Mollier h-x diagram (Web App): An open-source browser-based psychrometric calculator

Psychrometric charts are essential in HVAC design and education, but manual look-ups and spreadsheet workflows are slow and error-prone. This paper presents the Mollier h-x Diagram Web App , an open-source browser-based tool that performs fast, bidirectional moist-air calculations and visualizes results directly on a live Mollier h–x diagram. Users can enter any two of dry-bulb temperature, relative humidity, or humidity ratio to obtain the full psychrometric state, including dew-point temperature, enthalpy, entropy, and specific volume. The app is pressure-aware: barometric pressure is derived from user-selected altitude using the standard-atmosphere model, ensuring accuracy away from sea level. Implemented in vanilla JavaScript with Plotly.js, it runs entirely client-side, requires no installation, and functions offline after first load (MIT Licence). Validation against ASHRAE Fundamentals and benchmark spreadsheets shows agreement within 0.5 kJ kg⁻¹ in enthalpy and 0.3 g kg⁻¹ in humidity ratio. Performance tests confirm sub-millisecond solve times, enabling responsive interaction and rapid scenario analysis. The open architecture supports reuse in teaching, research, and HVAC practice.