How to Calculate Potting Soil for Any Container
If you've ever filled a planter and either run out halfway or had a mountain of leftover soil, you're not alone. Potting soil is sold by dry quarts or cubic feet, but pots are measured by diameter. The conversion math isn't intuitive, and the internet is full of conflicting charts. This guide gives you the actual formula — plus the shortcuts that make container gardening easier.
The Volume Formulas
Container volume depends on shape:
Square: V = Side² × Depth
Tapered: V = (π × Depth / 3) × (R₁² + R₁R₂ + R₂²)
The result is in cubic inches. Convert to dry quarts by dividing by 67.2 (1 US dry quart = 67.2 cubic inches). Convert to liters by dividing cubic inches by 61.02.
Quick Reference: Common Pot Sizes
- 6" pot: ~1.5 quarts
- 8" pot: ~3.5 quarts
- 10" pot: ~6 quarts
- 12" pot: ~10 quarts
- 14" pot: ~15 quarts
- 16" pot: ~20 quarts
- 18" pot: ~28 quarts
These assume standard proportions (depth ≈ diameter × 0.75). Deep pots need more; shallow bowls need less.
Pro Tip: For large pots (16"+), fill the bottom 1/3 with empty plastic bottles, packing peanuts, or a layer of landscape fabric over gravel. This saves soil, reduces weight, and improves drainage — plants don't need 18 inches of root depth.
Understanding Soil Bag Sizes
- 8 quart bag: Fills one 12" pot or two 8" pots
- 16 quart bag: Fills one 14" pot
- 1 cubic foot (≈25.7 quarts): Fills two 14" pots or one 18" pot
- 2 cubic foot bag: Fills about four 14" pots
Common Mistakes
1. Using Garden Soil in Pots
Garden soil compacts in containers, suffocating roots and holding too much water. Always use a potting mix — it's lighter, drains better, and is formulated for containers.
2. Not Accounting for Settling
Fresh potting soil settles 10–15% after the first few waterings. Fill pots to the top rim at planting time — they'll settle to the right level within a week.
Watch Out: "Moisture control" potting mixes hold extra water. They're great for forgetful waterers but can cause root rot in plants that prefer dry conditions (succulents, lavender, rosemary). Match the soil to the plant.
Use the calculator above for the exact number of quarts, then pick up the right bag size. Your plants will thank you for well-draining, properly-portioned soil.