# Chunk size for reading writing netcdfs

**URL:** <https://discourse.pangeo.io/t/chunk-size-for-reading-writing-netcdfs/1804>\
**Category:** Data\
**Created:** [September 28, 2021, 8:43pm UTC](https://discourse.pangeo.io/t/chunk-size-for-reading-writing-netcdfs/1804 "2021-09-28T20:43:12Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![WilcoT](https://avatars.discourse-cdn.com/v4/letter/w/7c8e57/32.png) [@WilcoT](https://discourse.pangeo.io/u/WilcoT)\
**Post date:** [September 28, 2021, 8:43pm UTC](https://discourse.pangeo.io/t/chunk-size-for-reading-writing-netcdfs/1804/1 "2021-09-28T20:43:12Z")

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Hi,

Is there any best practice for choosing chunk sizes when:

1. Using xr.open\_mfdataset to open 20 years of daily netcdf files (7305 files in total, leading to a dataset with dimensions: time: 7305, x: 1224, y: 1090). Dataset has two variables.
2. Doing some resampling over dimension time, e.g. monthly means
3. Writing the entire dataset from step 1 back to disk as one netcdf file (instead of 7305 individual files)
4. Writing the result from step 2 to disk as one netcdf file

The data is of dtype float32. All this is run on a simple standalone PC with 16 GB of RAM. If I don’t specify the chunk size it takes about 23 minutes to write step 3 to disk, which I find rather time consuming. The netcdf file size of step 3 is about 36 GB.

Any advice on how this can done best?