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Last updated: October 3, 2026
Potting soil calculator
Work out the potting mix to fill your containers. US units: feet, inches and cubic feet. Results update as you type.
How to use this calculator
Count your pots and pick the pot size. Pot sizes are usually printed on the base or the label in gallons.
Fill pots to just below the rim so there's room for watering.
Nursery pot sizes like #3 or "3 gallon" are trade sizes and usually hold a little less than a true 3 gallons, so this calculator slightly overestimates. That leaves a useful margin.
How it's calculated
Total volume = number of pots × pot volume. Gallons are converted to cubic feet (1 gallon ≈ 0.134 cu ft), then divided by the bag size.
Worked example
10 pots of 3 gallons each hold about 114 litres, or 4.01 cu ft. That's 3 bags of 1.5 cu ft.
Common questions
Should I buy extra?
Potting mix settles after the first few waterings, so it's worth keeping a little spare to top up.
Can I reuse last year's potting mix?
You can refresh it by mixing in fresh mix or compost, but for seedlings and heavy feeders fresh mix usually gives better results.
Do I need drainage material?
Most plants need a pot with drainage holes. Check the needs of your plants.