Container Garden Soil Calculator
Calculate exact potting mix for containers, raised pots, and planters, in quarts, cubic feet, and bag count. Pick a common pot size or enter custom dimensions, then see total bags and bag-size guidance.
For filling a new in-ground bed, use the Raised Bed Soil Calculator. To amend existing beds, use the Compost Calculator.
Calculate potting mix needed
Pick a common pot size, or set the shape and dimensions yourself. Add the fill depth, number of containers, and the bag size you'll buy. Results, bag count, and a recommendation appear instantly.
~5.5 qts needed
For 1 × 12-inch round pot filled 10 inches deep.
Container gardening tips
- Never use garden soil in containers. It compacts, drains poorly, and can introduce pests and disease. Always use a quality potting mix designed for pots.
- Lighten the load on big pots. Fill the bottom third of large containers with lightweight materials (perlite, foam peanuts, or upside-down nursery pots) to cut weight and save on expensive mix without affecting roots.
- Top off each spring. Potting mix compresses over time. Plan to add 1–2 inches of fresh mix each spring to keep the container at proper fill level.
- Boost drainage with perlite. Add 10–15% perlite to standard mix for improved drainage in containers with drainage holes, especially helpful for tomatoes and peppers.
- Mind the weight on balconies. A 16-qt bag of potting mix weighs 8–12 lbs; a half-barrel container can weigh 80–120 lbs filled and watered. Factor this into balcony or rooftop weight limits.
Common mistakes
- Trusting the bag's printed quart count. Some "16 qt" bags actually contain 12–14 quarts of compressed mix. When in doubt, buy one extra bag; most garden centers accept unopened returns on potting mix.
- Filling to the rim. Mix needs 1–2 inches of headspace for water to soak in without spilling. Subtract that from the container's full depth when entering Fill depth above.
- Reusing depleted mix without refreshing. Last year's mix is depleted of nutrients and may harbor disease. Mix in 30–50% fresh potting mix and a slow-release fertilizer before reusing.
- Skipping drainage. A container without drainage holes will drown roots regardless of how good the mix is. Drill holes if needed before filling.
How to use this calculator
Select a common pot size to auto-fill the dimensions, or choose a container shape and enter dimensions manually. Enter the fill depth (typically 1–2 inches below the rim) and the number of containers. Select your bag size to get the number of bags to purchase. Results are shown in quarts, cubic feet, and bags.
How much soil do common containers hold?
- 6-inch pot: ~0.5 quarts
- 10-inch pot: ~3 quarts
- 12-inch pot: ~5.5 quarts
- 16-inch pot: ~12 quarts
- 5-gallon bucket: ~0.67 cubic feet (~18 quarts)
- Half-barrel planter: ~4 cubic feet
- 24-inch window box: ~4 quarts
FAQ
Can I reuse potting mix from last year?
Yes, but with preparation. Old potting mix is compacted and depleted of nutrients. Remove any old roots, loosen the mix, and amend with compost and a slow-release fertilizer before reusing. Replace up to one-third with fresh potting mix for best results. Discard any mix that smells sour, is visibly moldy, or came from containers with diseased plants.
What's the difference between potting mix and potting soil?
Despite the name, quality potting mix contains no soil, it's typically a blend of peat moss or coco coir, perlite, and bark. "Potting soil" is a marketing term that can mean the same thing or refer to a product that contains actual soil. For containers, a soilless potting mix is always preferred because it's lighter, drains better, and resists compaction.
How deep should soil be for vegetable containers?
Root depth requirements vary by crop: leafy greens and herbs need 6–8 inches; tomatoes, peppers, and eggplant need 12–18 inches; root vegetables like carrots and beets need 12–16 inches. Shallow crops like lettuce can thrive in window boxes, while fruiting vegetables need deeper containers for best yields.
How do I reduce the weight of large containers?
Place a layer of crushed plastic bottles, foam packing peanuts, or a large nursery pot inverted at the bottom of deep containers. This reduces soil volume (and weight) significantly without affecting root growth of most vegetables. Don't use rocks, they add weight without improving drainage and can create a perched water table at the gravel layer.