Potting Soil Calculator

Article — Potting Soil Calculator

Potting Soil Calculator: How Much Mix to Buy

A potting soil calculator converts pot dimensions to soil volume in liters, cubic feet, quarts, and gallons. Pots fill at 90 percent of geometric volume because drainage gravel, root balls, and the 2 to 3 cm rim gap eat into total capacity. A 10-inch round pot needs about 10 liters or 0.36 cubic feet of mix.

The numbers matter because potting mix is sold in fixed bag sizes. A 1.5 cubic foot bag (about 42 L) is the retail standard. A 2 cubic foot bag (57 L) is the contractor size. Buying one bag too few means a return trip mid-project. Buying three too many means storing leftover mix that absorbs moisture, compacts, and develops fungal residents within a year.

What the potting soil calculator does

The potting soil calculator computes pot volume from inside dimensions, applies the 90 percent fill factor, multiplies by pot count, then converts the result to whatever unit your supplier uses. Round pots use π r² h. Rectangle uses length × width × height. Square uses side² × height.

Output includes liters, cubic feet, US gallons, quarts, the bag count for both 1.5 ft³ and 2 ft³ retail sizes, and an approximate weight estimate. The weight number matters if the pots sit on a balcony, indoor floor, or anywhere with a load limit.

Measuring pot volume correctly

Measure inside the pot, not outside. Decorative pottery often has 2 to 5 cm of decorative wall that does not hold soil. For round pots, the inside diameter at the rim is what feeds into π r². For rectangular pots, inside length and width. For tapered pots, measure halfway between rim and base for the most accurate result.

Round vs rectangular pot math

Round pots are easier to mix evenly because there are no corners for soil to wedge into. Rectangular pots fit more total soil for the same footprint — a 30 × 30 cm square pot holds more than a 30 cm round pot of the same height. The corners add 21 percent volume. Window boxes are usually rectangular for this reason.

Square pots used in greenhouse production stack and pack efficiently, but the corners create slight dead zones where roots circle. Round pots produce slightly stronger root balls for transplanting. Neither shape changes the soil math — the potting soil calculator handles all three.

Potting soil vs garden soil

Potting soil is a lightweight blend of peat moss, perlite, vermiculite, and composted bark or coir. It drains fast, holds nutrients short-term, and weighs about half what garden soil weighs. Garden soil (or topsoil) is heavier mineral soil with no peat, designed for in-ground beds.

Using garden soil in containers is one of the most common new-gardener mistakes. Garden soil compacts in pots after 2 to 3 waterings, suffocates roots, and never properly drains. Potting mix is the only correct choice for containers. Conversely, filling raised beds with bagged potting mix is wasteful — raised beds use topsoil and compost blends, with a smaller potting-mix top layer for seeding.

Bag counts and bulk volumes

Retail potting mix comes in three common bag sizes: 1.5 cubic feet (42 L), 2 cubic feet (57 L), and 3 cubic feet (85 L). Big-box stores stock the smaller bags; landscape supply yards stock the 2 and 3 cubic foot bags. Any project above 5 cubic feet should price bulk delivery — bulk is 40 to 60 percent cheaper per cubic foot than retail bags.

• 4-inch pot = 0.6 L (1/70 of a 1.5 ft³ bag)
• 6-inch pot = 2.1 L (1/20 bag)
• 8-inch pot = 5.1 L (1/8 bag)
• 10-inch pot = 10 L (1/4 bag)
• 14-inch pot = 27 L (2/3 bag)
• Window box (60 × 20 × 18 cm) = 17 L (less than 1/2 bag)
• Raised bed 4 × 4 × 1 ft = 16 ft³ (9.6 bags or 0.55 cubic yards bulk)

DIY potting mix recipes

Home-made potting mix saves 50 to 70 percent versus retail bagged mix for any project over 50 liters. The classic recipe is one part peat moss (or coconut coir), one part perlite, one part compost — sometimes called Mel's Mix when used in square-foot gardening. Adjust ratios for drainage: more perlite for cactus, more peat for moisture-loving ferns.

The University of Florida IFAS standard recipe for container vegetables is 50 percent peat, 35 percent composted pine bark, 15 percent perlite, plus 1 cup dolomitic lime per cubic foot to balance pH. Volume estimates from the potting soil calculator feed directly into ingredient quantities — calculate total soil needed, then split by mix ratio.

Weight and balcony load

Potting mix weighs roughly 0.5 kg per liter dry, up to 0.65 kg per liter wet. A 1.5 cubic foot bag is about 10 kg dry, 13 kg wet. A 16-inch hanging basket holding 30 L of saturated mix weighs nearly 20 kg — more than most plant hangers and balcony rails are rated for.

Reusing old potting soil

Used potting mix can be reused with refresh. Mix 50 percent old soil with 50 percent fresh, add a handful of slow-release fertilizer, and discard any soil that hosted diseased plants. Sterilize suspect batches by spreading 5 cm deep on a baking tray and heating at 75°C for 30 minutes — kills fungal spores, weed seeds, and most insect eggs.

Old mix that has dried out and shrunk to half its original volume is past reuse. Peat-based mix in particular becomes hydrophobic when fully dry — water beads off instead of soaking in. Re-wet by soaking in a tub for several hours, then test by squeezing a handful; it should hold shape when released without dripping water.