Two traps, and every other calculator falls into both
1. You specify it finished and you buy it loose
A sub-base is called for by its finished, compacted depth. That is what the spec says and what you measure at the end. But it arrives loose, and Type 1 loses roughly a fifth of its volume going under a whacker plate.
So a drive ordered by the finished figure turns up about a fifth short. Nobody notices until the last strip, which gets laid thin because there is not enough left and nobody wants to order half a bag.
This tool works both numbers and shows both: finished volume so you can check it against the spec, and loose volume to put on the order. The compaction factor is visible rather than buried in a fudged density, because a hidden allowance is impossible to argue with when a supplier quotes something different.
Materials that do not compact get no allowance at all. That covers decorative gravel, pea shingle and building sand. Applying one would just make you over-order.
2. A tonne bag is not a tonne
It is called a tonne bag, a bulk bag, or a jumbo bag, and merchants generally fill it to a nominal 850kg. The actual content moves with the material and how wet it is.
Work your bag count off a flat 1,000kg and you are short on every bag you order. Over a ten-bag job that is a bag and a half missing. This one calculates at 850 and lets you change it, and shows you exactly how much the other assumption would have cost.
Coverage figures are useless without a qualifier
"A tonne covers 10 square metres at 50mm" is the figure everybody quotes. It is roughly right, and it is loose, which is the word that never gets said.
That single omission is behind most short orders on a drive. It is why this tool asks what the material is doing rather than just how big the hole is.
Sharp sand and building sand are not the same thing
Worth stating because it costs people money. Sharp sand is coarse, and it goes in concrete, screeds and laying courses. Building sand, soft sand, is fine, and it goes in mortar.
Concrete made with building sand is weak. Mortar made with sharp sand is a nightmare to work. They cost about the same and they are not swappable, and the merchant will sell you whichever you asked for.
The densities are typical, and they are yours to change
Bulk density moves with grading, with the source, and with how much rain has fallen on the pile. No published table is authoritative and anyone claiming otherwise is selling certainty they do not have.
So the tool names the figure it used, and lets you swap in your supplier's. That is more useful than a precise-looking number arrived at by guessing.
Related: sub-base depths by job if you are not sure how deep to go, concrete volumes for what goes on top, and muck away for what comes out first, which bulks up on the way out, in the opposite direction to this.
Common questions
How to work out how much gravel I need, and what to ask for
Area times depth gives a volume, volume times density gives a weight, and it is sold by weight, so that is the number to order with. The part no calculator tells you is which product the weight is of. GRAVEL loosely means anything from pea shingle to crushed stone and suppliers will not guess for you. What goes under a base or a drive is MOT TYPE 1, a crushed graded stone with fines in it that locks solid under a whacker. What people call PEA GRAVEL or SHINGLE is rounded, has no fines, and never compacts, so it is a top layer or a border and nothing structural. ROCK and HARDCORE both get used for Type 1 in conversation and neither is what it says on a ticket. Ask for the product name and the tonnage, and the ticket matches what turns up.
How much gravel do I need?
Work in tonnes rather than bags and the answer stops moving about. Measure the area, decide the depth, and the tool turns that into a weight and a bulk bag count. The bigger thing worth knowing is that "gravel" covers several different products and they are not interchangeable: what a builder means by hardcore is usually MOT Type 1, a crushed graded stone that locks together under a whacker, and what a garden centre means is usually a decorative rounded gravel that never compacts at all. Ordering the second for a base under a shed or a drive gives you something that stays loose forever. The names to give a supplier are in the tool.
How much hardcore do I need under a base?
The same sum, and then the part everybody misses: 150mm of finished sub-base is not 150mm of delivered stone. Type 1 compacts by roughly a fifth, so you lay it deeper than the depth you want and whack it down to size. The tool adds that for you and shows both figures rather than hiding it, because the difference on a modest drive is most of a tonne, and being short means an extra delivery charge for a quarter of a load.
How many tonnes of Type 1 do I need?
Work the area by the finished depth, add about a quarter for compaction, then multiply by roughly 2 tonnes per cubic metre. A 10m by 4m drive at 150mm finished is 6 cubic metres compacted, about 7.5 loose, which is around 15 tonnes. The tool does all three steps and shows each one.
How much gravel do I need for a driveway?
More than for a path, and in two layers rather than one. A drive that takes a car wants around 150mm of compacted MOT Type 1 as a sub-base with a decorative or binding layer of 40 to 50mm over it, against 75 to 100mm of sub-base for something only walked on. Work the two layers out separately here, because they are different products at different depths and ordering one tonnage for the lot gets you the wrong stone for half the job. And the loose gravel on top is the layer that migrates: without an edge restraint it ends up on the pavement, which no calculator mentions and every drive demonstrates.
Why is the volume to buy bigger than the volume I measured?
Because Type 1 compacts. You specify a sub-base by its finished depth and you buy it loose, and it loses roughly a fifth of its volume going under a whacker. Order by the finished figure and you end up about a fifth short, which is why the last strip always gets laid thin.
How much does a bulk bag of aggregate weigh?
Not a tonne, whatever it gets called. Merchants generally fill to a nominal 850kg, and the real content moves with the material and how wet it is. Working out bags on a flat 1,000kg leaves you short on every single one, so this tool uses 850 and lets you change it.
How many square metres does a tonne of Type 1 cover?
About 10 square metres at 50mm, or 3.3 at 150mm, but that is loose, before compaction. Coverage figures quoted without saying whether they are loose or compacted are the reason so many orders come up short.
What is the difference between sharp sand and building sand?
Sharp sand is coarse and goes in concrete, screeds and laying courses. Building sand, soft sand, is fine and goes in mortar. They are not interchangeable, and concrete made with building sand is weak.
Are the densities exact?
No, and nobody’s are. Bulk density moves with grading and moisture, so the figures here are typical supplier values. The tool names the number it used and lets you replace it with your supplier’s, which is more useful than pretending to a precision the material does not have.