Overview
The maths is simple but unforgiving: bags are sold whole, so counts always round up.
How the calculation works
Bag count = total kilograms ÷ bag net weight, rounded up: 2,240kg of gravel is 2.64 × 850kg bags, so you buy 3.
'Tonne bags' are rarely one tonne: 850kg is the common UK bulk bag, with 800kg and 500kg half-bags also sold.
Because rounding up wastes up to a bag's worth of material, compare the price per tonne of loose loads once counts exceed about six bags.
Worked example
A driveway dressing needs 2.1 tonnes of 20mm gravel after allowances.
At 850kg per bulk bag: 2,100 ÷ 850 = 2.47 → 3 bags. At 25kg small bags: 84 bags.
If a third bulk bag is far more than needed, ordering 2 bulk bags plus a few 25kg bags tops the quantity up exactly.
20mm gravel / shingle at a glance
- Planning density1.60 t/m³
- Typical range1.40–1.70 t/m³
- CategoryGravel
- CompactionUsually not applied
- SourcePlanning value based on typical UK 20mm shingle bulk densities
- Reviewed2026-08-16
Planning values for typical UK supplies; your supplier product data takes precedence.
20mm gravel / shingle coverage guide
| Depth | Area per tonne | Area per 850kg bag | Weight per m² |
|---|---|---|---|
| 25mm | 25.0 m² | 21.3 m² | 40 kg |
| 40mm | 15.6 m² | 13.3 m² | 64 kg |
| 50mm | 12.5 m² | 10.6 m² | 80 kg |
| 75mm | 8.3 m² | 7.1 m² | 120 kg |
| 100mm | 6.3 m² | 5.3 m² | 160 kg |
| 150mm | 4.2 m² | 3.5 m² | 240 kg |
Computed from a 1.60 t/m³ planning density on level ground.
Assumptions and accuracy
Bag weights are nominal — check the printed net weight; moisture adds weight without adding volume.
Round-up is deliberate: the calculator never suggests part bags.
Delivery charges usually price per pallet or per drop, which the cost fields can include.
How to use this calculator
- Enter area and depth to derive a tonnage, or pick the material and enter a known weight using the converter.
- Tick every bag format you would consider — counts for all selected formats show together.
- Set a custom bag weight if your merchant uses a different fill.
- Add per-bag pricing to see the cheapest combination.
When bulk bags is commonly used
Bulk bags suit access-restricted sites without room for a loose tipper load. Dumpy bags, jumbo bags and tonne bags are all names for the same FIBC bulk bag format.
Frequently asked questions
How many bulk bags make 1 tonne?
One 850kg bulk bag is 0.85 tonnes, so you need two bags to exceed a tonne (1.7 tonnes total) — or one 850kg bag plus six 25kg small bags to make exactly 1,000kg.
Is a tonne bag a full tonne?
Usually not. Most UK 'tonne' or 'dumpy' bags are filled to 850kg, some to 800kg. Check the listing; the calculator has all three formats.
How many 25kg bags in a bulk bag?
An 850kg bulk bag equals 34 × 25kg bags — useful for topping up partial quantities.
Bulk bags or loose loads?
Roughly six or more bulk bags usually cost more per tonne than an equivalent loose delivery, but bags suit small sites, slower programmes and restricted access.
What size is a standard bulk bag?
The familiar FIBC "dumpy" bag is nominally around 90 × 90 × 90cm, but bags are filled by weight, not volume. An 850kg bag of gravel holds roughly half a cubic metre — always order by the printed net weight.
References and source notes
- Common UK bulk bag fill weights observed across supplier listings (850kg, 800kg, 1,000kg nominal).