How to measure for aggregates
Every loose aggregate is worked out the same way: the area times the depth gives the volume, and the volume times the density gives the weight. Measure the length (A) and width (B) of a rectangle, or split the area into rectangles, circles, triangles and L-shapes and add each one with Add another area. If you already know the area, type it as a rectangle 1 m wide: the length is then the area in square metres.
Then choose the material and type the depth. For a concrete base, use the thickness of the slab; for a sub-base, the compacted depth; for a sand laying course or a gravel border, the depth as laid. The density changes with the material you choose, so the weight follows the stone or sand you actually buy. If your supplier publishes a density, open Density under the depth and type it in.
If you measure in feet and inches, switch the units at the top. The answer still comes out in tonnes and bags, which is how UK merchants sell aggregates.
Typical depths for common aggregate jobs
These are the depths UK suppliers, paving makers and DIY chains give for the jobs this page is most often used for. The buttons set the material and the depth in the calculator.
| Use | Depth and material | Apply |
|---|---|---|
| Ballast for a concrete baseAbout 1 m³ of ballast per m³ of concrete, so the slab depth is the ballast depth. | 100 mm of all-in ballast | |
| Garden pathFoot traffic only. | 75–100 mm of MOT type 1 sub-base | |
| Block paving laying courseLaid loose; compacts to about 30 mm. | 50 mm of sharp sand | |
| Border or bedOver a weed membrane on firm soil. | 40–50 mm of 20 mm gravel |
Each of these jobs has its own page with more detail: the type 1 calculator for sub-bases, the sand calculator for laying courses and jointing sand, and the gravel calculator for decorative gravel with a weed membrane. Depths for bases and drives are a guide only: check loads, falls and drainage with your supplier or a professional.
How many bulk bags do I need?
Divide the weight by the size of the bag and round up. A standard UK bulk bag holds about 850 kg, some chains sell 800 kg, and the weight of any bag can vary by a few per cent with moisture, so the calculator always rounds up to whole bags. Choose your merchant’s size under Bulk bag size.
Bulk bags are often called tonne bags, but the ones UK merchants sell hold less than a tonne. Order by the weight on the listing, not the nickname.
As a bulk bag calculator it also tells you when small bags make more sense. Its best fit is small bags when you need less than half a bulk bag; otherwise whole bulk bags, topped up with small bags only when the top-up is no more than a quarter of a bulk bag, or else one more bulk bag. Change this under Buy it in.
Ballast calculator for a concrete base
All-in ballast is sharp sand and gravel already blended together, sold for making concrete. B&Q’s concrete guide uses about one cubic metre of ballast for each cubic metre of concrete, because the cement and water fill the gaps between the stones. So the Ballast for a concrete base button simply uses the slab thickness as the ballast depth, starting at 100 mm, the figure B&Q gives for a shed base.
B&Q mixes base concrete at one part cement to 4 parts ballast and adds 10% extra, which is the allowance this calculator starts at. This ballast calculator works out the ballast only. For the cement as well, the shed base calculator works out the whole mix for a slab. Concrete bases carry loads, so treat the depth as a guide and check it with your supplier or a professional.
Tonnage calculator: how much aggregates weigh
Used as a tonnage calculator, the figure that matters most is the density. Typical figures from UK suppliers are 1.5–1.8 t/m³ for all-in ballast, 1.6–1.8 t/m³ for sharp sand, 1.6–1.7 t/m³ for building sand and 1.6–1.8 t/m³ for 20 mm gravel. Sub-base materials are entered compacted, because the depth you give for a sub-base is the compacted depth: MOT type 1 at 2–2.2 t/m³ and scalpings at 1.9–2 t/m³.
The scalpings figures come from suppliers of recycled road planings. Crushed-rock scalpings may weigh differently, so ask your supplier for their figure and type it into Density.
Worked example: ballast for a concrete base
Here is the full calculation for a small concrete base using the Ballast for a concrete base preset and the suggested extra allowance. Every line comes from the same code that runs the calculator above. The cement is not included.
- Area 3 m × 2.4 m = 7.20 m²
- Volume of all-in ballast 7.20 m² × 0.1 m (100 mm) = 0.720 m³
- Weight of all-in ballast 0.720 m³ × 1.7 t/m³ = 1.224 t (1,224 kg)
- Plus extra 1,224 kg + 10% = 1,346 kg (1.35 t)
- In bulk bags 1,346 kg ÷ 850 kg = 1.58, rounded up = 2 bulk bags
On the list:
- 2bulk bags of all-in ballast (approx. 850 kg each)1.35 t needed
Notice that the calculator chooses bulk bags here. Small bags of ballast are easier to carry and store, but a base this size would take dozens of them: enter the same measurements above and the ways-to-buy table under the result shows the count. Remember the base also needs cement, and usually a compacted sub-base underneath, which the type 1 calculator works out.
UK aggregate bag sizes
Ballast, sand, scalpings, type 1 and gravel are all sold by weight. Bulk bags are delivered by lorry and set down on the drive or at the kerb; small bags come from DIY stores and builders’ merchants and fit in a car boot. Not every material comes in every size: type 1 and scalpings are sold in bulk bags only, and ballast’s small bag is the 22.5 kg major bag sold by DIY stores.
| Product | Size | Good for |
|---|---|---|
| Bulk bag, 850 kg | 850 kg | The most common merchant bulk bag. About 10 m² of gravel at 50 mm. |
| Bulk bag, 800 kg | 800 kg | Sold by several DIY chains and merchants; weight can vary by about 5% with moisture. |
| Bulk bag, 750 kg | 750 kg | Smaller decorative-aggregate bulk bags. |
| Small bag, 25 kg | 25 kg | Merchant “handy bag” or trade pack. |
| Small bag, 22.5 kg | 22.5 kg | A common DIY-store “major bag” size. |
| Small bag, 20 kg | 20 kg | Easy to carry; good for small borders and topping up. |
If a merchant lists a bulk bag by volume rather than weight, multiply the volume by the density to compare it, or ask how many kilograms it holds.
Common mistakes when ordering aggregates
- Using the wrong density. Compacted type 1 is heavier than loose ballast or gravel. Choose the material you are actually buying so the weight follows it.
- Taking tonne bags literally. Most bulk bags hold less than a tonne. Order by the weight on the listing.
- Forgetting the cement. The ballast preset covers the sand and stone in concrete, not the cement.
- Mixing up sharp and building sand. Sharp sand is coarse and gritty, for laying courses, patio beds and concrete. Building sand is soft and fine, for bricklaying mortar.
- Ordering for the depth of the dig. Enter only the depth of the layer you are buying. Each layer of a build-up is a separate order.
- Rounding down. A bag short is a pour or a path left half finished. Order whole bags and keep any spare for later jobs.
What else you'll need
- Cement for concrete, with the ballast: the shed base calculator works out the full mix for a slab.
- Sand and cement for a patio bed or mortar: use the sand and cement calculator.
- A compacted sub-base under paths, patios and drives: the type 1 calculator covers it in more detail.
- Decorative gravel and weed membrane: the gravel calculator adds the membrane rolls.
- A wheelbarrow, shovels and a tarpaulin to protect the drive where the bags are set down.
Last checked 28 October 2026