How Much Gravel Do I Need? Coverage Calculator & Guide
The exact formula, real coverage rates, and density data you need to order the right amount of gravel — without paying for a wasted second delivery or running short mid-project.
Ordering too little gravel means a second delivery charge (typically £30–£60 on top of the stone cost). Ordering too much means bags of surplus stone cluttering your garden for months. Neither is ideal.
The maths isn't complicated, but you do need accurate numbers — and gravel suppliers rarely spell out the formula clearly. This guide gives you the exact calculation method, real-world coverage rates per tonne, density figures for common UK gravel types, and recommended depths for every application.
Want the quick answer? Use our free interactive calculator at howmuchgravel.co.uk — plug in your measurements and it does the maths instantly. Read on if you want to understand the numbers behind it.
The Formula: How to Calculate Gravel Volume and Weight
Every gravel calculation follows three steps:
Step 1: Calculate Volume
Length (m) × Width (m) × Depth (m) = Volume (m³)
All measurements must be in metres. Convert depth from millimetres: divide by 1,000. So 50mm depth = 0.05m.
Example: A 4m × 3m garden border at 50mm depth = 4 × 3 × 0.05 = 0.6 m³
Step 2: Convert Volume to Weight
Volume (m³) × Density (kg/m³) = Weight (kg)
Gravel density varies by type — see the density table below. Most decorative gravels fall between 1,400–1,700 kg/m³.
Example: 0.6 m³ × 1,600 kg/m³ (pea gravel) = 960 kg
Step 3: Add 10–15% Extra
Weight × 1.10 (or 1.15) = Total to Order
Always add a buffer for compaction, spreading losses, uneven ground, and waste. 10% is adequate for simple rectangular areas on flat ground. Use 15% for irregular shapes, sloped ground, or areas with lots of edging where stone gets displaced during raking.
Example: 960 kg × 1.10 = 1,056 kg → order 2 bulk bags (each approx 850kg)
Quick Reference: The Full Calculation
L (m) × W (m) × D (m) × Density (kg/m³) × 1.10 = Total kg to order
Divide by 850 to get the number of bulk bags, or by 25 to get the number of 25kg handy bags. Always round up — you can't order half a bulk bag.
Coverage Rates Per Tonne at Different Depths
This is the table most people are looking for. These rates assume a typical decorative gravel density of approximately 1,500–1,600 kg/m³:
| Depth | Coverage per Tonne | Coverage per Bulk Bag (850kg) | Typical Use |
|---|---|---|---|
| 25mm | 25–28m² | 21–24m² | Light decorative topping |
| 30mm | 20m² | 17m² | Garden paths (10mm gravel) |
| 40mm | 15m² | 12–13m² | Standard borders and beds |
| 50mm | 12m² | 10–12m² | Decorative beds, pebble areas |
| 75mm | 8m² | 6–7m² | Large cobble areas |
| 100mm | 6m² | 5m² | Driveway sub-base, large cobbles |
The most common scenario — a bulk bag (approx 850kg) at 50mm depth covers roughly 10–12m². This is worth memorising for quick mental estimates at the planning stage.
Quick Area Estimates for Common Garden Features
| Feature | Typical Area | Recommended Depth | Approximate Stone Needed |
|---|---|---|---|
| Front garden path (10m × 0.9m) | 9m² | 40mm | ~650kg (1 bulk bag) |
| Small decorative border (3m × 1m) | 3m² | 50mm | ~250kg (10× 25kg bags) |
| Patio surround (12m total, 0.5m wide) | 6m² | 50mm | ~500kg (1 bulk bag) |
| Full front garden (5m × 4m) | 20m² | 50mm | ~1,700kg (2 bulk bags) |
| Single-car driveway (5m × 3m) | 15m² | 50mm surface | ~1,250kg (2 bulk bags) |
Gravel Density by Type: The Numbers You Need
Different stone types have different densities, which directly affects how much a given volume weighs — and therefore how much you need to order by weight.
| Stone Type | Density (kg/m³) | Notes |
|---|---|---|
| 10mm pea gravel | 1,600 | Most popular garden gravel size |
| 20mm gravel (rounded) | 1,500–1,700 | Varies by stone origin and shape |
| Decorative aggregate (mixed) | 1,400 | Lighter due to varied shapes and air gaps |
| Slate chippings (e.g. Black Slate Gravel) | 1,500 | Flat pieces stack with more air gaps |
| MOT Type 1 (sub-base) | 2,100 | Dense — compacts heavily, used under surfaces |
| Granite chippings (e.g. Silver Granite Gravel) | 1,600–1,800 | Dense, heavy stone |
| Limestone chippings | 1,500–1,600 | Medium density |
| Quartz pebbles | 1,550–1,650 | Dense, smooth stones with fewer air gaps |
If you don't know the exact density, use 1,500 kg/m³ as a safe default for decorative gravel and pebbles. You'll be within 10% of the real figure for most products, and the 10–15% buffer you add covers the difference.
Why Density Matters
A cubic metre of MOT Type 1 at 2,100 kg/m³ weighs 40% more than a cubic metre of decorative aggregate at 1,400 kg/m³ — yet they fill the same space. If you use the wrong density in your calculation, you could be off by hundreds of kilograms on a medium-sized order. Always check the product listing for the specific density if available.
Recommended Depths by Application
Using the wrong depth is one of the most common mistakes. Too shallow and the surface looks patchy and the membrane shows through. Too deep and you waste money, create an unstable walking surface, and make the area hard to maintain.
Garden Paths and Borders
25–40mm depth using 10mm pea gravel or 14mm aggregate. Small stone sits flatter and is more comfortable underfoot. For paths, use angular (crushed) stone rather than rounded — it interlocks and compacts into a firmer surface.
Decorative Beds and Feature Areas
30–50mm depth using 14–40mm gravel or pebbles. This is the most common application. 50mm gives full coverage with no membrane showing through, even after some natural settlement. See our size guide for help choosing between grades.
Driveways
40–50mm surface layer of 20mm angular gravel — such as Black Basalt Gravel or Silver Granite Gravel — over a 100mm MOT Type 1 compacted sub-base. Total depth: 150mm minimum. Never use rounded pebbles for driveways — they roll under tyres. Angular stone interlocks under compression and stays put.
Under Play Equipment and Garden Furniture
50–75mm depth using 20–40mm rounded gravel. Provides cushioning and drainage. For playground areas, some councils require a minimum 200mm depth of specific impact-absorbing material — check local regulations if this is a public or shared space.
For detailed instructions on the full laying process, see our step-by-step laying guide.
Pro tips
Measure twice, order once
Walk the area with a tape measure and write down every dimension. For irregular shapes, break the area into rectangles and triangles, calculate each separately, and add them up. A 5-minute measuring session prevents a £50+ second delivery charge.
Bulk bags are almost always better value
A bulk bag (approx 850kg) typically costs £60–£180 depending on stone type. The equivalent weight in 25kg handy bags would cost 30–50% more. Unless you need less than 250kg total, always order bulk bags. One bulk bag covers roughly 10–12m² at 50mm depth — enough for most single garden features.
Use the free calculator for instant answers
If you don't want to do the maths manually, head to <a href='https://howmuchgravel.co.uk'>howmuchgravel.co.uk</a>. Enter your length, width, and depth, select your stone type, and it calculates volume, weight, and number of bulk bags instantly — including the 10% buffer.
Frequently asked questions
How many bulk bags of gravel do I need for a driveway?
For a single-car driveway (roughly 5m × 3m = 15m²), you need approximately 2 bulk bags of 20mm angular gravel for the 50mm surface layer, plus 3–4 bulk bags of MOT Type 1 for the 100mm sub-base. A double driveway (5m × 5m = 25m²) needs roughly 3 bags of surface gravel and 5–6 bags of sub-base. Always add 10–15% for wastage and compaction.
What's the difference between a bulk bag and a tonne?
A standard bulk bag (also called a jumbo bag or dumpy bag) contains approximately 850kg of gravel — not a full tonne (1,000kg). Some suppliers sell by the tonne and deliver in larger bags; others sell by the bulk bag. Always check the listed weight per bag so your calculation is accurate. When using coverage-per-tonne figures, remember to adjust if you're ordering 850kg bags.
Can I mix different gravel sizes in one area?
You can, but smaller stones will migrate downwards through the gaps between larger ones over time — this is called 'grading segregation'. If you want a mixed look, choose sizes that are close together (e.g. 10mm and 14mm, or 20mm and 30mm). Avoid mixing 10mm with 40mm+ as the small stones will disappear into the gaps within a few months.
How do I calculate gravel for a circular area?
Use the formula: π × radius² × depth. So for a 2m diameter circle (1m radius) at 50mm depth: 3.14 × 1² × 0.05 = 0.157 m³. Multiply by density (e.g. 1,600 kg/m³) = 251 kg. Add 15% for circular shapes (more edge waste): 289 kg. That's roughly 12× 25kg bags or about a third of a bulk bag.
What area does an 800kg bulk bag of gravel cover?
An 800–875 kg bulk bag covers approximately 10–12 m² at 50 mm depth, or 12–14 m² at 40 mm depth. The exact coverage depends on the stone type — denser stones like granite gravel cover slightly less area than lighter slate gravel at the same depth. See our bulk bag delivery guide for full details.
Is 10mm or 20mm gravel better for garden borders?
For most garden borders, 20mm is the more popular and practical choice — individual stones are visible enough to appreciate the colour and character, while still sitting neatly against edging. 10mm works better for narrow, defined borders and areas where you want a tighter, more compact surface with superior weed suppression. Read our pebble size guide for a detailed breakdown by application.
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Get your exact gravel quantity with our free calculator at howmuchgravel.co.uk, then browse our gravel and pebbles collections at Stones4Gardens with next-day delivery across the UK.