Calculator
Potting Mix Calculator
Pick the gallon size on the pot or grow bag, how many pots you have, and the bag size at the store. The tool uses US liquid gallons (what most grow bags and buckets mean). Nursery trade gallons can be smaller; measure if the plant looks cramped. Fill with soilless potting mix, not garden soil or topsoil. Optionally split that volume into an Extension-based Equal-parts soilless mix or a Commercial-mix blend.
Your result
Potting mix to buy
1bag1.0 cu ft
- Volume
- 0.61 cu ft15.8 dry quarts
- Liters
- 17 L
- Bags needed
- 11.0 cu ft, rounded up
Fits: Determinate tomato, eggplant, trellised cucumber, a comfortable pepper.
Plastic: Check two inches down. Plastic stays wet longer than fabric after the same rain.
Equal-parts soilless mix · DIY (no garden soil)
- Finished compost0.2 cu ft
- Coarse vermiculite0.2 cu ft
- Peat moss0.2 cu ft
DIY recipe · soilless only
Optional potting mix recipe
Bagged soilless potting mix is fine. If you blend your own, stay soilless. Choose an Extension-based starting recipe below — not a universal formula. Raised beds can include screened topsoil; ordinary pots should not.
No garden soil or topsoil in pots. Those materials pack hard in a container and stay wet around roots. Use them in open raised beds instead.
An Extension-based starting recipe adapted from Penn State Extension’s equal-parts soilless mix: finished compost, vermiculite, and peat moss. One documented starting point — not the only valid potting mix. Compost and base mixes vary in fertility; follow product labels and watch plant response.
Best for: Most patio pots, grow bags, and herbs when you want a light soilless DIY blend
| Ingredient | Share | Cubic feet | Dry quarts | 2 cu ft bags |
|---|---|---|---|---|
| Finished compost | 33.3% | 0.2 | 5.3 | 1 |
| Coarse vermiculiteHorticultural grade, not craft-store fine dust | 33.3% | 0.2 | 5.3 | 1 |
| Peat mossPenn State’s cited formula names peat moss; coconut coir is sometimes used as a peat substitute, but that is not what that Extension recipe specified | 33.3% | 0.2 | 5.3 | 1 |
- Do not use garden soil or topsoil in pots. They are too heavy for ordinary containers and elevated garden boxes. Save screened topsoil for raised beds.
- Equal-parts soilless mix follows a Penn State Extension documented starting recipe of equal parts finished compost, vermiculite, and peat moss. Coconut coir is sometimes used as a peat substitute; that Extension formula names peat moss.
- Commercial-mix blend follows a University of Georgia Extension suggested starting mixture of half commercial soilless potting mix and half finished compost. Read the bag label — brands and fertilizer charges differ. Local prices determine cost.
- Compost and base mixes vary in fertility. Follow product labels and watch plant response rather than inventing fertilizer amounts. Compressed peat expands when wet — plan from the hydrated volume on the label.
- See potting mix vs garden soil and growing vegetables in containers.
How this calculation works
For a labeled gallon pot: cubic feet ≈ gallons × 0.1337, then about 8% off if you leave a watering rim. Bags = cubic feet ÷ bag size, rounded up. Custom round pots use π × radius² × fill height. Tapered pots use a frustum. Optional DIY Equal-parts soilless mix splits the volume into equal thirds of finished compost, coarse vermiculite, and peat moss. Commercial-mix blend uses half commercial soilless potting mix and half finished compost. Both are starting recipes, not universal formulas.
Example: two 5-gallon grow bags, 1 cu ft bags
One 5-gallon bag is about 0.67 cubic feet. Leaving a rim, call it 0.61 cu ft each. Two bags: 1.23 cu ft, so 2 bags of 1.0 cu ft potting mix (rounded up). A 16-quart bag is about 0.62 cu ft, so you would still need 2 of those. Equal-parts soilless mix for 1.23 cu ft is about 0.41 cu ft each of compost, vermiculite, and peat moss.
Frequently asked questions
Is a 5-gallon nursery pot really 5 gallons?
Not always. Hardware-store buckets and many fabric grow bags are close to true US gallons. Nursery “#5” plastic pots are often smaller. If the plant dries out every afternoon in a “5 gal” nursery pot, measure it or step up a size.
Can I use garden soil or raised-bed mix in a pot?
No. Garden soil and topsoil are too heavy for ordinary pots and elevated garden boxes. They pack, stay wet, and cut off air. Use a soilless potting mix and check the product label. The potting mix vs garden soil guide on this site explains the difference.
What DIY mix should I use?
There is no single correct blend. Equal-parts soilless mix follows a Penn State Extension documented starting recipe of equal parts finished compost, vermiculite, and peat moss. Commercial-mix blend follows a University of Georgia Extension suggested mixture of half commercial soilless mix and half finished compost. Compost and bags vary in fertility — follow labels and watch plants.
Does the pot material change how much mix I buy?
No. Material changes how fast the pot dries, not the interior volume. Fabric needs more frequent watering. Plastic holds moisture. Unglazed terracotta dries faster than glazed ceramic.
Sources and methodology
- Potting Media and Plant Propagation — Penn State Extension
- Starting your garden and containers off right — University of Minnesota Extension
- Soil for Home Gardens — University of Georgia Extension