Home & Construction⏱ 4 min read
How to Calculate Gravel for a Driveway or Garden Path
Gravel calculations trip people up because depth is usually stated in millimetres while area is in square metres. Here's the formula for any shape, the right depth for each use, and how much to order.
Ordering too little gravel is a common mistake because suppliers usually have a minimum order and a second delivery incurs another charge. Ordering too much is expensive and leaves you with tonnes of material to shift. Here's how to calculate precisely.
The Core Formula
Volume (m3) = Area (m2) x Depth (m)
Convert depth from mm to metres: divide by 1,000
50mm depth = 0.05m, 75mm = 0.075m, 100mm = 0.10m
Example: 6m x 4m driveway at 50mm depth
Volume = (6 x 4) x 0.05 = 24 x 0.05 = 1.2 m3
Add 15% for compaction and settling:
Order: 1.2 x 1.15 = 1.38 m3 → round up to 1.5 m3
Converting Volume to Weight
Gravel is typically sold by the tonne, not by volume.
Conversion depends on gravel type:
Pea gravel / decorative chippings: ~1.5 tonnes per m3
20mm angular gravel: ~1.7 tonnes per m3
40mm crushed stone: ~1.6 tonnes per m3
MOT Type 1 (sub-base): ~2.1 tonnes per m3
Example: 1.5 m3 of 20mm angular gravel:
Weight = 1.5 x 1.7 = 2.55 tonnes → order 3 tonnes
Recommended Depths by Use
ApplicationGravel DepthNotes
Decorative garden path (foot traffic)40-50mmCompact base optional
Driveway (car traffic)75-100mmNeed compacted Type 1 sub-base below
Driveway (van/lorry)100-150mm150mm compacted sub-base recommended
Resin-bound surface18-20mmApplied over existing concrete/tarmac
French drain / soakawayFill entire trench20-40mm angular best for drainage
Calculating a Circular or Irregular Area
Circular area:
Area = pi x radius^2 = 3.14159 x r^2
Example: circular feature, 3m diameter
r = 1.5m, Area = 3.14159 x 2.25 = 7.07 m2
At 50mm depth: 7.07 x 0.05 = 0.35 m3 = ~0.6 tonnes
Irregular shape:
Divide into rectangles or triangles, calculate each, add together.
Or: length x average width (reasonable estimate for irregular paths)
Budget Calculation
20mm landscaping gravel (2025 approximate prices):
Bulk bag (approximately 850kg / ~0.5 m3): £65-£90
Loose load, 6 tonnes delivered: £300-£450
For a 6m x 4m driveway (3 tonnes needed):
Bulk bags: 4 bags x £80 = £320 + delivery
Loose load: ~£200 + sub-base separately
For larger areas (3+ tonnes), loose load almost always
cheaper per tonne. For small areas, bulk bags are more practical.