Potting Mix Calculator
How much potting soil do your containers need? Enter pot diameter, height and how many pots to get liters, cubic feet and dry quarts.
Calculate your potting mix needs
Measure the inside top diameter — pots taper, so the top opening is what counts.
The cylinder math
Most pots are close enough to cylinders for this math: volume = π × radius² × height. A 12-inch-diameter pot that is 10 inches tall holds about 1,131 cubic inches. The calculator converts that into the three units bagged mix is sold in:
- Liters — the global standard on bag labels (cubic inches × 0.0164);
- Cubic feet — the US bulk unit (cubic inches ÷ 1,728);
- Dry quarts — the small-bag unit at garden centers (liters × 1.101).
So that 12×10 inch pot needs about 18.5 liters / 0.65 cu ft / 20 dry quarts — roughly two and a half 8-quart bags. Multiply by your pot count and buy once instead of making three trips.
Measure the right diameter
Pots taper — the top is wider than the base. Measure the inside diameter at the top opening, since that's the volume you're actually filling. For square planters, just use the raised-bed math (length × width × height); for tapered round pots, the cylinder formula runs a touch high, which is fine — a little extra mix always finds a use.
Fill it to the top (almost)
Fill to about an inch below the rim — enough headroom that water doesn't run straight off. And skip the old "gravel in the bottom for drainage" trick: research consistently shows a gravel layer raises the perched water table inside the pot, leaving roots wetter, not drier. One drainage hole plus a good freely-draining mix beats gravel every time.
Mix vs. soil: buy the right bag
True potting mix is soilless — peat or coco coir, perlite or vermiculite, plus compost — and it's lighter, fluffier and better-draining than bagged potting soil containing actual topsoil. In containers, soilless mix almost always wins: it doesn't compact into a brick by August the way topsoil-based products do. If a bag feels suspiciously heavy for its size, it's probably soil, not mix.
Reusing last year's mix
Don't throw it out. Dump pots into a wheelbarrow, pick out the old root balls, and refresh with 25–30% new compost plus a scoop of slow-release fertilizer — good for another season. The one exception: mix from diseased plants goes in the trash (not the compost pile) to avoid spreading pathogens.
Frequently asked questions
How much potting soil do I need for a pot?
Volume = π × radius² × height. A 12-inch-diameter, 10-inch-tall pot holds about 1,131 cubic inches — roughly 18.5 liters, 0.65 cubic feet, or 20 dry quarts.
How many dry quarts are in a cubic foot of potting soil?
About 31 dry quarts per cubic foot. An 8-quart bag fills roughly a quarter of a cubic foot.
Should I fill the whole pot with potting mix?
Yes — fill to about an inch below the rim. Skip gravel in the bottom; it raises the water table inside the pot instead of improving drainage.
Can I reuse potting soil next year?
Yes: remove old roots, mix in 25–30% fresh compost plus slow-release fertilizer. Discard (don't compost) mix from diseased plants.
What's the difference between potting soil and potting mix?
Potting mix is soilless (peat/coir + perlite + compost) — lighter and better-draining. For containers, soilless mix almost always performs better than topsoil-based potting soil.