Excel Converter: Convert between JSON, CSV, Parquet and Excel online for free, one file or a whole folder. It runs in your browser, so nothing is uploaded.
  • JSON to CSV Converter: Convert JSON to CSV in seconds. Drop a file or paste data and download the result. Private: it runs in your browser and nothing is uploaded. Large files too.
  • CSV to JSON Converter: Convert CSV to JSON in seconds. Drop a file or paste data and download the result. Private: it runs in your browser and nothing is uploaded. Large files too.
  • JSON to Parquet Converter: Convert JSON to Parquet in seconds. Drop a file or paste data and download the result. Private: it runs in your browser and nothing is uploaded.
  • Parquet to CSV Converter: Convert Parquet to CSV in seconds. Drop a file or paste data and download the result. Private: it runs in your browser and nothing is uploaded. Large files too.
  • Parquet to JSON Converter: Convert Parquet to JSON in seconds. Drop a file and download the result. Private: it runs in your browser and nothing is uploaded. Large files too.

How to use the CSV to Parquet converter: step by step

  1. Open the CSV to Parquet converter.
  2. Drop the file and turn on type detection.
  3. Check the types and the row count.
  4. Download Parquet and read it back to compare.

When to use the CSV to Parquet converter

  • Speed up loading of price history
  • Shrink a folder of CSV files
  • Prepare data for pandas, Polars or DuckDB
  • Share typed data with a colleague

Example

Input: id,name,age 1,Ada,36 2,Alan,41

Output: A Parquet file with the same columns (id, name, age) and typed values.

Frequently asked questions

How do I convert CSV to Parquet?

Drop your CSV file (or paste the data), check the options, click Convert and download the Parquet result.

How big can a file be?

It depends on your device. A few hundred MB usually works.

Which conversions does the Format Converter support?

Any direction between JSON, CSV, Parquet and Excel (.xlsx), for one file or a whole folder. Nested JSON is flattened into columns such as address.city. For Excel, the first sheet is read; old .xls files are not supported.

Why did my number or date change after converting?

Types are detected from the values. Turn off "Detect types" to keep everything as text, which also keeps leading zeros in IDs and phone numbers.

Tip: Read the Parquet back to CSV and compare the last rows. A cut file is easy to miss.