To create an online air cylinder capacity calculator, you can use HTML, JavaScript, and CSS. Here's a basic example of how you can structure your HTML and implement the functionality you described: ```html

Air Cylinder Capacity Calculator

Cylinder Capacity (mL):
Cylinder Service Pressure (Bar):
Average Breath Volume (mL):
Calculate

``` In this code, the user inputs the cylinder capacity (in mL), cylinder service pressure (in Bar), and average breath volume (in mL). When the "Calculate" button is clicked, the JavaScript function `calculateAirSupply` calculates the average expected breaths and average duration based on the provided values and displays the results. The calculations follow the formula you mentioned.