Measure the space you will actually fill
A bag of potting mix is sold by volume. To estimate what you need, begin with the inside of the container, not the outside dimensions printed on the box. Measure from the bottom to the intended soil level. Leaving room below the rim changes the volume, so use that fill depth in your calculation.
For a rectangular planter, multiply inside length by inside width by fill depth. With all three measurements in centimetres, divide the result by 1,000 to get litres. For a straight-sided round container, use π × radius² × fill depth, then divide by 1,000. Radius is half the inside diameter. These formulas describe simple shapes: a tapered pot needs a different model.
Worked example: one rectangular planter
Suppose the inside dimensions are 60 cm long and 30 cm wide, and the intended fill depth is 25 cm. The calculation is 60 × 30 × 25 ÷ 1,000 = 45 litres. These are example measurements, not a recommended planter size.
If you choose a 10% extra allowance, the shopping target becomes 45 × 1.10 = 49.5 litres. That allowance is a planning choice, not a claim about how much every mix settles. Two 25-litre bags provide 50 litres. Two 40-litre bags provide 80 litres, leaving substantially more unused mix.
The soil calculator starts with these measurements. Change the bag size to compare how many whole bags you would need. For multiple identical containers, multiply the container volume by the count before adding your allowance.
Compare litres with cubic feet
One cubic foot equals approximately 28.3168 litres. Multiply cubic feet by 28.3168 to compare a bag with a litre-based estimate. For example, a 1.5-cubic-foot bag holds about 42.48 litres; two hold about 84.95 litres. Keep more precision in the calculation and round the shopping quantity up only when choosing whole bags.
Do not treat litres as kilograms. Volume measures space; weight also depends on the material and its moisture. A volume calculator cannot establish whether a balcony, shelf, or railing can carry the load. Check the support and any building requirements separately.
Compare the cost of the amount you need
Write down the actual bag price at your supplier and divide it by the labelled volume. A hypothetical 25-litre bag costing 8 currency units costs 0.32 per litre. A hypothetical 40-litre bag costing 11 costs 0.275 per litre. These are arithmetic examples, not current prices.
The larger bag is cheaper per litre in that example, but two smaller bags cost 16 and cover a 49.5-litre target, while two larger bags cost 22. Compare both the total checkout cost and the amount you can realistically use or store. Also compare like-for-like mixes: a different product is not automatically suitable just because its unit price is lower.
Before you buy
- Record inside dimensions, container count, and fill depth.
- Allow for an existing root ball or other contents that occupy space.
- Choose an allowance deliberately rather than adding it twice.
- Read the bag volume and product-use instructions.
- Check how you will transport and store any unused material.
For choosing a container and setting up your first planting, continue with the first container garden guide. For watering afterwards, see the container watering routine.