Weight, Density & Bagged Quantities: kg, Tonnes and How Many Bags
A homeowner's plain-English decoder for the weights on extension deliveries and skips: kilograms and tonnes, density in kg/m3, and how to turn a cubic metre of sand, ballast or cement into a count of 25kg bags or bulk bags (~850kg).
On a build, weight turns up in three places: the delivery ticket the lorry driver hands you, the skip you pay to take waste away, and the order you place when you have to decide how many bags to buy. The same idea ties all three together. Density turns a volume into a weight, and a weight into a number of bags. Once you can move between cubic metres, kilograms and bag counts, you can order the right quantity and sanity-check what arrives against what you paid for.
Kilograms and tonnes
There are only two weight units to learn. The kilogram (kg) you already know, and the tonne, which is simply 1000 kg. Small bagged materials come in 25kg bags: a bag of cement, a bag of sand, a bag of postcrete. Anything bigger is quoted in tonnes. A bulk bag is rated at roughly a tonne, a tipper load of loose aggregate is sold by the tonne, and the muck-away firm weighs your skip and bills you in tonnes.
The skip is where the tonnes catch people out. A builder's skip is limited by weight, not by how full it looks.
Warning
Heavy waste such as soil, broken concrete and rubble hits a skip's weight limit long before the skip looks full. A standard builder's skip rated for a couple of tonnes can be barely half full of hardcore and already over its limit. Fill it to the brim and the firm will either refuse to lift it or charge a hefty overweight surcharge. Keep dense rubble to the bottom and spread it across more than one skip if you have a lot.
Density: why a cubic metre of one thing weighs far more than another
Density is the weight packed into each cubic metre of a material, written kg/m³. It is the number that explains why a cubic metre of one material is a one-person job and a cubic metre of another needs a machine.
The anchor to hold in your head is water: a cubic metre of water weighs about 1000 kg, one tonne. Almost every building material is heavier than that per cubic metre. Loose dry sharp sand sits at ~1600 kg/m³, so a cubic metre of it weighs about 1.6 tonnes. Ready-mix concrete is heavier still at 2400 kg/m³, which makes one cubic metre roughly 2.4 tonnes, the weight of a couple of small cars.
The same idea decides how a block feels in your hands. A dense concrete block runs at about 2000 kg/m³, which is why a single 100mm dense block is a deliberate two-handed lift. A lightweight aircrete (aerated) block, the grey foamed block used for the inner leaf of most cavity walls, comes in at around 600 kg/m³, roughly a third of the weight, which is why a bricklayer can flick one up one-handed. Same shape, same wall, completely different day's work.
One catch worth knowing: damp material weighs more per cubic metre than dry. Sand delivered wet can run a hundred kilograms or more heavier per cubic metre than the dry figure. Because a bulk bag is filled to a target weight, a wet-delivered bag holds less actual material than a dry-weight order implies. You are paying for water you cannot use.
Turning a volume into bags
Most ordering questions come down to one chain: you know roughly how many cubic metres you need from a take-off, and you need that as a number of bags. There are two bag formats. The 25kg bag, and the bulk bag, also called a jumbo bag or a dumpy bag, filled to roughly 850kg.
volume × density = weight
Weight in kg equals cubic metres multiplied by density in kg/m³. Once you have the weight, the number of 25kg bags is the weight divided by 25, and the number of bulk bags is the weight divided by 850 (roughly).
Work it through with a real order. Say you need about half a cubic metre of all-in ballast to mix some general-purpose concrete. At roughly 1600 kg/m³ that is about 800 kg of material, which is near enough one bulk bag, or about 32 of the 25kg bags. One 850kg sand bulk bag holds 0.5–0.55 m³ of material, which is the conversion that lets you jump straight from a cubic-metre take-off to a whole-bag order without doing the arithmetic each time.
That conversion also settles which format to buy. The 25kg bag is the worst value per kilogram by a wide margin, and the constant lifting and splitting open of small bags eats time on site. They earn their place only for small jobs and patch-up work. The moment you need more than a handful, a bulk bag or a loose tipper load wins on both price and effort. For more on what a bulk bag actually weighs and the imperial-versus-metric quirks behind these numbers, see UK building measurements: imperial vs metric.
A quick weight-and-bags table
| Material | Density (kg/m³) | Weight of 1 m³ | Roughly, in 25kg bags |
|---|---|---|---|
| Water (reference) | 1000 | 1 tonne | 40 bags |
| Loose dry sharp sand | ~1600 | ~1.6 tonnes | ~64 bags |
| MOT Type 1 / general aggregate | ~2000 | ~2 tonnes | ~80 bags |
| Ready-mix concrete | ~2400 | ~2.4 tonnes | ~96 bags |
| Aircrete (aerated) block | ~600 | ~0.6 tonnes | n/a |
The bag counts assume a 25kg bag, so they apply to loose materials you buy bagged. Blocks are bought as units, not by the bag, so the count reads "n/a": there a cubic metre is a stack of around 70 standard blocks, not a pile of bags.
Why the weights matter
Get the weights right and three things go your way. Ordering is the obvious one: a correct bag count means you are not paying twice for delivery because you ran short mid-pour, and not left with half a bulk bag of sand setting solid in the rain.
Access is the one people forget. An 850kg bulk bag cannot be manhandled once it is on the ground. It has to be lifted off the lorry by a crane arm or HIAB and placed where you want it first time, so the delivery point has to be reachable and the ground firm enough to take it. Skips are the same story in reverse: their weight limit governs how much rubble you can load, not the space inside.
Finally, the weights let you sanity-check what turned up. A bag sold as a "one tonne bag" is usually filled to about 850kg, not a full 1000kg, so read the actual figure on the delivery ticket. Multiply the cubic metres you expected by the density, compare it against the tonnage on the ticket, and you will know in a minute whether the load that arrived matches the load you paid for.
For the wider set of symbols you meet across a build, from kN on structural calculations to W/mK on insulation, see the master units and symbols decoder.