diff --git a/README.md b/README.md
index 92d62a1..6eda1dc 100644
--- a/README.md
+++ b/README.md
@@ -85,6 +85,7 @@ Do you know of an incredible green software repository that we missed? Let us kn
 - [AI Wattch](https://github.com/AIWattch/browser-extension): AI Wattch is a Chrome browser extension that estimates the carbon emissions of your ChatGPT conversations in real time.
 - [EnergiBridge](https://github.com/tdurieux/EnergiBridge): A cross-platform tool designed to measure the energy consumption of software across various operating systems and hardware architectures. It simplifies the process of gathering energy data, aiding developers in creating more sustainable software solutions.
 - [TCS Estimator](https://github.com/ScottLogic/sl-tech-carbon-estimator): Mapping an organization’s technology emissions using the Tech Carbon Standard structure.
+- [GreenCalculus](https://github.com/greencalculus/greencalculus-sdk): Python and JavaScript clients for greenhouse-gas emission factors published by DEFRA, the US EPA, ADEME, the IPCC and others. Each value is returned with the exact source cell it was read from, the publisher's citation and the data version, so an emissions figure in your software can be traced back to a primary document instead of hard-coded from a blog post. The corpus is readable with no API key, and a calculation can be pinned to a data version so the same inputs reproduce the same number a year later.
 
 ## Carbon Efficiency
 > Start making changes to your software and architecture to use less energy and emit less carbon.