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. Build from any machine; xllify compiles remotely. Your source is compiled to bytecode, so recipients get a working file but cannot read your logic. Function code runs in a sandbox with no access to the file system or network, safe to hand to clients without security holes.
Built 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.
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.
See how xllify compares to similar offerings →
See it in action
Create your custom functions directly in Excel
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
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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.