You buy a loose volume and you want a settled one
Order three cubic metres of topsoil and three cubic metres arrive — measured loose, in the back of a truck, after being dug, handled, dropped and tipped. Spread it, rake it, water it in, and a fortnight later the bed is lower than the level you left it at. Nothing was stolen. The soil simply stopped being loose.
This is the gap that catches people out. The volume you calculate from your bed dimensions is the **finished, settled** volume — the shape you actually want to end up with. The volume you buy is the **loose** volume as delivered. Those are two different numbers for the same soil, and the difference is real material you have to pay for.
Order to the finished depth and you will be short. Not by a catastrophic amount, but reliably, every time, on every job.
Where the air goes
Loose soil is full of voids. Excavating breaks the structure apart, screening fluffs it up further, and every transfer between bucket, truck and barrow introduces more air. The material that lands on your driveway is genuinely less dense than the same soil sitting undisturbed in the ground.
Once it is spread, several things close those voids up.
- Water: the single biggest driver. Rain or irrigation carries fine particles down into the gaps between larger ones, and the whole mass consolidates. This is why a bed can look perfect on the day and visibly drop after the first heavy rain.
- Gravity and time: the material settles under its own weight, slowly and continuously.
- Traffic: anything walking, kneeling or wheeling on the surface compacts it much faster than time alone.
- Decomposition: this one is specific to organic material. Bark, compost and wood chip break down, and their volume genuinely disappears rather than merely consolidating. That is a loss that keeps going for years, not a one-time settling.
The size of the effect depends on the material and how it was handled. Freshly screened topsoil settles noticeably. Compost and bark settle more, and keep going. Decorative stone barely settles at all, because there is nothing soft in it to consolidate and no organic content to break down.
The allowance, and why it is not the waste factor
The soil and mulch calculator on this site carries a settlement allowance defaulting to 15 percent, and a separate waste factor defaulting to 10 percent. They both increase the order, so it is tempting to think of them as one number. They are not, and keeping them apart is what makes the estimate adjustable.
- Settlement is a loss of volume after placement. The material is still there, it just occupies less space. It is a property of the material and of how much water and traffic it will see.
- Waste is a loss of material during placement. Spillage from the barrow, what stays stuck in the truck, what ends up on the path and gets swept up. It is a property of the site and how awkward the access is.
They deserve separate numbers because they move independently. Decorative stone through a wide side gate might warrant 5 percent settlement and 5 percent waste. Bark mulch barrowed through a house needs a much larger settlement allowance and a much larger waste allowance, for entirely unrelated reasons.
Both compound onto the finished volume. On a raised bed 4 metres by 1.2 metres at 100 millimetres deep, the finished volume is 0.48 cubic metres. With 10 percent waste and 15 percent settlement it comes out at about 0.61 cubic metres — the settlement allowance alone is worth roughly a bag and a half of the 50 litre bags you would buy it in.
The asphalt contrast
It is worth noting when a settlement allowance is exactly the wrong thing to add, because the reasoning makes the rule stick.
Asphalt compresses far more dramatically than soil does, and yet the asphalt calculator carries no settlement allowance whatsoever. That is not an oversight. Asphalt is sold **by weight at its compacted density**, so the compression is already inside the number you order. Soil and mulch are sold **by loose volume**, so the settling happens to volume you have already paid for.
The physics is the same in both cases. The unit of sale is what decides whether it belongs in your estimate.
Practical habits
- Overfill deliberately: fill a new bed proud of its final level, and let it settle down to where you want it. Topping up afterwards means a second delivery for a small quantity, which is the most expensive soil you will ever buy.
- Do not compact growing medium: the point of settlement allowance is to let soil find its own density, not to encourage you to tread it in. Compacted soil drains badly and roots struggle in it. Water it in and let it settle naturally.
- Compost is a repeat purchase: organic material that breaks down will need topping up annually. Budget for that rather than being surprised by it.
- Check what you are being sold: screened topsoil, as-dug topsoil and manufactured topsoil behave differently. Ask.