How Many Bulk Bags of Gravel Per Square Metre?
You have measured the path or driveway, the builders’ merchant sells gravel in “bulk bags”, and now you need to know one thing: how many bags per square metre? The short answer is that a typical bulk bag of gravel covers about 10 m² at a depth of 50 mm, which works out at roughly 0.1 bulk bags per square metre. The longer answer depends on how deep you lay it, what kind of stone it is and how much the bag actually holds. This guide shows you how to work it out properly, with worked examples in metric and imperial.
What is in a “bulk bag” of gravel?
In the UK, gravel is usually sold three ways: small bags of around 20–25 kg, bulk bags (also called jumbo bags, dumpy bags or “tonne bags”), and loose loads tipped from a lorry. Despite the “tonne bag” nickname, most bulk bags of gravel hold less than a tonne. Suppliers commonly quote somewhere between 750 kg and 900 kg, with around 850 kg being a typical figure. That is about 1,874 lb.
The bag weight matters more than its size, because gravel coverage is really a question of volume. To turn weight into volume you need the stone’s bulk density (its weight per cubic metre, air gaps included). Washed decorative gravel and shingle usually sit around 1.6–1.8 tonnes per m³. Using a middle value of 1.7 t/m³, an 850 kg bag holds:
0.85 t ÷ 1.7 t/m³ = 0.5 m³ (about 17.7 cubic feet, or 0.65 cubic yards)
Everything else in this article follows from that half a cubic metre. If your supplier’s bag weight or stone density is different, swap in their figures and the method stays the same.
In the US, gravel is more often sold by the ton or by the cubic yard. At 1.7 t/m³, one cubic yard of gravel weighs roughly 1.3 metric tonnes, or about 1.43 US (short) tons, which is why landscape yards often say “about one and a half tons per yard”.
How many bulk bags per square metre? (coverage table)
Coverage depends almost entirely on depth. The table below assumes an 850 kg bag of gravel at 1.7 t/m³ (0.5 m³ per bag).
| Depth | Gravel per m² | Bulk bags per m² | Area per bulk bag | Area per bag (sq ft) |
|---|---|---|---|---|
| 25 mm (1 in) | 42.5 kg | 0.05 | 20 m² | 215 sq ft |
| 40 mm (1.6 in) | 68 kg | 0.08 | 12.5 m² | 135 sq ft |
| 50 mm (2 in) | 85 kg | 0.10 | 10 m² | 108 sq ft |
| 75 mm (3 in) | 127.5 kg | 0.15 | 6.7 m² | 72 sq ft |
| 100 mm (4 in) | 170 kg | 0.20 | 5 m² | 54 sq ft |
A useful rule of thumb falls out of this: every 10 mm of depth needs about 17 kg of gravel per square metre. So a 50 mm layer is 5 × 17 = 85 kg/m², and a 25 kg bag covers roughly 0.3 m² at that depth.
How deep should the gravel be?
For decorative borders and paths, 40–50 mm of 10 mm or 20 mm gravel is a common choice: enough to hide the membrane underneath without being so deep that it is hard to walk on. For driveways, the gravel is normally a top layer of around 50 mm laid over a compacted sub-base such as Type 1 MOT, rather than a thick layer on its own. Laying gravel much deeper than this tends to make it shift underfoot and under tyres. Our guide on how to lay a gravel driveway covers the full build-up, and the Type 1 MOT calculator handles the sub-base.
Step-by-step: how to work out your bulk bags
- Measure the area. For a rectangle, multiply length by width in metres. For awkward shapes, split the area into rectangles and triangles, work out each one, then add them up.
- Choose a depth and convert it to metres. 50 mm = 0.05 m; 40 mm = 0.04 m.
- Find the volume. Area (m²) × depth (m) = volume (m³).
- Convert to weight. Volume (m³) × density (t/m³) = tonnes. Use 1.7 if you have no better figure, or your supplier’s stated density.
- Add an allowance. Add around 10% for levelling, settling and uneven ground.
- Divide by the bag weight. Tonnes ÷ bag weight in tonnes (for example 0.85) = number of bulk bags. Always round up, then decide whether a few small bags would cover the remainder more cheaply than another full bulk bag.
Worked examples (metric and imperial)
Example 1: a garden path, 8 m × 1.2 m at 50 mm
Area = 8 × 1.2 = 9.6 m²
Volume = 9.6 × 0.05 = 0.48 m³
Weight = 0.48 × 1.7 = 0.816 t (816 kg)
Plus 10% = 0.816 × 1.1 = 0.898 t (about 898 kg)
Bulk bags = 0.898 ÷ 0.85 = 1.06 bags
This is the classic awkward result. Rounding up to two bulk bags would leave you with about 800 kg spare. A more sensible order is one bulk bag plus two 25 kg bags (850 + 50 = 900 kg). Alternatively, skip the extra and lay the path a few millimetres thinner: one bag alone gives a depth of about 52 mm before any allowance, which is plenty.
Example 2: a driveway topping, 6 m × 5 m at 50 mm
Area = 6 × 5 = 30 m²
Volume = 30 × 0.05 = 1.5 m³
Weight = 1.5 × 1.7 = 2.55 t
Plus 10% = 2.55 × 1.1 = 2.805 t
Bulk bags = 2.805 ÷ 0.85 = 3.3 bags → order 4
At this size it is worth comparing four bulk bags against a loose tipper load of 3 tonnes, which some suppliers price lower per tonne. If a bulk bag cost £75 (an illustrative figure, not a quote), four bags would be £300; at roughly $95 or €85 a bag the same order would be about $380 or €340. Get real prices locally, including delivery, before deciding.
Example 3: a US patio, 20 ft × 12 ft at 2 inches
Area = 20 × 12 = 240 sq ft
Depth = 2 ÷ 12 = 0.1667 ft
Volume = 240 × 0.1667 = 40 cu ft
Cubic yards = 40 ÷ 27 = 1.48 cu yd
Weight = 1.48 × 1.43 = about 2.12 short tons
Plus 10% = 1.48 × 1.1 = 1.63 cu yd, so order about 1.75 cu yd or 2.3 tons
In bulk-bag terms, 40 cu ft is 1.13 m³, or 2.27 bags at 0.5 m³ each, so three bulk bags would cover it with room to spare.
Why your supplier’s figure might differ
If a supplier says one bag covers 8 m² or 12 m² at 50 mm, they are not necessarily wrong. Three things move the number:
- Bag weight. A 750 kg bag holds about 12% less than an 850 kg bag.
- Stone density. Dense, angular crushed stone and compacted sub-base materials weigh more per cubic metre than rounded pea shingle, so a bag of the same weight fills less space. Lighter decorative materials such as slate chippings generally go further per tonne.
- Moisture. Wet gravel weighs more, so a bag filled to a set weight in wet weather contains slightly less stone.
If the merchant states a coverage figure for their exact product, use it. If they only give a weight, the method above gets you close enough to order with confidence. Our cubic measurement converter is handy if they quote volume in cubic feet or yards and you think in cubic metres.
Frequently asked questions
How many square metres does a 1 tonne bag of gravel cover?
A true 1,000 kg of gravel at 1.7 t/m³ is about 0.59 m³, which covers roughly 11.8 m² at 50 mm or 14.7 m² at 40 mm. Most “tonne bags” actually contain around 850 kg, which covers about 10 m² at 50 mm.
How many 25 kg bags of gravel make a bulk bag?
For an 850 kg bulk bag, 850 ÷ 25 = 34 small bags. Small bags almost always cost more per tonne, so they make sense for small jobs or topping up rather than whole driveways.
How many bulk bags of gravel do I need for 20 m²?
At 50 mm deep, 20 m² needs 20 × 0.05 = 1 m³, or 1.7 tonnes. With a 10% allowance that is 1.87 tonnes, so about 2.2 bags: order three 850 kg bulk bags, or two bulk bags plus seven 25 kg bags (1,875 kg) if you are confident in your measurements.
Work it out in seconds
The arithmetic is simple, but it is easy to slip a decimal point when converting millimetres to metres or kilograms to tonnes. Enter your length, width and depth into the free gravel calculator for bulk bags and tonnes and it will give you the volume and weight straight away, in metric or imperial. If you are planning a full driveway, you can also browse the rest of our construction calculators for sub-base, concrete and paving.