Open source · Apache-2.0 · Alpha
OpenCalc is an open-source, Excel-compatible spreadsheet. It opens your
.xlsx files without quietly losing anything, keeps the ribbon
and the shortcuts your hands already know, and runs in your browser, on
your desktop, inside your own product — and under your AI agents.
No sign-up, nothing uploaded The engine runs on your machine
01 The product, not a mock-up
This is the real editor — the same ribbon, formula bar and grid you get at editor.html — calculating in this page with WebAssembly. Nothing you type leaves your browser.
=XLOOKUP, or open an .xlsx of your own from the File tab.
See it embedded with the SDK
02 What it is
Everything you reach for in a spreadsheet, with the file formats, ribbon and keyboard you already know — plus the parts a closed product can't give you.
Formatting, tables, charts, validation, protection, print setup and defined names survive the round-trip. Anything the model can't hold yet is carried through byte for byte or counted and named in a compatibility report — never silently discarded.
The same tabs, groups and dialogs in the same places, Alt-key KeyTips, and the keyboard shortcuts your hands learned years ago. Nothing to relearn.
Dynamic arrays — FILTER, SORT, UNIQUE, XLOOKUP — LET and LAMBDA with MAP, REDUCE and SCAN, structured references, and autocomplete with signatures.
Share a link and see each other's cursors and edits as they happen — on a collaboration server you run yourself, with presence and reconnect built in.
The engine as native Rust in its own window. Double-click an .xlsx, .xlsm, .ods or .csv to open it; no network needed.
Every line is on GitHub — the engine, the editor, the desktop shell and the servers. CI-gated, fuzzed and benchmarked; unsafe is forbidden. No licence check, nothing phones home, and fidelity bugs with a file attached are the most useful thing you can send.
03 Shipping now
A verified list, not a roadmap — everything here is exercised by the editor above. What is still pending is in the roadmap below.
Reads and writes .xlsx and .xlsm · reads and writes .ods (values and formulas; what it cannot carry is counted) · imports and exports CSV, TSV, PSV. Formatting, tables, charts, validation, protection, print setup and defined names survive the round-trip — and anything not yet modelled is carried through byte for byte rather than dropped.
04 Embed it
One custom element and your users get the editor above — .xlsx
in and out, 347 functions, pivot tables and charts, all client-side. React,
Next.js, Vue, Svelte or plain HTML.
The element and, on React, its wrapper. No UI framework peer dependency.
# the editor as a custom element
npm install @opencalc/sheetThe WebAssembly binary is copied into your build and served from your origin — never from a CDN we run.
# package.json → postinstall
opencalc-assets ./public/opencalcGive it a height and it boots. Open a workbook, get bytes back, listen for changes.
import "@opencalc/sheet";
// <opencalc-sheet style="height:600px"></opencalc-sheet>
const sheet = document.querySelector("opencalc-sheet");
sheet.theme({ light: { accentColor: "#7c3aed" } }); // typed CSS tokens
sheet.chrome({ statusbar: false }); // hide whole regions…
sheet.commands({ hidden: ["file.open"] }); // …or single controls
await sheet.configure({ access: "view" }); // "edit" | "view" | "preview"
await sheet.open(bytes, "budget.xlsx");
sheet.on("cellsChanged", async (e) => {
if (e.source !== "api") persist(await sheet.save()); // deterministic .xlsx
});
The element isolates itself in a shadow root: your CSS cannot reach in and
ours cannot reach out. Read-only and preview refuse writes in the engine,
not by hiding buttons. Alpha: published as 0.0.x — pin what
you test against.
05 For AI agents
casual-calc-mcp is an MCP server that lets Claude, or any MCP
client, open, read, write, recalculate and save spreadsheets with the same
engine — and get back the same compatibility report you would.
open_workbooklist_sheetsread_rangewrite_cellsevaluaterecalculaterunsaveclose
It touches files only under --root, every path canonicalised; with no root, every file tool refuses. It speaks stdio and has no network code.
File size, cells per call and open workbooks are all capped, so a runaway agent cannot exhaust the machine.
cargo build --release -p casual-calc-mcp claude mcp add opencalc -- ./target/release/casual-calc-mcp --root ~/Sheets
06 Three products, one engine
The engine is the same in all three — the same import, the same 347 functions, the same writer. What differs is where it runs and what it is allowed to reach.
| Question | SDK@opencalc/sheet · 0.0.1 | Desktop app0.0.3 | Self‑hostedthree images · 0.0.1 |
|---|---|---|---|
| Where it runs | Your web page, as WebAssembly | macOS (Apple Silicon), Windows x64, Linux x64 — the engine as native Rust | Your servers — amd64 and arm64 |
| How you get it | npm i @opencalc/sheet |
.dmg · .exe · .deb · .AppImage |
docker run casualoffice/calc |
| The whole engine — .xlsx, formulas, charts, pivots | Yes | Yes | Yes |
| Opening a file from the operating system | Through your own code — you hand it the bytes | Native dialogs, and double‑click on .xlsx .xlsm .ods .csv .tsv |
Through the document host, or your own storage |
| Several people in one sheet at once | No — a single‑user editing surface | Yes, as a participant | Yes — this is the server that orders and relays |
| Fits into | A web component, or the React wrapper | The file types above | Nextcloud, ownCloud, SharePoint, Moodle, Alfresco — one discovery URL |
| Signed by the operating system | Not applicable | Not yet — the bypass is two clicks, and the download page walks through it | Not applicable |
| Worth knowing | Each element carries its own engine, so page a long list in and out rather than mounting fifty. A pivot built here exports as its cells, not as a live Excel pivot. | Pasting from another application arrives as values without formatting. The .deb needs Ubuntu 23.04 or newer; the AppImage runs on 22.04. |
No Helm chart yet, and Redis is a single node. Prometheus metrics are on /metrics. |
Not yet, in any of the three. PDF export is written and tested in the engine but is not reachable from a published product — it ships when the Rust SDK does. .xls (the pre‑2007 binary format) is refused rather than half‑read; .xlsm opens, and its macros survive a round‑trip byte for byte, but nothing here executes them. Custom worksheet functions written in JavaScript are deliberately absent — they would make a recalculation unrepeatable. Menus and toolbar tooltips are localized; panels and dialogs are still English.
07 Under the hood
A layered Rust core. Parsing sits below evaluation, so import and editing use the formula AST without pulling in the calc engine at all — and the same input always produces the same model, values, layout and bytes.
A Tauri host where the whole engine — including calc — runs as native Rust at full speed.
The same core compiled to wasm32, driving a canvas grid entirely client-side. The editor above is the proof.
No UI at all — batch convert, recalculate, and rasterize a viewport to a deterministic PNG on a server.
// One session composes every layer: open, edit, recalc, render, save.
let mut session = WorkbookSession::open(bytes)?;
session.edit(Operation::SetCell { sheet: 0, at: a1("B2"), cell })?;
let png = session.render_png(0, &viewport)?; // virtualized grid → PNG
let xlsx = session.save()?; // deterministic bytes08 Self-hosted
Co-editing is not a feature you should have to take on trust. Two
containers, one command, and you are sharing a link with a colleague and
typing into the same sheet. Download gives you an .xlsx with
what you both typed — the server hands the finished bytes back to the host
rather than keeping them.
The collaboration server is what OpenCalc supplies — ordering, presence, the socket. The host is what your product supplies — documents, identity, tokens, saving: four endpoints and a signature, and the host you just ran is the worked example.
# change OPENCALC_SHARED_SECRET in .env$ cp .env.example .env$ docker compose up$ open http://localhost:8080# make a sheet, press Share, send the link# and a cluster, when one node is not enough$ docker compose -f docker-compose.cluster.yml upcasualoffice/calc, calc-host and wopi — pulled, not built. Add --build to build from your checkout.
Each with an SBOM and a build-provenance attestation naming the repository, workflow and commit it came from.
Nothing phones home, no licence check, and no document leaves your servers. Prometheus metrics on /metrics.
09 The trade-off everyone else makes
Most options force a compromise: a suite you can't embed, a converter that quietly drops what it doesn't understand, or a web grid that treats the DOM as the source of truth and calculates on a server.
Categories, not products — Excel and LibreOffice Calc remain our semantics and fidelity oracles, and we study IronCalc, calamine and Univer openly.
10 Honest status
Delivered in capability-gated phases. The workbook has to be read, modeled, preserved, written back, laid out and rendered before a single cell is evaluated.
No sign-up, no upload, no build step. Bring an .xlsx and watch it round-trip — then take the same thing to your desktop, your servers or your own product.