Free Potting Soil Matrix

🪴 FREE GARDEN TOOL

Potting Soil Matrix

Work out how much potting soil every pot, planter and container needs — per pot and in total — in cubic feet, litres and bags, with a handy size matrix for the pots you actually own.

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🧮 Potting Soil Calculator

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Your Results

Soil per pot—
Total soil needed—
Bags needed—
Fill tip—
Leave 1 inch (2-3 cm) of headroom below the rim for watering.
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How to Calculate Potting Soil

For a round pot, use the cylinder formula: pi x radius squared x height. For a rectangular planter, use length x width x height. That gives cubic inches — divide by 1,728 to get cubic feet, or multiply cubic inches by 0.0164 to get litres.

Worked example: a round pot 12 in wide and 12 in tall: 3.1416 x 6 x 6 x 12 = 1,357 cubic inches = 0.79 cu ft = 22.2 litres per pot. For 6 identical pots you need 4.71 cu ft (133 litres) — 4 bags of 1.5 cu ft potting soil.

Pot sizeApprox. soil per potBest for
4 in (10 cm) round0.03 cu ft (0.9 L)Seedlings, small herbs
6 in (15 cm) round0.10 cu ft (2.8 L)Herbs, small flowers
8 in (20 cm) round0.23 cu ft (6.6 L)Herbs, peppers
10 in (25 cm) round0.47 cu ft (13.4 L)Tomatoes (small), shrubs
12 in (30 cm) round0.79 cu ft (22.2 L)Tomatoes, dwarf fruit
14 in (35 cm) round1.22 cu ft (34.6 L)Large shrubs, small trees

Common Mistakes to Avoid

  • Using garden soil in pots — it compacts, drains poorly and may carry pests. Use a light potting mix instead.
  • Filling pots to the brim — leave 1 inch (2-3 cm) of headroom, or water runs straight off the top.
  • Forgetting drainage — every pot needs drainage holes. A layer of gravel at the bottom does not replace them.
  • Reusing old mix without refreshing it — old potting soil is depleted. Mix in at least one-third fresh compost before replanting.
💡 Pro tip: For large pots, fill the bottom third with lightweight filler (upturned nursery pots or packing peanuts in a mesh bag) and use potting mix only in the top two-thirds — it saves soil and makes heavy pots easier to move.

Frequently Asked Questions

About 0.79 cubic feet (22 litres) if the pot is 12 inches tall. A shorter 12-inch pot, 10 inches tall, needs about 0.65 cubic feet (18 litres).
A 1.5 cu ft bag fills about two 12-inch pots, four 10-inch pots, or about fifteen 6-inch pots.
It is heavier and denser than potting mix, so it drains slowly in containers. If you use it, lighten it with perlite and compost (about one-third each).
Replace or heavily refresh it every 1-2 years, or whenever you replant. Top-dress with fresh compost each spring in between.
Most good mixes already contain some. Add extra perlite (10-20%) for succulents, cacti and any plant that hates wet feet.

Related Garden Tools

Pair this with the Raised Bed Soil Calculator, Compost Estimator and Mulch Calculator. See all tools on the Free Tools page.