---
path: /guides/excel-rater-to-api
title: "How to convert an Excel rater into an API"
description: "How to convert an Excel insurance rater into a governed, callable API: tag inputs and outputs, preserve the logic, add versioning and an audit trail."
section: Resources
priority: 0.6
changefreq: monthly
source_file: pages/marketing/seo/articleData.ts
---

# How to convert an Excel rater into an API

Most insurance rating logic still lives in Excel — transparent, but stuck on laptops and hard to deploy. Turning that workbook into a governed, callable API lets every system price consistently while keeping Excel as the source of truth. Here is the approach, and the governance it requires.

## The basic mechanics

At its simplest, converting a rater to an API has three steps:

- Tag inputs and outputs — name the cells that take parameters (state, limits, exposure) and the cells that return results (premium, factors).
- Extract the logic — load the workbook into an engine that executes the formulas server-side, or translate them to production code.
- Expose an endpoint — publish a REST API that accepts a JSON payload of inputs and returns the calculated outputs.

## What a naive conversion misses

Wrapping a spreadsheet in an endpoint is the easy part. For insurance, the hard part is governance: every rate change has to create a new version, previous versions must remain available for audit and comparison, and each result needs a reproducible record of inputs, outputs, and timestamp.

Tesora’s Rating Agent rebuilds Excel raters into versioned, self-testing models, then deploys them to an API or front end — with author, reviewer, and approver kept separate so the process stays SOX-friendly.

## Why an audit trail beats a custom rewrite

A common objection to keeping Excel in production is the audit-trail problem: if a calculation ran on someone’s laptop, how do you reproduce it? Running the logic through a governed API with per-request capture actually produces better auditability than a hand-coded rewrite — inputs, outputs, timestamp, and the exact model version, every time.

## Frequently asked questions

### Do I have to rewrite my Excel formulas?

No. The goal is to preserve the existing logic exactly. Tesora rebuilds the rater so the formulas and factor tables are inspectable and versioned, not re-typed into opaque code.

### How do rate changes work once it is an API?

Each change should produce a new version rather than overwriting what is live, so you can review exactly what changed and roll back if needed.