Potting Soil Calculator

Background.

A bag of potting mix is labelled in dry quarts, cubic feet or litres; the pot it is going into is measured in inches across and inches deep. Getting from one to the other is where people come home a bag short, or with two spare bags leaning against the shed.

Shape is the first trap. Nursery pots and most patio planters taper, narrower at the base than at the rim, and the soil line sits below the rim, where the pot is narrower still. A pot 14 inches across the rim and 10 across the base, filled 11 inches deep, holds about 0.7057 cubic feet. Treat it as a straight-sided 14-inch tub and you would estimate 0.9799, almost 39% too much. The tapered option models the pot as a cone with its tip cut off and measures the soil surface at the fill line, not at the rim.

Units are the second. The quart on a soil bag is the US dry quart, about 16% larger than the liquid quart in a measuring jug, and a cubic foot holds 25.714 of them or 28.3168 litres.

Then there is the blend. University of Maryland Extension's container recipe is half soilless mix and half compost, and if the compost comes from your own heap only the bagged half counts towards the bags, which are rounded up and priced whole. No weight is given, because mixes run from airy peat-and-perlite blends to dense compost-rich ones and change again with moisture.

What is potting soil calculator?

Potting mix, usually sold as potting soil, is a growing medium for containers, and most commercial mixes hold 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 amounts of lime and starter fertiliser. Clemson Extension reproduces the Cornell foliage plant mix by the bushel: half a bushel of sphagnum peat and a quarter bushel each of No. 2 vermiculite and perlite. What a container needs is its interior volume up to the fill line — the inside depth minus the gap left below the rim so water soaks in rather than spilling over.

How to use this calculator.

1. 01Choose the shape: tapered for nursery pots and most planters, cylinder only for genuinely straight-sided tubs, rectangular for window boxes, troughs and small raised beds, bowl for a hemispherical hanging basket.
2. 02Measure inside the container, not across the outer lip: rim diameter, base diameter for a tapered pot, length and width for a box (a 4 by 8 ft bed is 48 by 96 in), and inside depth. A bowl needs only the rim diameter.
3. 03Enter how many identical containers you are filling and the gap you will leave below the rim, or 0 to fill to the brim. For pots of different sizes, run each size separately and add the totals.
4. 04Set the share you will supply yourself: 50 for Maryland's half-and-half blend using your own compost, 0 if every bit is coming from bags.
5. 05Enter the bag size and unit exactly as printed on the bag, then the price per bag, or 0 to skip the cost.

The formula.

Everything is worked in cubic inches. With inside depth d, rim gap g and fill depth h = d − g, a rectangular box holds L × W × h and a cylinder of diameter D holds π(D/2)²h. A tapered pot is a frustum: since its wall rises in a straight line, the radius at the soil surface is r_f = r_b + (r_t − r_b) × h/d, and the volume is (πh/3)(r_b² + r_b·r_f + r_f²). Using the rim radius instead of r_f would overstate the example pot by about 3%. A hemispherical bowl of rim radius R is as deep as R, so it is filled to h = R − g and holds a spherical cap, πh²(3R − h)/3. Totals are converted with NIST Handbook 44's tables: 1,728 cubic inches to the cubic foot; 67.200625 cubic inches to the dry quart, one thirty-second of the 2,150.42-cubic-inch bushel; and 28.316846592 litres to the cubic foot. Your amendment share comes off the total, the bagged remainder is divided by the bag volume and rounded up, and the cost is that whole number of bags times the price.

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.