How the calculation works
The formulas
Enter an area or rectangular dimensions and a uniform depth. The allowance increases loose volume. Density converts that volume to weight; bags can be specified by volume or weight.
Net volume = area × depth
Order weight = net volume × (1 + allowance/100) × density
Bags = ceil(order volume ÷ bag volume) OR ceil(order weight ÷ bag mass)
Assumptions to check
- The 1,500 kg/m³ density default is approximate, not a universal gravel/soil/mulch specification. Replace it with supplier data.
- Moisture, compaction, and particle grading can change density; no structural bearing performance is inferred.
A worked example
20 m² at 50 mm is 1 m³. With 5% allowance and 1,500 kg/m³ density, order volume is 1.05 m³ and estimated weight is 1,575 kg; 25 L bags require 42 bags.