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How the garden estimate works
Two calculations that share a measurement of the ground. Soil is a volume; seed is a rate applied to an area. Confusing the two is why people order forty bags of compost for a job that wanted a bulk bag.
Formula
volume (litres) = area × depth in metres × 1000 · bags = ceil(volume ÷ bag volume) · seed = area × sowing rate · seed bags = ceil(seed ÷ bag weight)
Worked example
A 4 × 1.2 m border is 4.8 m². Topping it up by 15 cm needs 4.8 × 0.15 = 0.72 m³, which is 720 litres, or 15 bags of 50 litres. At that point a bulk bag — usually around 0.7 to 1 m³ — is cheaper and lighter to move than fifteen trips from the car. Sowing the same area with a new lawn at 35 g/m² needs 168 g of seed: one small bag covers it several times over.
Common mistakes
- Buying soil by area. A square metre tells you nothing until you decide a depth; 5 cm and 30 cm differ by a factor of six.
- Ignoring settlement. Fresh compost settles by roughly 10-20% in the first season, so fill slightly proud rather than level.
- Missing the bulk threshold. Above about 1 m³, a bulk bag or a loose delivery is almost always cheaper per litre than bagged compost, and far less carrying.
- Using the same sowing rate for everything. A new lawn wants roughly 35 g/m², overseeding an existing one nearer 25, and the bag's own figure beats both.
- Sowing at the wrong time. Rate and quantity are right all year; germination is not. Spring and early autumn are the windows.
- Confusing topsoil with compost. Topsoil fills and levels, compost feeds and lightens. Most beds want a mix, and only you know the ratio.
FAQ
Questions
How deep should the soil be?
About 10 cm to top up an established bed, 15 cm for a new planting bed, and 30 cm or more for a raised bed or anything with root vegetables. Deeper is not automatically better: most bedding plants root in the top 20 cm and the rest is expense.
Bags or a bulk delivery?
The crossover is usually around a cubic metre — roughly twenty 50-litre bags. Below that, bags are convenient. Above it, bulk is cheaper per litre and saves a great deal of carrying, though you need somewhere the delivery can be tipped.
Why does the result show cubic metres as well as bags?
Because suppliers quote bulk in cubic metres or cubic yards, and bags in litres, and you cannot compare two prices until both are in the same unit. Seeing them side by side is usually what decides the order.
Does the sowing rate change with the seed mix?
Yes, considerably — a fine ornamental mix and a hard-wearing ryegrass differ. Always use the figure on your own bag; the default here is a common new-lawn rate, not a universal one.
Estimates for planning a purchase, based on a rectangular area and an even depth. Soil settles and ground is rarely level, so treat the volume as a lower bound. Check the sowing rate on your own seed bag.
About this material
Topsoil, compost and grass seedWhat each product is for, bags against bulk, and the sowing rates printed on a seed bag.