Hi @gardenspecial,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
I'd suggest using the Tuscan Path Pebbles Calculator for this calculation.
You can select the option to calculate by area. For a circle with a 4m diameter, the area is approximately 12.57 m².
If you plug this into the calculator, at a depth of 60mm, which is fairly common, it says you would need 88 bags.
Let me know what you think and if you have further questions, please don't hesitate to ask.
Jacob
