Potting Soil Calculator

Background.

A potting soil calculator turns the size of a pot, planter, window box or hanging basket into the amount of growing medium it will actually swallow, and then into the number of bags you have to put in the trolley. It sounds like a small question and it is not. Bagged potting mix in the United States is labelled in dry quarts, bulk soil is quoted in cubic yards, gardeners measure pots in inches, and the rest of the world buys mix in litres. Four units, none of which convert in your head, is why so many people come home with three bags too few and one bag too many.

The geometry is the other half of the problem. A pot is almost never a cylinder. The standard nursery container and the standard patio planter both taper: the base is meaningfully narrower than the rim, so treating the pot as a straight-sided tub overstates what it holds by a fifth or more. A hanging basket is not a cylinder either — it is closer to a bowl, and a bowl filled to a given depth is a spherical cap, not a disc. This calculator treats each shape as the solid it really is. The tapered mode uses a conical frustum, and it goes one step further: because you stop filling an inch below the rim, the soil surface is narrower than the rim, so the radius at the fill line is interpolated down the wall rather than taken from the rim. That single correction is the difference between an answer that is right and an answer that is optimistic.

Everything is computed in cubic inches, then converted once with the unit definitions in NIST Handbook 44-2026, Appendix C. One cubic foot is 1,728 cubic inches. One US dry quart — the quart on the bag, which is a different and larger quart than the one in your measuring jug — is 67.200625 cubic inches, one thirty-second of the 2,150.42 cubic inch bushel. That makes a cubic foot 25.71405 dry quarts, so a 2 cubic foot bag is a little over 51 dry quarts and a 16 dry quart bag is half a bushel. A cubic foot is 28.316846592 litres exactly.

The calculator also splits the fill if you are blending. Extension guidance for containers and raised beds is usually written as a ratio rather than a volume: the University of Maryland's container page uses 50% soilless mix with 50% compost, and its raised-bed page suggests compost to purchased topsoil at 1:1 or 1:2, with organic matter landing somewhere between a quarter and half of the finished volume. Enter the share you are supplying yourself and the bag count is calculated on the remainder, so you do not buy mix you were going to replace with your own compost anyway.

Two things this page deliberately does not do. It reports no weight, because the bulk density of a peat- or coir-based mix varies far too much with recipe and moisture to convert honestly, and a guessed density is worse than no density. And it is not a bulk-delivery tool: if you are filling a large bed from a truckload of screened soil measured in feet and quoted by the cubic yard, the topsoil calculator is the right page and it is linked below.

What is potting soil calculator?

Potting soil, more accurately called potting mix or growing medium, is the material a container plant roots into. Most commercial mixes contain little or no mineral soil. University of Maryland Extension describes them as blends of sphagnum peat moss, perlite, vermiculite, composted bark, compost or coconut coir, with small additions of lime and starter fertiliser. Clemson Extension's HGIC 1456 lists worked recipes in parts — two parts peat to one part perlite to one part coarse sand, for example — and reproduces the Cornell foliage plant mix, which is specified per bushel: half a bushel of sphagnum peat, a quarter bushel of No. 2 vermiculite, a quarter bushel of perlite.

The vocabulary that matters for the arithmetic is: interior dimensions, fill depth, rim gap, dry quart and bag volume. Interior dimensions are measured inside the container, not across the outside of the lip. Fill depth is the interior depth minus the rim gap — the lip of empty space you leave so that water pools and soaks in instead of sheeting over the side. The dry quart is the US unit of dry capacity used to label bagged soil products; it is 67.200625 cubic inches, roughly 16 percent larger than the 57.75 cubic inch liquid quart, and confusing the two is the single most common unit error in this subject. Bag volume is whatever the bag says, in cubic feet, dry quarts or litres.

Depth is a horticultural question as much as a geometric one. UGA Extension's raised-bed circular states that most garden crops need at least 10 inches of soil to thrive, and Maryland's raised-bed page gives a minimum of 8 inches for leafy greens, beans and cucumbers and 12 to 24 inches for peppers, tomatoes and squash when the bed sits on a hard surface. Those numbers tell you what depth to type in; the calculator tells you what that depth costs.

How to use this calculator.

1. 01Pick the shape that matches your container. Tapered is the right answer for most nursery pots and patio planters. Use cylinder only for a genuinely straight-sided tub, rectangular for window boxes, troughs and small raised beds, and bowl for a hemispherical hanging basket.
2. 02Measure inside the container, not outside. For a tapered pot you need the inside rim diameter, the inside base diameter and the inside depth. For a bowl you need only the inside rim diameter — a hemisphere is as deep as its rim radius, so the depth field is ignored.
3. 03Enter how many identical containers you are filling. If your pots are different sizes, run the calculator once per size and add the totals.
4. 04Set the space you leave below the rim. One inch is a common choice for a mid-sized pot. Enter 0 if you fill to the brim.
5. 05Enter the share of the finished fill you are supplying yourself as compost or another amendment. Enter 0 if you are buying everything in bags.
6. 06Type the number printed on the bag and pick its unit — cubic feet, US dry quarts or litres. Then enter the price per bag, or 0 to skip the cost line.
7. 07Read the note beside the result. It names the solid the shape was treated as, the fill depth used, and the two things this page does not attempt.

The formula.

Let d be the inside depth, g the rim gap and h = d − g the fill depth, all in inches. A rectangular container of inside length L and width W holds L × W × h cubic inches. A straight-sided round pot of inside diameter D holds π(D/2)²h.

A tapered pot is a conical frustum. With base radius r_b and rim radius r_t, the radius at the fill line is r_f = r_b + (r_t − r_b)(h/d), because the wall rises in a straight line from base to rim and you stopped short of the rim. The frustum volume is then (πh/3)(r_b² + r_b·r_f + r_f²). Using r_t in place of r_f is the mistake that makes a tapered pot look bigger than it is: in the worked example below it inflates a 0.7056900808 cubic foot pot to 0.7266145046 cubic feet, just under three percent on every pot in the order.

A hemispherical bowl of rim radius R filled to depth h from the bottom is a spherical cap: V = πh²(3R − h)/3. Here the interior depth is not a free variable — a hemisphere is exactly as deep as its rim radius — so the calculator uses R and ignores whatever is in the depth field, rather than silently computing a cap taller than the sphere it came from.

Units are converted once, at the end, from cubic inches. Cubic feet = in³ ÷ 1,728. US dry quarts = in³ ÷ 67.200625. Litres = cubic feet × 28.316846592. All three factors are from NIST Handbook 44-2026, Appendix C; the dry quart is derived as one thirty-second of the 2,150.42 cubic inch bushel, which the handbook defines exactly.

The blend split is proportional: amendment volume = total × (amendment share ÷ 100), and bagged mix = total − amendment. Bags = ceiling(bagged mix ÷ bag volume), because you cannot buy part of a bag, and cost = bags × price per bag, because you pay for whole bags. Those two roundings are the only ones before the final answer; every other quantity is carried at full precision and rounded once at the end.

A worked example.

Frequently asked questions.

References& sources.

1. [1]National Institute of Standards and Technology, Handbook 44 – 2026, Appendix C, "General Tables of Units of Measurement". Units of Volume: 1,728 cubic inches = 1 cubic foot. Units of Capacity or Volume, Dry Measure: 2 pints = 1 quart = 67.200 6 cubic inches; 4 pecks = 1 bushel = 2,150.42 cubic inches exactly. Dry-measure conversion table, cubic-foot row, Dry Quarts column: 25.714 05. Source of every unit factor on this page. Retrieved 2026-07-31; open access PDF, downloaded and text-extracted in full.
2. [2]University of Maryland Extension, "Growing Media (Potting Soil) for Containers", updated 23 March 2026. Source of the composition of commercial soilless mixes (sphagnum peat moss, perlite, vermiculite, composted bark, compost, coconut coir, lime and starter fertiliser) and of the 50% soilless mix plus 50% compost blend used as the default amendment share. Retrieved 2026-07-31; open access.
3. [3]University of Maryland Extension, "Soil to Fill Raised Beds", updated 6 November 2024. Source of the blend ratios (compost to purchased topsoil at 1:2 or 1:1; compost to soilless growing mix at 1:1 on hard surfaces), the 25%–50% organic matter by volume target, and the minimum depths of 8 inches for leafy greens, beans and cucumbers and 12–24 inches for pepper, tomato and squash on hard surfaces. Retrieved 2026-07-31; open access.
4. [4]Clemson Cooperative Extension, Home & Garden Information Center, HGIC 1456, "Indoor Plants – Soil Mixes", Shaughnessy, D. and Pertuit, A., updated 21 October 2024. Source of the parts-based mix recipes and of the Cornell foliage plant mix stated per bushel (½ bushel sphagnum peat, ¼ bushel No. 2 vermiculite, ¼ bushel perlite), which is used as the independent check on the cubic-inch to dry-quart conversion. Retrieved 2026-07-31; open access.
5. [5]University of Georgia Cooperative Extension, Circular 1027-4, "Raised Garden Bed Dimensions", Berle, D. and Westerfield, B., published 14 December 2022. Source of "Most garden crops need at least 10 inches of soil to thrive." Retrieved 2026-07-31; open access.