Potting Soil & Container Calculator
Calculate exact potting mix volume for round tapered flower pots (circular frustums), fabric grow bags (1 to 30 gallon), rectangular window boxes, straight cylinders, and square tapered planters. Compare US Dry Quarts, US Liquid Gallons, Liters, Cubic Feet, and retail potting soil bag counts with watering headspace factored in.
Quick Container Presets
Standard Tapered Pots (Inside Dimensions)
Popular Fabric Grow Bags
Standard nursery & patio pots wider at the top than the base
Potting Mix Settling, Retail Bag Size & Optional Cost
Default is 0% (net geometry with headspace). Add 5%–10% if using fluffy peat or coir mixes that compress after initial watering.
Standard Round Tapered Flower Pot Soil Volume & Bag Chart
Most nursery and patio flower pots taper inward from the rim to the base (a circular frustum). The chart below compares full-to-the-rim volume against practical net fill volume with watering headspace left below the rim, using exact NIST US dry quarts (67.20 in³) and US liquid gallons (231 in³).
| Pot Size | Inside Top × Bottom × Height | Headspace | Full-Rim (US Dry Qt) | Net Fill (US Dry Qt) | Net Fill (US Liq Gal) | Net Fill (Liters) | Net Fill (Cu Ft) | Pots per 2.0 ft³ Bag |
|---|---|---|---|---|---|---|---|---|
| 4-inch Standard Pot | 4" top × 2.75" base × 3.5" H | 0.5 in | 0.47 dry qt | 0.38 dry qt | 0.11 liq gal | 0.4 L | 0.015 cu ft | ~134.7 pots |
| 6-inch Standard Pot | 6" top × 4.25" base × 5.5" H | 0.5 in | 1.70 dry qt | 1.50 dry qt | 0.44 liq gal | 1.6 L | 0.058 cu ft | ~34.3 pots |
| 8-inch Standard Pot | 8" top × 5.75" base × 7" H | 0.75 in | 3.90 dry qt | 3.36 dry qt | 0.98 liq gal | 3.7 L | 0.131 cu ft | ~15.3 pots |
| 10-inch Patio Pot | 10" top × 7.25" base × 9" H | 1 in | 7.89 dry qt | 6.76 dry qt | 1.97 liq gal | 7.4 L | 0.263 cu ft | ~7.6 pots |
| 12-inch Patio Pot | 12" top × 8.75" base × 10.5" H | 1 in | 13.32 dry qt | 11.68 dry qt | 3.40 liq gal | 12.9 L | 0.454 cu ft | ~4.4 pots |
| 14-inch Planter Pot | 14" top × 10.25" base × 12" H | 1 in | 20.78 dry qt | 18.54 dry qt | 5.39 liq gal | 20.4 L | 0.721 cu ft | ~2.8 pots |
| 16-inch Planter Pot | 16" top × 11.75" base × 13.5" H | 1 in | 30.61 dry qt | 27.68 dry qt | 8.05 liq gal | 30.5 L | 1.076 cu ft | ~1.9 pots |
| 18-inch Tub Planter | 18" top × 13.5" base × 15" H | 1.5 in | 43.78 dry qt | 38.24 dry qt | 11.13 liq gal | 42.1 L | 1.487 cu ft | ~1.3 pots |
| 20-inch Tub Planter | 20" top × 15" base × 16.5" H | 1.5 in | 59.46 dry qt | 52.61 dry qt | 15.30 liq gal | 57.9 L | 2.046 cu ft | ~1.0 pots |
| 24-inch Half-Barrel Tub | 24" top × 18" base × 18" H | 1.5 in | 93.41 dry qt | 83.52 dry qt | 24.30 liq gal | 92.0 L | 3.248 cu ft | ~0.6 pots |
Note on Pot Mold Variations: Decorative ceramic, terracotta, and resin planters vary in wall thickness and taper angle. Measure your pot's interior rim diameter, interior base diameter, and interior depth to the drainage base for exact results.
Fabric Grow Bag Soil Volume & Bag Count Reference Chart (1 to 30 Gallon)
Fabric grow bags are soft-sided cylinders whose nominal gallon label is a trade class rather than an exact liquid-gallon fill line. In the table below, the 3, 5, 7, 10, 15, and 20-gallon dimensions are documented in the Johnny's Selected Seeds Smart Pots Comparison Chart, while the 1-gallon and 30-gallon dimensions are based on separate Smart Pots manufacturer specifications. Because fabric grow bags flex and vary in actual filled dimensions, all preset dimensions remain editable in the calculator above.
| Grow Bag Size | Diameter × Height | Full-Rim (Cu Ft) | Net Fill w/ 1" Rim Gap (Cu Ft) | Net Fill (US Dry Qt) | Net Fill (US Liq Gal) | Net Fill (Liters) | Whole 2.0 ft³ Bags (1 Bag) |
|---|---|---|---|---|---|---|---|
| 1 Gallon Grow Bag | 7" D × 6" H | 0.13 cu ft | 0.11 cu ft | 2.9 dry qt | 0.83 liq gal | 3.2 L | 1 bag |
| 3 Gallon Grow Bag | 10" D × 7.5" H | 0.34 cu ft | 0.30 cu ft | 7.6 dry qt | 2.21 liq gal | 8.4 L | 1 bag |
| 5 Gallon Grow Bag | 12" D × 9.5" H | 0.62 cu ft | 0.56 cu ft | 14.3 dry qt | 4.16 liq gal | 15.8 L | 1 bag |
| 7 Gallon Grow Bag | 14" D × 9.5" H | 0.85 cu ft | 0.76 cu ft | 19.5 dry qt | 5.66 liq gal | 21.4 L | 1 bag |
| 10 Gallon Grow Bag | 16" D × 11.5" H | 1.34 cu ft | 1.22 cu ft | 31.4 dry qt | 9.14 liq gal | 34.6 L | 1 bag |
| 15 Gallon Grow Bag | 18" D × 13.5" H | 1.99 cu ft | 1.84 cu ft | 47.3 dry qt | 13.77 liq gal | 52.1 L | 1 bag |
| 20 Gallon Grow Bag | 20" D × 15.5" H | 2.82 cu ft | 2.64 cu ft | 67.8 dry qt | 19.72 liq gal | 74.6 L | 2 bags |
| 30 Gallon Grow Bag | 24" D × 15.5" H | 4.06 cu ft | 3.80 cu ft | 97.6 dry qt | 28.40 liq gal | 107.5 L | 2 bags |
Why Full-Rim vs. Net Fill Matters: According to the Smart Pots medium guide and Smart Pots soil calculator, a 5-gallon fabric bag filled to the top rim holds ~0.62–0.65 cu ft (~16 dry quarts), while leaving 1 inch of watering headspace reduces the net fill to ~0.56 cu ft (~14.3 dry quarts).
Container Soil Volume Mathematical Formulas & Worked Example
Estimating a tapered flower pot as if it were a straight cylinder using its top rim diameter systematically overestimates soil volume by 25% to 40%. GardenCalc models tapered round and square containers as true geometric frustums and interpolates the narrower soil surface diameter at your exact fill height.
1. Round Tapered Pot (Circular Frustum)
h_fill = max(0, h - g)
D_fill = D_bot + (D_top - D_bot) × (h_fill / h)
V (in³) = (π × h_fill / 12) × (D_fill² + D_fill×D_bot + D_bot²)
Where D_top is inside rim diameter, D_bot is inside base diameter, h is inside height, and g is watering headspace.
2. Square Tapered Planter (Square Frustum)
h_fill = max(0, h - g)
a_fill = a_bot + (a_top - a_bot) × (h_fill / h)
V (in³) = (h_fill / 3) × (a_fill² + a_fill×a_bot + a_bot²)
Where a_top and a_bot are inside top and bottom square side widths, and a_fill is the interpolated width at the soil line.
3. Straight Cylinder & Fabric Grow Bag
V (in³) = π × (D / 2)² × max(0, h - g)
Used for straight-walled ceramic pots, 5-gallon utility buckets, and cylindrical non-woven fabric grow bags.
4. Rectangular Planter / Window Box
V (in³) = Length × Width × max(0, h - g)
Used for rectangular deck rail boxes, trough planters, and sub-irrigated patio boxes.
Worked Example: Four 12-Inch Tapered Patio Pots vs. Naive Cylinder Estimate
Suppose you are planting 4 patio pots with an inside top diameter of 12.0 in, inside bottom diameter of 8.75 in, inside height of 10.5 in, and a 1.0 in watering headspace gap:
- Step 1 (Effective Fill Height & Surface Diameter):
h_fill = 10.5 - 1.0 = 9.5 in. At9.5 inabove the base, the interpolated pot diameter isD_fill = 8.75 + (12.0 - 8.75) × (9.5 / 10.5) = 11.6905 in. - Step 2 (Net Frustum Volume per Pot):
V_net = (π × 9.5 / 12) × (11.6905² + 11.6905 × 8.75 + 8.75²) = 784.73 in³per pot, which equals 11.68 US dry quarts (784.73 ÷ 67.200625), 3.40 US liquid gallons (784.73 ÷ 231), or 0.454 cubic feet. - Step 3 (Four Pots Total & Bag Count): Four pots require
4 × 11.68 = 46.71 US dry quarts(1.82 cubic feet). That fits inside one 2.0 cu ft bag (which holds51.43 US dry quarts) or two 1.5 cu ft bags. - Why Cylinder Math Fails: A straight cylinder using the 12-inch top diameter and 10.5-inch height predicts
1,187.52 in³(17.67 dry quarts) per pot — overestimating your soil need by 51.3% compared to the true net frustum fill!
Why Gravel in the Bottom of Pots Hurts Drainage (The Perched Water Table Myth)
A common gardening myth claims that placing a 2-inch layer of gravel, rocks, or broken pottery shards at the bottom of a pot improves drainage and reduces the amount of potting mix you need. Soil physics research proves the opposite: water moves through fine-pored potting media by capillary action and resists crossing into coarse gravel air spaces until the soil immediately above the gravel is completely saturated.
As documented by Washington State University Extension horticulturist Dr. Linda Chalker-Scott in The Myth of Drainage Material in Container Plantings (WSU Extension PDF), bottom gravel simply raises the perched water table closer to plant roots, shrinking the aerated root zone and increasing root rot risk. Fill containers all the way from the drainage holes with quality potting mix and use a porous mesh screen if needed to keep soil from washing out.
US Dry Quarts vs. Liquid Gallons & Nursery Trade Gallons Explained
Potting mix bags in the United States are regulated under NIST Handbook 130 and sold by dry volume (dry quarts, cubic feet, or liters), whereas liquid jugs and many container capacities are quoted in US liquid gallons:
- 1 US Dry Quart: Exactly
67.200625 in³(1.101 L). One cubic foot (1,728 in³) equals 25.71 US dry quarts. - 1 US Liquid Quart: Exactly
57.75 in³(0.946 L) — about 16.4% smaller than a dry quart. - 1 US Liquid Gallon: Exactly
231 in³(3.785 L). One cubic foot equals 7.48 US liquid gallons, and a standard 5-gallon bucket holds0.67 cu ft. - Nursery "Trade Gallon" (#1 Container): Under the American Standard for Nursery Stock (ANSI Z60.1), a black plastic
#1("1 gallon") nursery pot actually holds only 0.63 to 0.75 US liquid gallons (~2.5–2.8 Lor~2.2–2.6 US dry quarts), and a#5nursery pot holds roughly 3.5 to 3.8 US liquid gallons.
For successful container vegetable and flower growing depths, see University of Minnesota Extension: Container Gardening and our Editorial Policy & Verification Standards.