What is the NSF file format?
An .nsf file is a database file that stores data in the Notes Storage Facility (NSF) format - a proprietary binary NoSQL document store. The .nsf file is used to store various types of data in the form of "notes" (documents), specifically mail, calendar, and general application data created by HCL Notes software (formerly IBM Lotus Notes). The file contains metadata, user data, and design information such as forms, views, and agents. Generally, .nsf files are stored on the HCL Domino server or on HCL Notes clients, and can be accessed in offline mode. Optional encryption is tied to the user's .id identity file. Database templates that share the same on-disk structure use the .ntf extension.
Security & safety
RISK: MEDIUMThe NSF format itself is business data, and databases can run agents/LotusScript, but the biggest real risk to users is the conversion ecosystem: searches for "free NSF viewer/converter" lead to adware-bundled and outright fake tools. Use only well-known vendors (HCL, Kernel, Stellar, SysTools, BitRecover) from their official sites. Also note NSF can be encrypted with a Notes ID - without the right ID and password the data cannot be read by any tool, legitimate or otherwise.
Format details
in a nutshellFULL NAMENotes Storage Facility databaseaka Lotus Notes database, HCL Notes database
DEVELOPERHCLSoftware (formerly IBM Lotus; originally Lotus Development)since 1989 (Lotus Notes 1.0)
CATEGORYDatabase Files
MIME TYPEapplication/vnd.lotus-notes
TYPEProprietary NoSQL document database (binary)
This extension is also used by…
- NES Sound Format - Chiptune audio format containing ripped Nintendo NES/Famicom music played by NSF players/emulators - unrelated to Lotus Notes.
- NetShelter / NetSuite export - .nsf is occasionally seen for assorted vendor export/config files; not the Notes database documented here.
MAGIC BYTES · FILE SIGNATURE
OFFSET
000102030405
HEX
1A0000040000
ASCII
······
NSF (and the template variant NTF) is a proprietary binary database with no public, stable magic string; it begins with a small binary header followed by the On-Disk Structure. There is no documented portable signature, so files are identified by extension and by the Notes/Domino API, not by a reliable leading byte sequence.
Programs that open NSF files
Windows3 apps
HCL Notes
Paid Install HCL Notes, set up your Notes ID, then File > Open > Notes Application (or open a local replica) to read the NSF.
HCL Domino server
Paid Host the NSF on a Domino server and access it via Notes/web (HCL iNotes/Verse) - the server-side way to open application databases.
SysTools NSF Viewer / Kernel NSF Viewer
Freemium Free viewers that read NSF without a full Notes setup (often still need Notes installed for encrypted files); useful for inspecting old archives. Verify the download source. 
macOS1 app
Technical details
deep spec| Format type | Proprietary binary NoSQL document database |
| MIME type | `application/vnd.lotus-notes` |
| Byte order | Little-endian |
| File signature / magic bytes | Binary header at offset 0 (hex: `1A 00 00 04 00 00`); no printable ASCII signature - identified by extension and Notes/Domino API |
| On-Disk Structure (ODS) version | Incremented with each major Notes/Domino release (e.g. ODS 43, ODS 52, ODS 53); controls feature support and compatibility |
| Encryption | Optional local database encryption tied to the user's Notes `.id` file; per-document and field-level encryption also supported |
| Compression | Optional LZ1 or Huffman compression for documents and attachments; separate design element compression |
| Typical file size | A few MB to many GB; a single `.nsf` can historically approach ~64 GB |
| Data model | Schema-free collection of "notes" (documents), each composed of named, typed items/fields - no fixed schema |
| Design elements | Forms, views, folders, agents, LotusScript, Java, and JavaScript code stored as design notes within the same `.nsf` file |
| Operating systems | Windows, macOS, Linux (Domino server), IBM i, AIX |
| Offline replication | Full offline mode; changes replicate bidirectionally with HCL Domino server when client reconnects |
| Integrity / repair tools | `nfixup` (consistency check) and `ncompact` (compaction / defragmentation) command-line utilities |
| Related template format | Database design templates use the `.ntf` extension and share the identical NSF on-disk structure |
| Developer | HCLSoftware (formerly IBM Lotus, originally Lotus Development Corporation) |
| Access control | Built-in Access Control List (ACL) defines user and group rights at database, document, section, and field level |
| Released | 1989 (Lotus Notes 1.0) |
| Latest version | NSF as used by HCL Notes/Domino 14 (2023); on-disk structure (ODS) versions evolve per release |
| Specification | www.hcl-software.com |
NSF conversions
Possible
.NSF .PSTGUIDE
Yes - use a commercial converter such as Kernel for Lotus Notes to Outlook, Stellar Converter for NSF, or…
How to convert
.NSF .MBOXGUIDE
Yes, NSF mail databases can be converted to MBOX using dedicated migration tools such as SysTools NSF to MBOX…
How to convert
.NSF .EMLGUIDE
Yes -- the fastest way to get individual .eml files from an NSF is to drag messages out of HCL Notes directly…
How to convert
.NSF .CSVGUIDE
Yes, you can convert NSF to CSV using the built-in export in HCL Notes - no third-party tool required.
How to convert
.NSF .DXLGUIDE
Yes -- use HCL Notes/Domino's built-in DXL export (NotesDXLExporter), which is the only supported method and…
How to convert
Not possible
Community Q&A
asked by usersNo questions yet - be the first to ask about NSF files.
Frequently asked questions
How do I open an NSF file?
Open it in HCL Notes (formerly Lotus Notes) with the matching Notes ID, or place it on a Domino server. NSF is proprietary and can be encrypted, so a generic viewer often still needs Notes installed.
How do I convert NSF to PST for Outlook?
Use a reputable NSF-to-PST converter (Kernel, Stellar, SysTools, BitRecover); most require HCL Notes installed to read the data. Be careful - this niche has many adware-laden fake 'free' tools.
Can I open an NSF without Lotus/HCL Notes?
Sometimes: free NSF viewers can read unencrypted databases, but encrypted NSF files (tied to a Notes ID) require Notes installed. There is no reliable way around the Notes security model.
What is an NSF file used for?
It is a Lotus/HCL Notes database - it can hold email, contacts and calendars, or an entire business application (forms, views, code). Many are old mail archives from companies that used Notes.
My NSF file plays music / is a chiptune - why?
That is a different format: NES Sound Format, also using .nsf, holds ripped Nintendo NES music. It is unrelated to Lotus Notes and is played by NSF chiptune players, not Notes.
My NSF is corrupt - how do I repair it?
Use the Notes/Domino tools nfixup and ncompact (Fixup and Compact) to repair and rebuild the database, or a recovery tool like Stellar Converter for NSF for severe damage.
References
1Wikipedia - HCL Notes (Lotus Notes)en.wikipedia.org
2Wikipedia - HCL Domino (Lotus Domino)en.wikipedia.org
