Potting Soil Calculator

Frequently Asked Questions

Why does my pot need more soil than the calculator says?

Potting mix settles after the first few waterings, sometimes by 10-15%. Filling to exactly the calculated volume may leave the soil level dropping below the rim. Always buy one extra bag for large planters and use the leftover to top off after the mix settles.

What fill percentage should I use for raised beds?

For raised beds, fill to about 90-95% of depth - you want the soil close to the top so plants can use the full rooting depth. The 80% default is more appropriate for containers where headspace prevents overflow during watering. Raised beds do not have the overflow problem of a container, so you can fill higher.

Can I reuse potting soil from last season?

Yes, with care. Remove dead roots and debris, check for disease or pests, and refresh with 20-30% new mix plus slow-release fertilizer. The structure degrades over seasons as the organic components break down, but the bulk is reusable for 2-3 seasons before full replacement makes sense.

When is it worth buying bulk soil instead of bags?

Once you need more than about 0.5 cubic yards (roughly 13 cubic feet or about 9 bags of 1.5 cu ft), bulk soil delivered by the cubic yard is usually cheaper per cubic foot than bagged products and saves the labor of splitting and carrying dozens of bags. Compare the price per cubic foot from both options before ordering.

Provided by AllCalculators.io
Free online calculators for everyday. No registration required.

Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

JavaScript is required to use the interactive calculator above. The questions and answers below remain readable without JavaScript.

How This Calculator Works

The calculator converts your container dimensions to cubic inches, applies the fill percentage to account for leaving headspace, converts the result to cubic feet, and then divides by the bag size to find how many bags you need. For cylindrical pots it uses the formula for the volume of a cylinder (π × radius² × height). For rectangular containers it uses length × width × depth. The fill percentage defaults to 80% because most containers should not be filled to the very top - leaving about 2 inches of headspace prevents soil from washing over the rim when watering and gives room for a light mulch layer.

Potting Mix vs. Garden Soil

Garden soil and potting mix are not interchangeable for containers. Garden soil compacts in pots, squeezing out the air pockets that roots need and blocking drainage. Most garden soils also carry weed seeds, fungal pathogens, and insects. Potting mix is formulated to stay loose and well-drained in the confined environment of a container: it typically contains peat moss or coco coir for water retention, perlite or vermiculite for drainage and aeration, and bark fines for structure.

For raised beds, a blend of 60% topsoil, 30% compost, and 10% perlite or coarse sand is more practical and economical than filling with bagged potting mix alone - the cost would be enormous and it dries out too fast at larger scale.

Bag Sizes and Coverage

Standard bag sizes and what they typically fill:

1 cu ft bag: a 12-inch diameter pot about 14 inches deep, or a 12 × 12 inch planter box 12 inches deep

1.5 cu ft bag: a 14-inch diameter pot at standard depth, or a window box about 36 inches long and 8 inches wide

2 cu ft bag: the most economical size for large planters; fills roughly a 16-inch diameter pot or a half-barrel planter

Big-box stores often sell 1.5 cu ft as the standard consumer size. Garden centers and farm supply stores carry 2 cu ft bags, which cost less per cubic foot. Buying in bulk (loose yards delivered by truck) is worth considering for more than 20-30 bags.

Soil Amendments for Containers

For most vegetables and herbs in containers, start with a quality potting mix and consider these amendments:

Perlite (10-20% by volume): improves drainage and prevents compaction in heavy-feeding crops like tomatoes

Compost (10-20%): adds slow-release nutrients and beneficial microbes; mix into the lower two-thirds of the container

Slow-release granular fertilizer: mix into the top half of the container at planting; container plants need more frequent feeding than in-ground plants because nutrients leach out with every watering

Water-retaining crystals: useful for hanging baskets and window boxes that dry out quickly, though they do not replace regular watering

Container plants are watered frequently, which accelerates nutrient leaching. Plan to supplement with liquid fertilizer every 2-3 weeks during the growing season regardless of what you mixed in at planting.

Settling, Reuse, and Top-Up

Potting mix settles after the first few waterings, sometimes by 10-15%. If you filled to exactly the calculated amount, the level may drop below the rim. Adding a small reserve bag to top off after settling is common practice. For this reason, always buy one extra bag beyond the calculated number for large planters.

Old potting mix can be reused for 2-3 seasons with proper preparation: remove dead roots and debris, check for signs of disease or pests, and refresh with 20-30% new mix plus a dose of slow-release fertilizer. Old potting mix loses structure over time and perlite degrades, but the bulk can be economically reused rather than discarded.

Potting Soil Calculator

Frequently Asked Questions

Related Garden Calculators