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NII Converter
Convert NII files right in your browser, with no upload and no size limit. reaConverter for Windows adds 700+ file formats and batch mode.
How to Convert .nii Files in Bulk
1. Install reaConverter
2. Select NII Files
3. Start Conversion
Step by step
See It in Action
For quick and effortless NII file handling on your PC, reaConverter offers powerful conversion tools with batch processing. Best of all, it works entirely offline — ensuring your sensitive data never leaves your computer.
Extra Tools
Do more with your files
Converting NII files for use in a larger project? reaConverter offers many ways to process files, so you can customize them according to your requirements.
Our range of editing options includes auto-processing options such as auto-rotate, auto-crop, watermarking and more. Change Color Space, copy Metadata, set DPI, manage ICC Profile, control Bit Depth.
User reviews
From those who tried it
Average User Rating:Last updated on June 29, 2026
Questions & Answers
NII files: Frequently Asked Questions
What are NII files?
NIfTI, which usually carries the .nii or compressed .nii.gz extension, was created by the National Institutes of Health (NIH) to replace the older, cumbersome Analyze format. A NIfTI file can store 3D structural volumes or 4D time-series data (like functional MRI or diffusion-weighted imaging), keeping the raw voxel intensity matrix bundled with a critical header. This header contains spatial affine matrices (qform and sform) that map the image coordinates directly to real-world physical space, preventing common orientation errors like flipping left and right sides of the brain. While it is the absolute standard for neuroimaging research and works natively in tools like FSL, SPM, and FreeSurfer, NIfTI files lack patient-centric clinical metadata, meaning they are frequently converted from clinical DICOM files for analysis and subsequently exported to standard formats or specialized meshes for visualization.
Can reaConverter open NII files?
reaConverter fully supports the NII file format – open, edit, and export NII files to any other popular file format with ease.
How do I change NII to another format?
reaConverter is a user-friendly tool for converting NII files. Once installed on your computer, it allows you to convert all your .nii files at once with just a few clicks. You can even convert nii images directly from Windows Explorer using the right-click menu — fast and convenient.
Can I convert NII online?
If you only want to convert a few NII files, you are welcome to try our free online NII converter.
How can I turn NII files on my computer to another format?
The best way to convert NII files on your PC is to get a quality piece of software, such as reaConverter. This software is extremely efficient in managing a wide range of batch files conversions, including conversion from NII.
How can I automate NII files conversion?
reaConverter supports all the essential automation features, such as right-click сonversion and Watch Folders, for the most efficient automated file conversion in any software of its kind.
Can I convert NII files using scripts or the command-line interface?
reaConverter provides full access to all its features from the Windows command-line shell, including the command-line nii converter.
What else can I do with reaConverter?
Convert DICOM imagesCrop photoConvert PDFWatermark photosConvert vector images
What formats can I convert a NII file to?
Discover More
Beyond NII conversion
Convert .nii files in batches of any size, with editing and processing applied along the way. The same tool also handles 700+ other formats, including CAD, medical imaging, and GIS.
