How the calculator works
- Container volume (inner): cylinder
V = π·(d/2)²·h, boxV = l·w·h. - Drainage volume: footprint area × drainage thickness.
- Soil volume used: (total − drainage) × fill %.
- Soil weight: soil volume × dry bulk density × moisture factor.
- Water weight: (irrigation × retained %) + reservoir water, using 1 kg/L.
- Total load: soil + drainage + water + pot + plant + additives.
- Load per footprint: total load ÷ footprint area.
- Recommended rating: total load × safety factor.
Steps for accurate results
- Select unit system and container shape.
- Enter inner dimensions and drainage thickness.
- Set fill percentage for your mix height.
- Enter soil density and moisture factor for conditions.
- Add pot, plant, and additive weights if known.
- Estimate irrigation added and retained percentage after drainage.
- Choose a safety factor for moving or shelf design.
- Press calculate to see totals and breakdown.
Sample container scenarios
| Scenario | Shape & size | Mix & moisture | Water retained | Pot + plant | Estimated total |
|---|---|---|---|---|---|
| Balcony herb pot | Round, 30×30 cm | 0.60 kg/L × 1.15 | 2.0 L × 60% | 3.0 kg | ~18 kg |
| Tomato container | Round, 40×35 cm | 0.65 kg/L × 1.30 | 3.0 L × 70% | 5.5 kg | ~34 kg |
| Planter box | Box, 60×25×25 cm | 0.55 kg/L × 1.15 | 2.5 L × 60% | 4.0 kg | ~27 kg |
Practical guidance
Container load drivers in small gardens
Container load is dominated by media volume, retained water, and pot material. A 30 L container adds about 30 kg of water-equivalent mass at full saturation, even before counting the pot itself. Lightweight mixes typically range from 0.55–0.75 kg/L when dry, while dense compost blends can exceed 0.85 kg/L. Plastic pots often stay under 2 kg, while ceramic and concrete can add 5–25 kg.
Estimating soil mass with bulk density
This calculator converts inner container volume to usable soil volume after subtracting the drainage layer and applying the fill percentage. Soil weight is then computed as soil volume × dry bulk density × moisture factor. For example, 20 L of mix at 0.60 kg/L and a 1.15 moisture factor yields about 13.8 kg of soil load. Measuring a known bucket volume and weighing it is the fastest way to calibrate your real mix.
Water retention after irrigation and storms
Not all applied water remains in the container, but most mixes hold 50–70% as pore water immediately after drainage slows. A routine watering of 2 L with 60% retention adds 1.2 kg. Self‑watering bases and saucers may store an additional 0.3–2.0 L. For rooftop and balcony planning, use a higher moisture factor (1.30–1.55) to reflect rainfall saturation and reduced evaporation.
Footprint pressure for shelves and balconies
Total weight matters for lifting, but supports fail from pressure concentration. The calculator reports load per footprint area, letting you compare a narrow rail planter against a wide floor pot. Two containers with equal total weight can produce very different pressure if their footprint differs by 2×. When in doubt, spread the load with trays, wider stands, or additional brackets.
Using a safety factor for handling and hardware
The recommended rating multiplies total load by a safety factor (commonly 1.10–1.30). This buffer accounts for measurement error, uneven loading, dynamic forces when moving, and seasonal wet periods. If your setup is elevated, exposed to heavy rain, or placed on a cantilevered shelf, increase the factor and verify fasteners, anchors, and material condition.
FAQs
1) What soil bulk density should I start with?
Use 0.60 kg/L for most potting mixes. If your mix is compost‑heavy, try 0.70–0.85 kg/L. For accuracy, weigh a known volume bucket of your media.
2) How do I choose the moisture factor?
Dry indoor pots: 1.00–1.10. Regular outdoor moisture: about 1.15. Wet season or frequent irrigation: 1.30. After storms or waterlogging risk: up to 1.55.
3) Should I include a drainage layer?
Include it only if you actually use one. It reduces soil volume but adds its own weight. Lightweight clay pebbles are often modest load, while gravel can add significant weight.
4) How do I estimate retained water percent?
Start with 60%. Increase to 70% for fine, peat‑based mixes and dense root zones. Decrease to 40–50% for very coarse mixes with high drainage and airy structure.
5) What does load per footprint mean in practice?
It is the weight divided by the base area. It helps compare pressure on shelves, railings, and decking. Higher values mean the support must resist more concentrated force.
6) Why is the recommended rating higher than total load?
It applies a safety factor to cover wet periods, measurement uncertainty, and movement forces. Use it when selecting plant caddies, shelves, hooks, and brackets to avoid overstress.