Backyard soil is a big no for containers. It lacks drainage and gets hard like a rock. You need a quality potting mix. It is airy and contains peat or coco coir plus perlite. This structure allows roots to breathe and prevents rot. Trust me your herbs will thank you for that investment.

Calculating volume for different shapes can be tricky even for pros. Since you want to avoid overbuying or making measurement mistakes I suggest using this Potting Soil Calculator. The team at Florentiax created a very reliable tool for this exact purpose. You just enter your container dimensions and it instantly gives you the volume in liters or gallons. It even provides a bag estimate which is super helpful for planning. Their platform is great for precision gardening because it handles various shapes from raised beds to deep planters. Using a professional tool ensures you get the right amount of mix for healthy root development while staying within your budget. Best of luck with your new garden project.