Planter Soil Volume: Four Depth Bands Set the Litres

Root depth first, litres second

Nursery pot names are trade sizes rather than measured volumes, and that gap is where planter soil volume calculations go wrong before anyone opens a bag. Start from the pot and you inherit its error. Start from the plant and you don't.

So size the container to the root depth the crop needs, then convert that depth into litres, then buy. That order matters most to anyone growing food on a balcony or a patio, where every pot is a compromise and a shallow pot limits what a deep-rooted crop can do no matter how good the mix is. What follows lets you name a crop, get a minimum volume, and check whether the pot you already own clears it.

The depth figures themselves are contested. The Old Farmer's Almanac says most vegetables want 12 to 18 inches, with lettuce managing in 6. EarthBox scales lower: 6–8 inches for small plants, 10–12 for medium, 18-plus for large. The Almanac's own rooting scale calls 12–18 inches shallow, which is where EarthBox's small-plant range ends. I use the lower numbers as absolute floors and the higher ones as targets.

The conversions, so you never touch cubic feet: the standard US retail unit the Almanac tells you to divide by is the 1.5 cubic foot bag. For a round planter, Vego Garden's formula is radius squared × 3.14 × height.

The short version

• The pick: Compressed, for filling a few small pots or window boxes when you have nowhere to store a full bag of mix and want to know the finished volume before you buy.
• Also worth it: Doter, for topping off a single herb pot or repotting one small houseplant, where a cubic-foot bag would be mostly waste.
• How this was decided: published specifications and buyer reports across 5 products, compared below.

Why your calculator's number is wrong

Several problems sit between the calculator output and the soil you actually need, though no source quantifies how far off the result ends up.

• Measure the pot, not its name. For pots, the Old Farmer's Almanac says the measurement to take is the diameter across the top of the pot plus the depth.
• Formulas treat a planter as a full geometric solid. None of the calculators here accounts for pot taper, a drainage layer, root ball displacement, or the aeration screen and water reservoir in a wicking system, so the geometric figure is an upper bound.
• Settling is real, but nobody publishes a factor for it. The Old Farmer's Almanac says its volume figures are based on loose fill so beds start full, since rain and watering settle and compact soil over time; no source states how much loose bagged mix shrinks on filling and first watering.
• Buy by volume, not weight. The bag unit the Almanac works from is 1.5 cubic feet.

Buy by cubic feet or litres. Treat any bag count derived from weight as an estimate, and expect to top up: EarthBox recommends refreshing container soil every one to two years, which makes volume a recurring cost rather than a single purchase. The same discipline applies to any size decision you can only make once the piece is already in place.

Product	Item Weight	Style	Style Name	Volume	Right for
Compressed	2 pounds	All Purpose, Compressed, Potting and Planting Soil, Soil Granules	All Purpose, Compressed, Potting and Planting Soil, Soil Granules	3 gallons	Best compact storage
Doter	—	Modern	Modern	1 quarts	Best single-pot top-up
Espoma	0.27 g	Potting Soil Mix	Potting Soil Mix	2 cubic feet	Best mid-size volume
BuildASoil Light	30 pounds	Potting Soil Mix	Potting Soil Mix	1 cubic feet	Best for deep roots
Back to the Roots	1900 pounds	Garden Soil	Garden Soil	46.66 cubic feet	Best bulk fill

Every spec above is quoted from the listing; the “Right for” column is our judgement.

The two containers that hold 30 litres and behave nothing alike

Two containers of the same volume can be very different growing environments: a wide shallow bowl and a narrow deep column can price the same and behave nothing alike, because the calculators here measure volume and say nothing about depth. None of these sources describes the drainage behaviour of a shallow container, so the size of that effect is not documented here.

A wider container also presents more exposed surface for evaporation, though none of these sources quantifies the difference in watering frequency. Nothing in a bag fixes this — Proven Winners notes that moisture-control soils break down over the season and often give little benefit by mid to late summer, so the geometry you buy in April is the geometry you water in August. Pick depth for the roots, then accept the watering interval that shape gives you.

Bagged or bulk: where the line actually falls

None of these sources prices soil per cubic foot or per cubic yard, so the crossover volume at which bulk delivery beats bagged mix is undocumented here. What is documented is only that bulk soil is commonly sold by the yard, which is why Vego Garden tells you to convert your cubic-foot figure to yards.

The sanity check that keeps you honest: one cubic yard is 27 cubic feet, and the Old Farmer's Almanac puts a standard bag at 1.5 cubic feet. So a yard is 18 bags.

Which resolves the 87-bag complaint. A commenter on the Almanac's soil calculator, someone who builds beds for a living, reported that the calculator's mix for a 4×8×2-foot bed came to 2.37 cubic yards, or 87 bags, and that ordering that much would leave a significant amount left over. The arithmetic does not reconcile: 2.37 cubic yards at 1.5 cubic feet per bag is about 43 bags, not 87, and none of these sources explains the discrepancy.

Bags suit containers, a single small bed, and anyone without a driveway a truck can tip on. Bulk suits multiple beds and people who can receive a load. Back to the Roots 100% Organic Garden Soil is sold by the pallet according to its listing, so you need somewhere to put it.

Compressed — Best compact storage

• Compressed granules expand up to 4x to make up to 3 gallons of soil from a 2 lb, 32 oz bag
• Resealable bag means leftovers can be kept dry between plantings
• But: 3 gallons is roughly 0.4 cubic feet, so it fills only small planters — not a deep tomato pot or raised bed
• But: Requires mixing with water before use, an extra step versus ready-to-use mix

Doter — Best single-pot top-up

• 1 quart volume suits one small container or a top-up rather than a whole planting
• Peat moss, perlite, coconut coir and worm castings blend aimed at aeration and drainage
• But: 1 quart is a fraction of a gallon — far too little for any planter over about 6 in.
• But: Contains peat moss, which rules it out for gardeners avoiding peat

Espoma — Best mid-size volume

• 2 cubic feet is about 15 gallons, enough for several 12–14 in. containers from one bag
• Sphagnum peat moss, humus and perlite enriched with earthworm castings, alfalfa, kelp and feather meal
• But: Peat-based, so not an option for peat-free gardens
• But: A 2 cubic foot bag is bulky and heavy to move up stairs or onto a roof terrace

BuildASoil Light — Best for deep roots

• 1 cubic foot (about 7.5 gallons) at 30 lb suits filling one large, deep pot completely
• High in worm castings and compost to feed roots through a long season
• But: 1 cubic foot fills only one large planter, so multi-pot projects need several bags
• But: At 30 lb per cubic foot it is a dense mix — heavy for hanging baskets or railing planters

Back to the Roots — Best bulk fill

• Pallet quantity listed at roughly 46.66 cubic feet — enough for multiple large raised beds
• 100% organic and peat-free, made from upcycled plant matter and wood fines
• But: Around 1,900 lb on a pallet — needs somewhere to receive and unload it
• But: Listing describes 60 cubic ft in the title but 46.66 cubic feet in the specs, so plan volume from the lower figure

Matching a bag to the pot you actually own

Five bags, five different jobs, and only one of them belongs anywhere near a raised bed. Judge them on volume, density, peat content, and whether you can physically receive the delivery — bearing in mind that no source here gives a settling factor or a weight per cubic foot of finished mix.

Volume first, because it disqualifies fastest. The product listings differ widely in bag size, from a single quart at one end to pallet quantities at the other, but none of the sources checked here documents the pack sizes, expansion ratios or weights for these specific products, so verify each figure on the listing before buying.

Density matters for balcony, rooftop and railing planters, but no source here states the weight of a cubic foot or cubic yard of finished mix, so the load a given volume puts on a structure — or on the person carrying it upstairs — cannot be worked out from these pages. Peat content is worth checking on each label for the same reason.

Match the pack size to the job: a small tub for a single top-up, a compressed bag where you have no storage, a pallet only if you have a driveway and a wheelbarrow. And whatever the pack, remember the UC Master Gardener Program says to keep garden soil out of containers regardless of quality, because it lacks the aeration container plants need.

Whatever the pot is called, buy for its measured volume, not its label.

Our call

Compressed, best for filling a few small pots or window boxes when you have nowhere to store a full bag of mix and want to know the finished volume before you buy.

A different situation changes the pick:

• For topping off a single herb pot or repotting one small houseplant, where a cubic-foot bag would be mostly waste: Doter.
• For planting out a balcony's worth of medium containers in one go, where you want a familiar organic mix in a volume you can actually plan around: Espoma.
• For one deep container for a heavy feeder like tomatoes, where root depth means the pot needs a nutrient-dense fill rather than a light mix: BuildASoil Light.

One caveat on the Compressed: 3 gallons is roughly 0.4 cubic feet, so it fills only small planters — not a deep tomato pot or raised bed