Raised Bed Soil Calculator
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Potting Soil Calculator
Liters and bags of potting mix for any number of pots.
Potting Soil Calculator
Chlorova · updated for 2026
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Click “Calculate” to see your results.
Cylinder volume at 100% fill — real pots need 0.5 to 1 inch of headroom below the rim, and fresh fluffy mix settles about 15%. A pot's 'gallon' label is outer volume and is never the mix volume you buy.
The potting soil calculator sizes a mix purchase from pot geometry: diameter × depth as a cylinder, multiplied by pot count, converted into the units you actually shop in — quarts per pot, cubic feet and liters total, and bags at the store's bag size. A dozen 12-inch pots drink about 1.2 cubic feet each, which is why 'eight bags of 2-cu ft' materializes out of what looked like a small patio project. The number that surprises most growers is per-pot: it is what tells you whether that 5-gallon bucket label means 5 gallons of mix. It does not.
How to Use the Potting Soil Calculator
- Measure the pot's inside diameter and its usable depth in inches — depth to about an inch below the rim.
- Enter how many pots of that size you are filling; mix one size at a time for clean math.
- Set the bag size the store stocks: 0.5, 1, 2 and 3 cu ft are the common formats.
- Type the bag price so the total cost sits next to the bag count.
- Compare the quarts-per-pot output with your seed or transplant plan before buying.
- Add a cushion for spillage and top-up after settling — bags, not arithmetic, round you up anyway.
The Pot Volume Formula, Explained
A pot is a cylinder, so mix volume = π × (diameter ÷ 2)² × depth, computed in inches and divided by 1,728 (cubic inches in a cubic foot). From there it is pure unit shopping: cu ft × 25.82 = quarts, cu ft × 28.3168 = liters, and bags = total ÷ bag size, rounded up.
Two realities bend the ideal cylinder. First, headroom: every pot needs an inch or so below the rim for watering, which is why usable depth rather than the pot's full height is the input. Second, settling: fresh potting mix is fluffy and drops 10 to 20% after the first waterings, so a brim-full pot sinks below it by the second week. Tapered pots (a true cone) hold roughly 10 to 15% less than the cylinder math predicts, and a '5-gallon' bucket holds its outer volume of water but maybe four gallons of mix once root ball and headroom are accounted.
Worked Example: Four Patio Tomato Buckets
- Four pots, 14 in inside diameter, 13 in of usable depth (an inch of headroom shy of full).
- Per pot: 3.1416 × 7² × 13 ÷ 1,728 ≈ 1.16 cu ft — call it 30 quarts.
- Total: 4 × 1.16 = 4.65 cu ft, which is 131 liters of mix.
- In 2-cu ft bags: ceil(4.65 ÷ 2) = 3 bags — and at $8 each, $24 for the season's planting.
- A 5-gallon bucket label would have promised 25.7 quarts and delivered about 30 — pot labels describe water volume to the brim, not mix volume with headroom.
- Budget the settle: fill to the brim, water, and top a dry handful or two per pot a week later.
- Reusing last year's mix? Subtract nothing from the volume math, but plan a third of fresh compost stirred in for the nutrients the crop took out.
Mix Volume for Common Pot Sizes
Usable volume per pot at one inch below the rim, in quarts — the unit bagged mix is sold in. One 2-cu ft bag (51.6 qt) fills roughly the number of pots implied by dividing down.
| Pot | Usable qt | Approx cu ft |
|---|---|---|
| 2-in cell (plug) | 0.05 | 0.002 |
| 4-in pot | 0.7 | 0.03 |
| 6-in pot | 1.9 | 0.07 |
| 10-in / gallon pot | 7 | 0.27 |
| 12-in hanging basket | 11 | 0.43 |
| 14-in patio pot | 30 | 1.16 |
| 5-gallon bucket | 34 | 1.3 |
Key Terms
- Potting mix
- A soilless blend of peat or coir with perlite and bark — lighter and fluffier by volume than garden soil, and the right product for containers.
- Headroom
- The inch or so left below the rim so watering has somewhere to go — subtracted from depth in this calculator.
- Settling
- The 10–20% volume drop as dry mix compacts under its first soaks; top up rather than pack.
- Quart format
- Retail bag sizes quoted in quarts (8, 16, 50 qt) — 25.82 qt to the cubic foot is the conversion doing the work here.
- Plug/cell count
- Seedling tray vocabulary: a 36-cell flat holds about 1.8 cu ft of media total, which a single 2-cu ft bag covers.
What This Means on a Real Patio
- Buy in the biggest bag format your hands can carry: cost per cubic foot drops sharply from 8-quart pouches to 3-cu ft bales, and re-bagging mix is the exact chore container gardening is supposed to avoid.
- Peat-based mix hydrophobically betrays you when it dries fully — wet it thoroughly at fill time rather than assuming the first watering soaks evenly.
- For seedlings, the volume that matters is per-cell: over-filling a 72-cell tray by a inch is two extra bags across a spring; this calculator's cell math keeps flats honest.
- Weight is the hidden constraint: 1.2 cu ft of saturated mix is 60-plus pounds, so a tall pot of garden soil on a balcony is both a drainage problem and a load problem — stay with light mix above ground level.
- Deck the cost against per-season reuse: good mix refreshed with compost and a slow-release feed gardens three years, which is the number a bag price never shows.
Common Mistakes & Fixes
- Scooping garden soil into pots — it compacts to a brick in a container and drowns roots; potting mix is a manufactured structure, not dirt with manners.
- Sizing the purchase off the pot's gallon label, which is outer brim volume, not the mix with headroom the roots get.
- Measuring outside diameter on a tapered pot, which overstates every row of the plan by the wall thickness and taper.
- Filling to the absolute rim and then washing half the mix out the first watering — leave the headroom this calculator already subtracted.
- Assuming one bag equals its printed volume poured dry — compressed bales expand, and the honest figure is what fits your pots after fluffing.
When to Use This Estimate (and When Not To)
Use it for any set of same-shaped containers: patio pots, hanging baskets, window boxes (treat the box as a rectangle and do the cubic-inch division yourself), and cell flats. It does not fit raised beds — those want the bed calculator's blend fractions — nor grow bags and fabric pots whose effective volume is smaller than their stated dims, nor self-watering planters where a reservoir displaces mix. For mixed pot sizes, run the calculator once per size and add the bag counts together.
Potting Soil Calculator: Frequently Asked Questions
How many quarts are in a cubic foot of potting soil?
Almost exactly 25.82 dry quarts, so a 2-cu ft bag holds about 52 qt. That conversion is the whole trick to comparing bag sizes and pot demands written in different units.
Does a 5-gallon bucket need 5 gallons of potting mix?
No — the label is water volume to the brim. With an inch of headroom and the tapered or straight walls of real buckets, you fill it with roughly 4 to 4.5 gallons of mix, and it settles below that after the first soaks.
How many bags for a 12-inch hanging basket?
A 12-in basket takes about 11 qt, roughly 0.43 cu ft — so one 2-cu ft bag fills four baskets, and a 50-qt bag fills nearly five. Buy one size up from your count for the settling top-up.
Is potting mix the same as raised-bed soil?
No, and the difference is structural: pots need a light, free-draining soilless mix because drainage is a single hole, while beds can carry a heavier blended soil with real topsoil in it. Using bed soil in pots suffocates roots; using pot mix in beds drains and dries too fast.
Can I reuse last year's potting mix?
Yes for most crops — refresh it with about a third part fresh compost or aged manure and a slow-release organic feed, and rehydrate fully before refilling. Skip reuse where the previous crop died of a soil-borne disease like verticillium.
Why does my new bag look half-empty?
Bagged mix is compressed for shipping and expands when fluffed — the printed volume is the poured, aerated figure. Break up any clods as you fill and the bag will stretch to its claim.
Sources & Further Reading
- Container Potting Mixes and Fertilization — Michigan State University Extension. Mix composition, moisture and reuse guidance.
- Growing Plants in Containers — University of Minnesota Extension
- Potting Mix Science for Home Gardeners — Cornell University
Chlorova Gardening Team
Builds, checks and documents every tool we publish
Chlorova editorial review
Checked against extension-service publications
Formula verified against published references · Updated