When customers ask what volume of soil our planter box factory recommends for large planters, we do not give one number. The right volume depends on internal dimensions, root depth, plant type, drainage, and climate. As a factory, we use a simple guideline: measure the actual interior space, then fill 90–95% with a light, free-draining mix.
For typical large planters, our recommended soil volumes are:
- 24–30 in (61–76 cm) wide, 18–24 in (46–61 cm) deep: 80–120 L (21–32 gal) for annuals, herbs, and compact perennials.
- 30–36 in (76–91 cm) wide, 20–28 in (51–71 cm) deep: 150–250 L (40–66 gal) for shrubs, roses, and larger perennials.
- 36–48 in (91–122 cm) wide, 24–36 in (61–91 cm) deep: 300–500 L (79–132 gal) for small trees, Japanese maples, and mixed plantings.
- 48 in+ (122 cm+) wide or extra-large rectangular boxes, 30–40 in (76–102 cm) deep: 600–1,200+ L (158–317+ gal) for specimen trees and long-term tree plantings.
To calculate your own planter, use the interior dimensions. For a rectangular box: length × width × depth in centimeters ÷ 1,000 = liters. For inches: length × width × depth × 0.016387 = liters. For a round planter: 3.14 × radius² × depth ÷ 1,000 = liters when using centimeters. Subtract any drainage layer, false bottom, or filler. If you add a 5 cm (2 in) gravel layer, that space does not count as soil volume. Most large planters with drainage holes do not need gravel; a geotextile screen over the holes and a coarse, airy soil mix is better.
Root depth matters as much as volume. Annuals and shallow herbs can grow in 25–30 cm (10–12 in) of soil. Most shrubs need 40–50 cm (16–20 in). Small trees and large evergreens need at least 60–90 cm (24–36 in). If the planter is shallower than these depths, choose smaller plants or treat it as a seasonal display.
For large planters, we recommend a soil mix rather than pure garden soil. A good blend is roughly 60–70% quality topsoil or compost, 20–30% peat moss or coconut coir, and 10% perlite, pumice, or pine bark. This holds moisture without becoming heavy or waterlogged. Add slow-release fertilizer at planting and refresh the top 5–8 cm (2–3 in) of soil each year. Do not fill the planter to the absolute rim; leave 2–5 cm (1–2 in) of headspace for watering and mulch.
If you are ordering from our factory, send us the exterior size, interior depth, drainage hole layout, and the plants you want to grow. We will recommend the exact soil volume and, if needed, design the planter with the right depth and drainage for your project. In short, for large planters our factory recommends 80–120 L for medium-large boxes, 150–250 L for large boxes, 300–500 L for extra-large boxes, and 600–1,200+ L for tree-sized planters, always based on internal dimensions and filled to 90–95%.
