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 potting mix, not garden soil. Optionally split the volume into a Classic or Budget DIY soilless recipe.
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.
Classic · DIY (no garden soil)
- Finished compost0.18 cu ft
- Coco peat (coir) or peat moss0.18 cu ft
- Horticultural perlite0.18 cu ft
- Coarse vermiculite0.06 cu ft
DIY recipe · soilless only
Optional potting mix recipe
Bagged potting mix is fine. If you blend your own, stick to compost + peat or coco + perlite + a little vermiculite. That is different from raised-bed blends, which can include screened garden soil. These ratios split the volume above — starting points, not one perfect formula.
No garden soil in pots. Dirt from the yard packs hard in a container and stays wet around roots. Use it in raised beds instead.
Compost, coco peat (or peat moss), and perlite as the base, plus a little vermiculite for moisture. Soilless, light, and ready for everyday pots.
Best for: Most pots, vegetables, and herbs
| Ingredient | Share | Cubic feet | Dry quarts | 2 cu ft bags |
|---|---|---|---|---|
| Finished compost | 30% | 0.18 | 4.7 | 1 |
| Coco peat (coir) or peat mossUse one of them, not both at full strength | 30% | 0.18 | 4.7 | 1 |
| Horticultural perliteCoarse perlite, not craft-store fine dust | 30% | 0.18 | 4.7 | 1 |
| Coarse vermiculiteHelps the mix hold water between waterings | 10% | 0.06 | 1.6 | 1 |
- Do not use garden soil or topsoil in pots. Native dirt packs, drains poorly in a closed container, and can bring pests and weeds. Save screened soil for raised beds.
- Base is compost + peat or coco + perlite + a little vermiculite. Perlite keeps air in the mix; vermiculite holds moisture between waterings. Swap peat for coco if you prefer. Budget uses more compost and less of the other bags when you are filling a lot of pots.
- Compressed coir bricks expand a lot when wet. Plan from the hydrated volume on the label, then match the cubic feet / dry quarts in the table.
- DIY potting mix has little fertilizer. After seedlings are established, feed lightly — or start with a small charge of slow-release according to the product 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. DIY Classic splits the volume into 30% compost, 30% coco peat or peat moss, 30% perlite, and 10% vermiculite. Budget uses 50% compost, 20% coco or peat, 20% perlite, and 10% vermiculite.
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. DIY Classic for 1.23 cu ft is about 0.37 cu ft each of compost, coco or peat, and perlite, plus about 0.12 cu ft vermiculite.
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 mineral-heavy raised-bed mix pack in a closed container, stay wet, and cut off air. Use potting mix (peat or coir, bark, perlite). The potting mix vs garden soil guide on this site explains the difference.
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.