LeanRows: a free large CSV viewer for Windows

Open a 1.5 GB CSV with 20 million rows using 18 MB of RAM. No installer, no runtime, no admin rights.

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20,000,001 rows, 1,532,454,643 bytes, fully indexed. Peak working set: 18.4 MB.

Excel stops at about 1.05 million rows. Most tools that go further are paid, need an installer, or live in a terminal. LeanRows is a single 987 KB executable you can copy onto a locked-down work machine and run.

It is strictly read-only, so it cannot alter the file it is inspecting. There is no telemetry and no network access of any kind.

Install

Download the portable ZIP from the latest release, extract it, and run leanrows.exe. Outside that folder, LeanRows writes only its theme and zoom settings, under HKCU\Software\LeanRows, and a scratch folder for search results, under %LOCALAPPDATA%\LeanRows. Column sorting also keeps temporary sort data there.

Microsoft Store

LeanRows is also in the Microsoft Store. The Store build is signed by Microsoft, so Windows installs it without the SmartScreen warning that the unsigned GitHub ZIP can show. It can also be installed from the Store with winget:

winget install --id 9NMV8GKJXSN0 --source msstore

Scoop

scoop bucket add lean https://github.com/abooodbah/scoop-bucket
scoop install lean/leanrows

Chocolatey

From an administrator PowerShell:

choco install leanrows -y

The LeanRows package installs the portable Windows ZIP from the GitHub release, checks its SHA-256, and adds a Start menu shortcut and a command-line shim. To move an existing Chocolatey installation to the newest release:

choco upgrade leanrows -y

A new version reaches Chocolatey once its moderators have approved the package, which can take a few days after the GitHub release.

Android (closed test)

LeanRows for Android is in a closed test on Google Play before its public release. To try it:

1. Join the Lean Apps testers group with the Google account you use on your phone.
2. On your phone, open the LeanRows tester page, tap Become a tester, then install LeanRows from Google Play.

If the tester page says the app is not available yet, Google is still reviewing the test; try again a day later. Google asks testers to keep the app installed for 14 days. The APK is also attached to each Android release.

Supported formats

CSV, TSV, JSONL, NDJSON, log, plain text, and .xlsx workbooks (stored cell values, read-only), on Windows x64. CSV and TSV record boundaries are quote-aware across read blocks, including quoted fields containing newlines. JSONL, NDJSON, log, and text files are treated as line-oriented records.

How it stays small

The file is indexed progressively as you move through it, and only the rows currently in the viewport are materialized. Peak memory is governed by window size rather than file size, which is why the figure above stays flat whether the source is 15 MB or 1.5 GB.

LeanRows is for viewing large row-based data files without loading everything into memory. LeanMark, a Markdown viewer, is for reading Markdown documents and Mermaid diagrams in a clean, focused interface, and LeanDoc, a Word document viewer, is for reading .docx files without opening an office suite. All three are read-only tools designed to make local files easier to inspect without changing them.

Why LeanRows

• Progressive viewing: the first bounded viewport is prepared before the background index reaches the end of a large file.
• Bounded data structures: fixed-capacity checkpoints, viewport caches, previews, and coalescing worker queues prevent memory use from scaling with the source file by design.
• Read-only snapshots: LeanRows never edits, imports, or rewrites the source.
• Focused inspection tools: inline literal search, absolute-row navigation, and bounded clipboard copy operate without constructing a whole-file model.
• Native Windows interface: a responsive command bar and virtual owner-data grid use Win32 controls rather than a browser engine or bundled web runtime.
• Local operation: the application contains no telemetry, update checker, account system, or network retrieval path.
• Visible malformed data: invalid bytes are escaped and oversized values are marked as truncated instead of being silently normalized.

Download

The official package is attached to the GitHub release as:

LeanRows-v0.5.0-windows-x64-unsigned.zip
LeanRows-v0.5.0-windows-x64-unsigned.sha256

LeanRows 0.5 is not Authenticode-signed. Windows may therefore show a security warning. Verify the downloaded ZIP before running it:

Get-FileHash .\LeanRows-v0.5.0-windows-x64-unsigned.zip -Algorithm SHA256

The result must match the value in the accompanying .sha256 file from the same release.

Run or install

The extracted ZIP is portable. Launch leanrows.exe directly, pass one or more supported file paths on the command line, use Open file or Ctrl+O, or drop supported files onto the window. Each file opens in its own tab.

While LeanRows is running, a file opened from Explorer or the command line joins the running window as a new tab, so every open file shares one process. A copy started from a different folder, such as a portable copy next to an installed one, keeps its own window.

An optional per-user installation is also included:

.\install.ps1 -ValidateOnly
.\install.ps1

-ValidateOnly checks the package manifest and file hashes without changing files or the registry. Installation places LeanRows under %LOCALAPPDATA%\Programs\LeanRows and registers it as an available handler for .csv, .tsv, .jsonl, .ndjson, and .log. Where an extension has no protected Explorer UserChoice, the installer records its prior direct per-user default and makes LeanRows the direct per-user default. Protected choices and .txt remain untouched. The installer also adds LeanRows to the current user's Start menu and Installed Apps list.

Run the installed uninstaller with:

& "$env:LOCALAPPDATA\Programs\LeanRows\uninstall.ps1"

See the installer contract for the exact package and registry behavior.

Supported input

Format	Extensions	v0.5 behavior
Comma-separated data	.csv	Quote-aware logical records in independently bounded native columns
Tab-separated data	.tsv	Quote-aware logical records in independently bounded native columns
JSON Lines	.jsonl, .ndjson	One physical line per displayed record; no schema or JSON-tree view
Logs and text	.log, .txt	One physical line per displayed record
Excel workbooks	.xlsx	One worksheet at a time, stored values as text; see XLSX cell text. Opened from LeanRows or the command line; the installer does not register .xlsx.

Format selection is extension-based in 0.5. Input is treated as bytes and decoded for display as UTF-8; invalid bytes are shown as \xNN. Long records, fields, and display strings use bounded previews and are marked when truncated.

LeanRows 0.5 deliberately excludes editing, saving, export, filtering, regular expressions, SQL, charts, archives, remote or UNC paths, reparse points, tail/follow mode, plug-ins, scripts, and arbitrary JSON documents remain excluded.

Column sorting

Click a data-column header to sort the entire document ascending; click it again to reverse direction. Right-click the header for text or numeric ordering. The More menu exposes the same commands for the last selected sort column (initially the first column). Click Row or choose Restore source order to reset. More > Cancel sort stops pending work while keeping the last completed view.

Sorting runs in the document worker, with a process-wide 16 MiB managed-memory limit and 8 GiB temporary-disk quota. Only one sort runs at a time; a sort in another tab waits for it rather than cancelling it. A sort needs about 256 bytes of temporary disk space per row at its peak (about 2.4 GB for 10 million rows), so files above roughly 33 million rows cannot be sorted. LeanRows checks the free space before it starts and says so if the drive is too full. No database is created and the source is never rewritten. Closing/reloading releases the sort data.

Text order compares complete decoded bytes, case sensitively and without locale collation; displayed truncation does not truncate the sort key. Numeric mode compares decimal integers, fractions and scientific notation exactly, without floating-point rounding. Numbers precede text and empty/missing values in both directions. Equal keys retain original row order. CSV/TSV's first record remains data. Row numbers, Find results and Go to row identify the original source rows. Sorting is not available for .xlsx workbooks yet; it reports that instead.

Snapshot behavior

An open document is a read-only snapshot. On Windows, the retained file handle allows reads and atomic replacement but does not share in-place writes. An editor that appends to, truncates, or rewrites the same file may receive a sharing violation while it remains open in LeanRows. Editors that save by atomic replacement can proceed; LeanRows detects the changed path identity and requires a reload before continuing.

This behavior prevents indexed offsets from being combined with different file bytes. Closing the document releases the retained handle.

Memory model

LeanRows does not load the complete document, store an offset for every row, or create a full in-memory object model. It reads positioned blocks, maintains a fixed-capacity adaptive index, and decodes only bounded record previews for the active viewport. A single large record is scanned incrementally rather than allocated as one buffer.

Literal Find uses fixed read and result-page buffers. Its result positions are stored in an application-owned scratch file capped at 64 MiB under %LOCALAPPDATA%\LeanRows\query-cache, never beside the source. Find is demand-driven: it advances only far enough to answer the current next or previous request. Owned scratch files are removed after use, and stale-file recovery is coordinated across concurrent LeanRows processes.

Every open file shares one process. Each tab has its own background worker, file handle, index, and cached row window; a tab that is not showing keeps its rows in the same allocation its worker already holds, so switching tabs copies nothing. At most 32 files can be open at once. A window with no open file runs no worker thread, and minimizing the window asks Windows to page out the process until it is used again.

These bounds apply to data-dependent application structures, not to a fixed whole-process RAM guarantee. Executable pages, Windows controls, thread stacks, allocator overhead, and the operating-system file cache are separate. Release qualification records full process-tree memory independently; see Acceptance and assurance.

Keyboard and accessibility

• Ctrl+O opens the native file picker.
• Ctrl+F focuses the inline literal-search field. Enter or F3 moves to the next match; Shift+Enter or Shift+F3 moves to the previous stored match.
• Ctrl+G jumps to a checked, one-based absolute row number.
• Ctrl+C copies selected cached rows as tab-delimited Unicode text. One operation is limited to 4,096 rows and 1 MiB, including the terminator.
• F5 reloads the active path as a new snapshot.
• Ctrl+Plus, Ctrl+Minus and Ctrl+0 zoom the rows in, out and back to actual size, as does Ctrl with the mouse wheel over the rows. The steps are LeanMark's, from 50% to 300%. Only the rows and their column titles zoom; the toolbar and tabs keep their size. The zoom is remembered.
• Ctrl+/ or F1 opens the settings panel, and Escape closes it. The panel, as in LeanMark, has the theme, the zoom, and the list of every keyboard shortcut and mouse action, sorting included. The settings button next to Open does the same.
• Ctrl+Shift+T changes the theme, as the theme button does. The button shows a coloured dot for the current theme, and its tooltip names the theme.
• Ctrl+Tab and Ctrl+Shift+Tab (or Ctrl+PgDn and Ctrl+PgUp) move between tabs. Ctrl+1 to Ctrl+8 pick a tab by position and Ctrl+9 picks the last one. Ctrl+W or Ctrl+F4 closes the current tab; a middle click closes any tab.
• Standard list navigation keys move through the virtual row control.

Find compares raw UTF-8 bytes. Its optional case-insensitive mode folds ASCII letters only; non-ASCII bytes remain exact. Regular expressions are not supported in 0.5.

The interface provides five themes, the same as LeanMark: System, Light, Dark, Dim, and Dusk. Dim and Dusk are softer dark themes. System follows the active Windows app theme. The application also provides per-monitor DPI handling, visible keyboard focus, and Microsoft Active Accessibility names for the window, grid, and row items. When Windows high contrast is active, application colors yield to the corresponding system colors.

Build from source

The workspace pins Rust 1.97.1:

rustup toolchain install 1.97.1 --profile minimal --component rustfmt,clippy
cargo +1.97.1 build --release --locked -p leanrows-app

The release executable is written to target/release/leanrows.exe. Building the Windows application requires the Windows SDK resource compiler (rc.exe) because the manifest and application icon are embedded at build time.

Run the repository checks with:

cargo +1.97.1 fmt --all -- --check
cargo +1.97.1 test --workspace --all-targets --locked
cargo +1.97.1 clippy --workspace --all-targets --locked -- -D warnings
.\scripts\Test-ReleaseEngineering.ps1

Project documentation

• Architecture
• Acceptance and assurance
• Validation status
• LeanRows 0.5.0 release notes
• LeanRows 0.4.0 release notes
• LeanRows 0.3.0 release notes
• LeanRows 0.2.0 release notes
• LeanRows 0.1.3 release notes
• LeanRows 0.1.2 release notes
• LeanRows 0.1.1 release notes
• Security policy
• Roadmap
• Contributing
• Changelog
• Citation metadata

License

LeanRows is available under the MIT License.

Privacy policy

This program will not transfer any information to other networked systems unless specifically requested by the user or the person installing or operating it.

In practice LeanRows makes no network connections at all. It has no telemetry, update checker, account system, or network retrieval path, and it refuses remote and UNC file paths.

Uninstallation

LeanRows is portable. To remove it, close it and delete the folder you extracted it to. Outside that folder it keeps two things, which you can delete as well:

• the theme and zoom settings, under HKCU\Software\LeanRows
• a scratch folder for search results, under %LOCALAPPDATA%\LeanRows

Remove-Item -Path 'HKCU:\Software\LeanRows' -Recurse
Remove-Item -LiteralPath "$env:LOCALAPPDATA\LeanRows" -Recurse

If you installed LeanRows with Chocolatey, remove it with choco uninstall leanrows from an administrator shell instead of deleting the folder. The two items above stay until you delete them.

If you ran the optional install.ps1, uninstall it first, either from Installed apps in Windows Settings or with & "$env:LOCALAPPDATA\Programs\LeanRows\uninstall.ps1". That removes the installed files, the Start menu shortcut, the file associations, and the Installed apps entry. It leaves the theme and zoom settings and the scratch folder, which you can then delete with the two commands above.