A container potting mix calculator takes the guesswork out of buying soil for pots and planters. Enter your container dimensions, count, and plant type to get the exact volume you need, a custom ingredient breakdown (peat/coir, perlite, compost, vermiculite), and how many bags to buy — so you never run out or overbuy.
Container Setup
Standard half whiskey barrel: ~23.5 inches diameter × 11.5 inches deep ≈ 23 gallons capacity (about 87 liters). Dimensions are fixed for this shape.
How to Use the Container Garden Mix Calculator
Getting the potting mix right is one of the most important factors for container garden success. Too little and you run short mid-filling. Too much and you waste money on unused bags. Our free container potting mix calculator gives you the exact volume by container shape and size, a plant-specific ingredient breakdown, and a cost estimate.
Step 1: Choose Container Shape and Dimensions
Select from round pots, square planters, rectangular window boxes, or the standard half barrel. Enter dimensions in inches — all dimensions are converted to cubic inches internally, then to gallons or liters depending on your unit preference. The calculator deducts 10% for headspace (the gap between soil and pot rim needed for watering).
Step 2: Set Number of Containers and Plant Type
If you are filling multiple identical containers, set the count to get total volume. Plant type affects the mix recipe — succulents need fast-draining gritty mix, vegetables need nutrient-rich compost-heavy blends, and tropicals need moisture-retentive mixes with good aeration.
Step 3: Review the Mix Breakdown
The ingredient breakdown shows what percentage of each component to use. For vegetables, a typical mix is: 60% peat/coir (moisture retention), 20% perlite (drainage), 20% compost (nutrients). For succulents, perlite increases to 40-50% to prevent root rot. The table shows exact volumes per container and in total so you can calculate how much of each component to purchase separately.
Understanding Component Functions
Peat moss / Coconut coir: The bulk ingredient. Retains moisture and provides structure. Coir is the more sustainable choice. Perlite: White volcanic glass that improves drainage and prevents compaction. Compost: Provides slow-release nutrition and beneficial soil microbes. Vermiculite: Improves moisture retention and helps retain nutrients — used more for seed starting than established plants.
Buying Guide
Standard bagged potting mix comes in 1 cubic foot bags (about 7.5 gallons) or 2 cubic foot bags. For larger projects, buying by the cubic yard from a landscape supplier is much cheaper. Always buy 10-15% extra to account for settling and to have material for top-dressing later in the season.
FAQ
Is this container mix calculator free?
Yes, the container potting mix calculator is completely free with no limits. Calculate soil volumes for as many containers as you like. No signup or account required, and all calculations run locally in your browser.
Is my data private?
Yes. Every calculation runs locally in your browser. No container dimensions, plant types, or personal information are ever sent to a server. Your data stays completely private.
What is the difference between potting mix and garden soil?
Potting mix is specifically formulated for containers — it is lightweight, well-draining, and sterile. Garden soil is too heavy for containers, compacts easily, and can introduce pests and diseases. Always use potting mix or a custom blend in containers, never straight garden soil.
How much potting soil do I need for a 5-gallon pot?
A 5-gallon round pot needs approximately 5 gallons (about 19 liters) of potting mix. However, you should fill to about 1-2 inches below the rim to prevent overflow during watering, so in practice you will use 4-4.5 gallons. The calculator accounts for this headspace automatically.
What components should I use in a potting mix?
Most quality potting mixes contain: peat moss or coconut coir (water retention), perlite (drainage and aeration), compost (nutrients and biology), and sometimes vermiculite (additional moisture retention). The ratios vary by plant type — succulents need more perlite, vegetables need more compost, and tropicals need a balanced blend.
Can I reuse potting mix from last year?
Old potting mix can be refreshed by removing any roots, mixing in 20-30% new mix, and adding compost and a slow-release fertilizer. Potting mix loses its structure and nutrients over time. After 2-3 seasons, it is usually better to replace it entirely rather than trying to amend it.
What size pot do I need for tomatoes?
Tomatoes need a minimum of 5 gallons (about 19 liters) per plant, and 10-15 gallons is better for full-sized indeterminate varieties. Compact or patio tomato varieties can thrive in 3-5 gallon pots. Larger containers hold moisture longer and provide room for the extensive root system tomatoes develop.
Does the calculator support metric units?
Yes. Use the unit toggle to switch between gallons (US) and liters. All volume outputs update automatically. The calculator also shows how many standard bag sizes you need to purchase.