Gravel Calculator

Two small underestimates in a gravel order compound to exactly double

Gravel volume is area times depth, and depth is the number people guess. Going from 5 cm to the 8 cm a driveway actually needs is 60% more material — a change nobody would notice by eye. Add the 20% that loose gravel settles and you need 10 cm laid, and the two independent errors multiply out to twice the order.

How it works

• Converts an area and a finished depth into volume and tonnage.
• Accounts for compaction, so the figure is what you must lay rather than what you want left.
• Lets you compare depths directly, since depth is where the cost actually sits.

volume (m³) = area (m²) × depth (m)
tonnage = volume × density

typical gravel density ≈ 1.5 t/m³ — varies by stone type and grading

loose gravel settles roughly 20%:
  depth to lay = finished depth ÷ 0.8

Worked example

A 50 m² driveway at each depth people consider.

1. 3 cm → 1.50 m³ = 2.25 t
2. 5 cm → 2.50 m³ = 3.75 t
3. 8 cm → 4.00 m³ = 6.00 t
4. 10 cm → 5.00 m³ = 7.50 t
5. 15 cm → 7.50 m³ = 11.25 t

Ordering for 5 cm when the job needs 8 cm finished leaves you 60% short. Allowing for settlement means laying 10 cm to finish at 8, which is another 25% — so 5 cm assumed against 10 cm laid is exactly twice the material.

Reading the result

• Depth is dictated by use, not by preference. A footpath is fine at 3 to 5 cm; a driveway carrying cars needs 8 to 10 cm or the tyres cut ruts through to the base. Ordering the path depth for a driveway is the single most common way these jobs fail.
• Density of 1.5 t/m³ is a working figure and not a constant. Pea shingle, angular crushed stone and limestone chippings all differ, and moisture content shifts the weight further — order by volume where you can, since volume is what you are actually filling.
• The 20% settlement figure is a rule of thumb and depends on whether the material is compacted mechanically or simply driven over. Either way, budgeting the finished depth without adding for settlement guarantees a short order.
• A sub-base is a separate layer with its own depth and is not included in this figure. Calculating only the decorative top layer and forgetting the compacted base beneath it understates a driveway job substantially.

Common questions

How deep should gravel be?

Around 3 to 5 cm for a footpath and 8 to 10 cm for a driveway that takes vehicle weight. The difference is not cosmetic: too shallow and tyres push the stone aside and expose the base, which costs more to fix than the extra material would have cost.

Why did I run out despite ordering the right area?

Almost certainly depth or settlement. Both are easy to under-count and they multiply rather than add — assuming 5 cm when 10 cm needs laying is double the material, from two errors that each looked minor on their own.