Potting Soil Calculator
Work out how much potting soil your pots and planters need, in quarts, litres and bags.
Potting soil is sold in quarts and litres while pots are measured in inches, so every spring someone stands in the aisle doing hopeless maths. This calculator does it properly: it works out the real volume of round, tapered and rectangular pots, leaves an inch of watering space at the rim, and totals the bags for the whole shelf or patio.
Tell it the pot shape and count. If you are matching plants to pots rather than filling them, the Container Size Matchmaker picks the pot size for the plant.
How this is worked out
A tapered round pot is a cone frustum: volume = pi x fill height / 3 x (R squared + R x r + r squared), where R and r are the top and base radii. A cylinder is pi x radius squared x height; a trough is length x width x height. Every pot is filled to 1 in below the rim, because a pot filled to the brim sheds water instead of taking it in. One dry quart is 57.75 cubic inches.
Assumptions:
- Measurements are inside measurements. Thick-walled ceramic pots hold noticeably less than their outside size suggests.
- The root ball of the plant displaces some mix, roughly a quart on a gallon-sized plant. It is covered by the settling allowance in practice.
- Bag volumes are dry quarts as printed; 1.5 cu ft is about 45 dry quarts.
Worked examples
1. Six 12 in tapered pots. Top 12 in, base 9 in, 11 in tall, filled to 10 in: about 15 qt each, 92 qt total. Six 16 qt bags, with a little left over.
2. A 24 x 8 x 8 in window box. Filled to 7 in: 24 x 8 x 7 = 1,344 cu in = 23 qt. Two 16 qt bags, or one bag plus last year’s leftovers.
3. One big 20 in patio pot. Top 20 in, base 14 in, 18 in tall: about 86 qt. That is two 1.5 cu ft bags for a single pot, which is why big pots get a filler of empty bottles or a false bottom in practice.
Standard pot sizes and what they hold
| Trade size | Top diameter | Approx volume |
|---|---|---|
| 4 in pot | 4 in | about 1 qt |
| 6 in pot | 6 in | about 2.5 qt |
| 1 gallon | 7 to 8 in | about 4 qt (a trade gallon, not a full one) |
| 3 gallon | 10 to 11 in | about 11 qt |
| 5 gallon | 12 to 13 in | about 19 qt |
| 10 gallon | 16 to 17 in | about 38 qt |
Trade gallons are a nursery convention, not a liquid measure: a ‘1 gallon’ pot holds about 0.7 of a real gallon. Measuring your actual pot beats trusting the label.
Common mistakes
- Filling to the brim. Water runs off a brim-full pot. The 1 in gap this calculator leaves is the watering reservoir.
- Judging by outside dimensions. A chunky terracotta rim can eat 20% of the volume. Measure inside.
- Buying for the pot count, not the volume. Doubling pot diameter roughly octuples volume. Three sizes up is not ‘a bit more soil’.
- Reusing tired mix unchanged. Last year’s mix has lost structure and nutrients; refresh it with a third new mix and a slow-release feed, or replace it.
Putting the result to work
Fill pots in layers, firming gently, and water each layer in so there are no dry pockets. Plant at the same depth the plant grew in its nursery pot. For choosing which pot a plant actually needs, the Container Size Matchmaker gives minimum sizes per plant, and the Liquid Fertilizer Dilution Calculator handles feeding once the pots are growing.
How much potting soil do I need?
- Measure the pot inside: top width, base width (round pots), and height, in inches.
- Pick the shape: tapered round, straight cylinder, or rectangular trough.
- The calculator fills to 1 in below the rim and works out quarts per pot.
- Multiply by the pot count and round up to whole bags.
- Keep the spare mix for topping up after the first waterings settle the level.
Why fill pots 1 inch below the rim?
That gap is a watering reservoir. A pot filled level with the rim sheds most of the water you pour on it, especially once the surface dries and turns water-repellent. One inch holds a proper drink in place long enough to soak in.
How many quarts are in a 1.5 cu ft bag?
About 45 dry quarts. A cubic foot is 29.9 dry quarts, so 1.5 cu ft is roughly 45 qt, whatever the bag art suggests. Bags labelled only by weight vary, because wet mix weighs far more than dry.
Does the plant’s root ball change the amount?
A little. A gallon-sized root ball displaces about a quart of mix. In practice the settling of fresh mix (about an inch over the first weeks) matters more, and keeping spare mix from the last bag covers both.
Can I reuse last year’s potting soil?
Yes, with a refresh: break it up, pick out old roots, and blend in about a third fresh mix plus a slow-release fertilizer. Replace it entirely if last year’s plants had root disease, or if the mix has turned to dust and drains instantly.
How much soil does a raised bed need instead?
Raised beds are measured in cubic feet and yards, not quarts. Use the Raised Bed Soil Calculator, which also splits the total into topsoil, compost and aeration fractions.
Volumes are standard geometry (cone frustum, cylinder, box). Conversions: 1 dry quart = 57.75 cu in; 1 cu ft = 29.9 dry quarts. Trade pot sizes follow common US nursery conventions.
