Create your custom functions directly in Excel
See it in action
Who is this for?
Data & software vendors
You already have the product, so it is not a huge reach to give your customers native Excel functions that pull your data or run your models live in their spreadsheets. An additional revenue stream and no C++ or .NET to maintain.
Analysts & finance teams
You have calculations that live in formulas spread across hundreds of cells. Package them as functions your team calls by name: consistent, auditable, impossible to accidentally overwrite.
Trading desks
You're fielding need-this-yesterday requests from traders. Capture their requirements, test, and roll out fast. Every desk gets the same locked-down logic running at native speed.
We work with teams on a consultancy basis to deliver add-ins end to end. Because we built and maintain xllify and its MIT-licensed core ZigXLL, delivery time and cost tend to be a lot lower than you'd expect. Get in touch.
Custom functions vs formulas
Formulas are the right tool most of the time. Custom functions extend what's possible, unlocking benefits that formulas alone can't provide.
Your logic in code, not cells
Store functions in files, version them in git, test before you deploy. Everything your team already does for software, now for your Excel logic too.
Process data outside Excel
Pull from APIs, query databases, or run heavy calculations elsewhere. Your function returns only the result.
Faster to change
Fix or improve the function, rebuild, and every sheet that uses it is updated. Test your logic outside of Excel before deploying.
Distributable and self-contained
Package your functions into a single installable file. No formulas to copy between workbooks, no risk of them breaking in transit.
What xllify solves
Maximum performance, sane dev experience
Live data, hundreds of cells, every tick recalculating. Native add-ins handle this better than anything else: your function runs directly inside Excel, across all cores. The catch is the C API, which Microsoft describe as "impractical for most users". xllify shields you from this relic. You just provide the logic, as code or plain English.
Cloud build service
xllify compiles your functions into native Windows XLLs and Office Add-ins remotely. Build from a Mac, a Chromebook, or an iPad. No toolchain to install, no Windows VM required.
Test before you ship
Run your functions outside Excel to test logic, catch regressions, and verify behaviour before you touch a spreadsheet.
Your IP stays yours
Source code is compiled to bytecode before it goes into the add-in. Recipients get a working .xll file; they cannot read or copy your logic.
Safe to share
Native XLLs can in principle access files and the network. xllify's sandboxed design means builds cannot. Safe to hand to clients or colleagues without a security review.
Verified standard library
Common operations are built in and verified, not generated on the fly. The assistant draws on this library first, giving you higher-quality suggestions and functions you can trust. More →
No lock-in
ZigXLL, the framework that powers xllify, is MIT licensed. Read and audit every line of code that runs inside your add-in. No black boxes, no vendor lock-in on the runtime.
Build at any skill level
Write code directly, get help where you need it, or describe what you want and iterate from there. Paste in existing VBA and let the assistant convert it. Use as much AI assistance as you like.
See how xllify compares to similar offerings →
Pricing
Free
- Build in your browser or Excel
- Office add-in hosted previews and downloadable builds
- Native XLL builds with 30-day expiry
- AI assistant included
- xllify standard library access
- Support for xllify issues
- Personal, non-commercial, non-profit use only
Early adopter
- Permanent XLL builds
- No xllify branding
- Multiple add-in projects
- Higher AI coding assistance usage rates
- Priority email support for xllify issues
- Distribute to unlimited end users
Thank you! After clicking, sign in with your preferred account (Google, GitHub, or Microsoft). You'll land on your Account page where you'll find the button to complete your one-off payment.
xllify is built and maintained by one person. New features are shipping regularly. We'll be in touch personally after you sign up.
Roadmap
Included as they land, initially for early adopter subscribers.
External data
Pull live data from APIs and databases directly into your functions. HTTP requests, SQL queries, and common data sources available from within your Lua scripts.
RPC
Call out to local or remote services from Excel functions. Expose any backend logic as a custom function without running computation inside the add-in itself.
Async functions
Run long calculations without blocking Excel recalculation. Return results when ready, keeping your spreadsheet responsive while work happens in the background.
RTD / streaming
Push live values directly into Excel cells. Pre-written integrations with common brokers and streaming data services, or connect your own source.
From the blog
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zigxll: building Excel XLL add-ins in Zig
xllify's native XLL runtime was originally built in C++. It has been replaced with one written in Zig. This post explains why.
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Excel's fastest custom functions are also the hardest to build. xllify changes that.
Why native XLL add-ins are the fastest way to run custom functions in Excel, and why building them has historically required deep C++ expertise.
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How xllify uses Claude to suggest code
A look at how xllify integrates Claude to turn plain-English descriptions into working Lua functions, with a verified standard library to keep suggestions grounded.
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Why Lua?
Why xllify chose Lua as its scripting language: fast, embeddable, sandboxed by design, and a natural fit for the kinds of calculations Excel users actually write.
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Ever wish you could just add a function to Excel?
Adding a truly custom function to Excel used to mean C++, the XLL SDK, and weeks of work. xllify makes it a few minutes' work.