ProjectEstimate

Gravel calculator

Work out gravel, chippings and sub-base quantities by volume, weight, bags, bulk bags or tonnes — including the compaction allowance most calculators leave out.

Your project

Region
Units

Quick start

Area to cover

Add as many sections as you need. Subtract anything that should stay uncovered, such as a pond or a manhole.

Area

Measured in metres.

  • Add · section 1

    Area = length × width

Material and depth

The default choice for paths and decorative areas.

40–50 mm for decorative areas, 75 mm or more where vehicles will drive.

Adds the extra loose volume needed to reach the finished depth after whacking down.

How you will buy it

Sold as "1 tonne bags" but typically filled to 800–900 kg. Check with your merchant.

5% is enough for a simple area. Increase it for awkward shapes and spillage.

Extras

Estimate updated. 3 bulk bags of 20 mm gravel required.

Shopping list

2 lines
  • 3bulk bags20 mm gravel50 mm finished depth over 24.00 m² (258.33 ft²)
  • 27m²Weed control membraneIncludes a 10% allowance for overlaps and turn-ups at edges

Estimate results

What you measured

Area
24.00 m²
258.33 ft²
Finished depth
50 mm
Volume
1.20 m³ (1.57 yd³)

What to buy

5% waste included
Materials required, with the quantity to purchase, how it was rounded, and the estimated weight and cost of each line.
MaterialBuyWeight

20 mm gravel

50 mm finished depth over 24.00 m² (258.33 ft²)

Rounding: The calculation requires 2.26 bulk bags. Adding 5% waste gives 2.37. Rounded up to 3 bulk bags.

Main material

3bulk bags2,550 kg

Weed control membrane(optional)

Includes a 10% allowance for overlaps and turn-ups at edges

Rounding: The calculation requires 26.4 m². No waste allowance was added. Rounded up to 27 m².

Base and membrane

27m²—
Assumptions behind each line

20 mm gravel

  • Loose bulk density taken as 1,600 kg/m³ (range 1,500–1,700).
  • No compaction allowance applied — this is a loose decorative layer.
  • Sold as "1 tonne bags" but typically filled to 800–900 kg. Check with your merchant.

Weed control membrane

  • Membrane is sized on the gross added area, before subtractions, because you still cut around obstacles.
  • Overlap joints by at least 100 mm (4 in) and pin them down before spreading stone.

Plan and build-up

Plan and build-up — planPlan view of 1 area section totalling 24.00 m², with a cross-section showing 50 mm of 20 mm gravel over a weed control membrane on a prepared subgrade.24.00 m²Total area: 24.00 m²
Schematic of the sections you entered. Positions are illustrative; only the dimensions affect the calculation.
Build-upCross-section, top to bottom: 20 mm gravel — 50 mm, then Weed control membrane, then Prepared, firm subgrade.20 mm gravel — 50 mmWeed control membranePrepared, firm subgrade
Layer thicknesses are drawn to a compressed scale so thin layers stay visible.

How this was calculated

  1. Area

    plus Main area (6 × 4 m) = 24.00 m²

    = 24.00 m² (258.33 ft²)

  2. Finished volume

    24.00 m² (258.33 ft²) × 50 mm depth

    = 1.20 m³ (1.57 yd³)

  3. Approximate weight

    1.20 m³ (1.57 yd³) × 1,600 kg/m³

    = 1.92 tonnes (4,233 lb)

    • Density range 1,500–1,700 kg/m³. Wet material sits at the top of that range.

  4. Purchase quantity

    The calculation requires 2.26 bulk bags. Adding 5% waste gives 2.37. Rounded up to 3 bulk bags.

    = 3 bulk bags

Weight and delivery

Approximate weight
2.55 tonnes

Likely delivery
tipper or grab lorry

  • 2.55 tonnes needs a tipper or grab lorry. Confirm access width, overhead clearance and where the driver can tip.
  • Check that the delivery route can take the weight — block paving, drainage covers and soft verges are easily damaged.

Note: Weight varies with moisture and grading

Aggregate weights are based on typical loose bulk densities. A wet load can be 10% heavier than a dry one, so treat the weight as a planning figure rather than a delivery note.

Safety and compliance

Check for buried services before digging

Before digging, check for buried services. Free utility plans are available from the relevant network operators, and a cable avoidance tool (CAT) scan is strongly recommended.

Data behind this estimate

Material assumptions used, with their value, range, source and review date.
AssumptionValueSource

20 mm gravel / ¾ in gravel

Loose bulk density as delivered, not compacted.

1,600 kg/m³range 1,500–1,700Mineral Products AssociationReviewed 2026-07-01

Default waste allowance

Covers cutting, breakage, setting-out error and the odd unusable unit.

10 %range 5–15Project EstimateReviewed 2026-07-01

Cost

Enter the prices your supplier quoted. This site does not hold or estimate supplier prices, so totals appear only once you fill these in.

UK standard-rate VAT is 20%. Builders' merchant prices are often quoted excluding VAT.

No prices entered yet. Everything else in this estimate — quantities, weight, delivery and the shopping list — works without them.

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How this calculator works

Every figure below is derived rather than looked up, so you can check it against your own numbers. Nothing is hidden behind a black box.

Volume

Every section you add is converted to square metres, subtracted sections are removed, and the total is multiplied by the finished depth to give a volume.

volume = total area × finished depth

Compaction

Granular materials lose volume when they are compacted. If you tick the compaction option, the loose volume ordered is increased by the material's compaction factor — 1.3 for MOT Type 1 and dense graded base, 1.1 for angular chippings and sharp sand.

Skipping this is the most common reason a sub-base order comes up short.

loose volume = finished volume × compaction factor

Weight

Weight is volume multiplied by the material's loose bulk density. The engine stores a range for each material and shows it alongside the result, because moisture content alone can move the delivered weight by 10%.

weight = loose volume × bulk density

Purchase units

Bags and bulk bags are defined by weight in the UK and by volume in the US, which is why the same project produces a different pack count in each region. Loose deliveries are rounded to whole tonnes or half cubic yards to match how suppliers actually sell.

Common questions

How deep should gravel be?
40–50 mm (about 2 in) for decorative areas and paths, and 75 mm (3 in) or more for a driveway surface over a properly compacted sub-base. Going deeper than about 75 mm on a footpath makes walking harder, not better.
How many bulk bags are in a tonne?
In the UK a bulk bag is usually filled to around 850 kg despite being sold as a 'one tonne bag', so a tonne is roughly 1.2 bulk bags. Always check the filled weight with your merchant before ordering by the bag.
Why does the calculator ask about compaction?
Because a 100 mm compacted sub-base needs about 130 mm of loose material. If you order by the finished depth you will be roughly 30% short.