Topsoil Calculator

Key Takeaways

• A topsoil order is worked out as length times width times depth. Every m² takes 1L of topsoil for each mm of depth, so a 5m by 4m lawn at 150mm deep needs 3,000L before any allowance for settlement is added on top.
• Topsoil depth depends on the job. A new lawn or a bed or border takes about 150mm, a raised bed takes 200 to 300mm, a wildflower area takes 100mm of low fertility topsoil, and a top dressing over an existing lawn takes about 10mm.
• A bulk bag of topsoil is not a standard size. UK suppliers sell bulk bags holding anything from 500L to around 1,000L, which is why the same lawn can come back as four bags on one topsoil calculator and seven on another.
• Topsoil settles after it is laid and watered in. Screened topsoil settles 10 to 15% and a compost rich blend settles 15 to 25%, so the order needs that much extra on top of the measured volume, with a top up after two to four weeks.
• A 700L builders bag of topsoil covers 7m² at 100mm deep and 4.7m² at 150mm deep. A 25L bag covers a quarter of a square metre at 100mm, so one builders bag is the same soil as 28 small bags.

Topsoil is ordered by volume: length times width times depth gives the litres, and every m² takes 1L for each mm of depth. Enter your measurements in this topsoil calculator (some people search for a top soil calculator, it is the same thing) and it turns them into bags, with the cheapest order worked out from our shop prices. So this calculator works in litres first, then picks from the bags we sell.

How Do I Calculate How Much Topsoil I Need?

The amount of topsoil you need is the area in m² multiplied by the depth in mm, which gives the answer in litres. Then add the settlement allowance.

Two worked examples, using bags we stock:

1. A new lawn, 5m by 4m, at 150mm deep. 5 x 4 = 20m². 20 x 150 = 3,000L. Add 15% for settlement and it is 3,450L, which is five 700L builders bags of Premium Topsoil.
2. A raised bed, 2.4m by 1.2m, and 300mm deep. 2.4 x 1.2 = 2.88m². 2.88 x 300 = 864L. Add 15% and it is 994L, which is one 40 x 25L pack of Vegetable Topsoil.

If you think in m³, divide the litres by 1,000. That lawn is 3m³ before the allowance.

How to Use the Top Soil Calculator

The top soil calculator needs four things from you: the shape of the area, its measurements, the depth and the product (you only need to know the product if you want to calculate the cheapest and closest fit options we offer).

1. Choose the product, or leave it on "Not sure yet" to see the common bulk bag sizes.
2. Pick the shape. A circle needs the distance across (diameter). A triangle needs a base and a height. An L shape is measured as two rectangles.
3. Type the measurements in cm, or switch to feet and inches.
4. Tap a job to fill the depth in, or type your own.
5. Leave the settlement allowance on 15% unless you have a reason to change it.
6. Untick Bulk Bags or Mini Bags if one of them will not suit your access.

The answer shows the cheapest order and the closest fit, and either one can go straight into your basket.

For levelling an existing lawn, I only ever measured the dips. Working out the whole lawn at the depth of the worst hollow is how people end up with two bags they did not need.

How Deep Should Topsoil Be?

Topsoil depth runs from about 10mm for a top dressing to 300mm for a raised bed, with 150mm the working figure for lawns and borders.

Before a new lawn, the existing ground gets loosened to 13 to 15cm and the stones raked out. Then the topsoil goes on.

Wildflowers are the one job where I would steer you away from rich soil. Give them 100mm of low fertility topsoil and no compost, or the coarse weeds win.

On a new build, dig a trial pit before you order anything. The turf is often sitting on a few centimetres of soil over clay subsoil and builders' rubble, and it is better to find that out before the lorry is booked.

How Much Does a Bag of Topsoil Cover?

A 700L builders bag of topsoil covers 7m² at 100mm deep, and a 25L bag covers a quarter of a square metre.

Coverage is the litres in the bag divided by the depth in mm.

These figures have no settlement allowance in them. The calculator adds it for you.

Outdoor Aggregates sells nine of its eleven topsoils by the litre, most of them in a builders bag of about 700L. The other two, Landscaping Topsoil and Beds and Borders Topsoil, come in an 850kg bag that holds about 600L. You can see all eleven in our topsoil range.

Why Do Topsoil Calculators Give Different Answers?

Topsoil calculators give different answers because each one assumes its own bulk bag size, and a UK bulk bag holds anything from 500 to about 1,000L.

In October 2026 we ran the same lawn, 5m by 4m at 150mm deep, through eleven UK topsoil calculators. Every one that showed a volume agreed on 3m³. After that they went their own ways.

1. The bag count ran from 4 bulk bags to 7.
2. Only two of the eleven said what size of bag they were counting.
3. One added 15% for settlement without being asked.

Worked backwards, the bags behind those answers hold anything from 500L to about 950L. None of them is wrong for the bags that seller fills. The trouble starts when you take the bag count from one site and buy from another.

Some suppliers sell topsoil by the tonne, and the same soil weighs more wet than dry, so a tonne is not a fixed amount of ground. Litres are. Outdoor Aggregates sells every topsoil by the litre, and this calculator works in litres from start to finish.

How Much Extra Topsoil Should I Order for Settlement?

Screened topsoil needs 10 to 15% extra for settlement, and a compost rich blend needs 15 to 25%, on top of the measured volume.

Loose soil out of a bag is full of air. Once it is spread, trodden and watered, it sinks. The more compost in the mix, the further it goes, because compost carries on breaking down after it is laid.

My routine was to fill, leave it two to four weeks, then top up. The calculator starts on 15%. Drop it to 10% for a plain screened topsoil, and push it to 20 or 25% for a compost rich blend.

A raised bed gets filled to the top on day one. At 10 to 15%, a 300mm bed drops 30 to 45mm within the month, and that is what the top up is for.

If one spot keeps sinking after that, more topsoil is not the fix. Something under it is rotting or washing away.

Is It Better to Buy Topsoil in Bags?

Bulk bag topsoil costs less per litre than small bags, but small bags are the better buy where access does not allow for a bulk bag. Untick Bulk Bags in the calculator and it reworks the whole order in packs.

A bulk bag is dropped at the kerb and from there it's you and a wheelbarrow. If the only way to the back garden is through the house, 25L bags are worth the extra. One builders bag is the same as 28 of them.

Next Steps

Run your measurements through the calculator at the top of the page, then add the order to your basket or browse the full topsoil range. The same calculator covers our composts and soil improvers. For bark and mulch, use the Bark & Wood Chip Calculator.

If you would rather talk your topsoil order through with a person, ring Outdoor Aggregates on 0330 043 7585. The office is open 7am to 7pm, seven days a week.

Have you ever worked out topsoil for a lawn or a raised bed? Tell us how deep you went and whether you ran short or had a pile left over, we would love to hear.

FAQs

How much area does 25L of top soil cover?

A 25L bag of top soil covers 0.25m² at 100mm deep, or 2.5m² as a 10mm top dressing. A raised bed 1.2m square and 200mm deep takes twelve of them.

How much topsoil do I need for 100m2?

Topsoil for 100m² comes to 10,000L at 100mm deep and 15,000L at 150mm deep, before settlement. That is 15 or 22 builders bags of 700L, and at that size it is worth ringing us for a bulk price.

How many litres are in a bulk bag of topsoil?

A bulk bag of topsoil holds anywhere from 500 to 1,000L depending on the supplier, so the litres on the listing matter more than the word 'bulk'. Most Outdoor Aggregates topsoil comes in a 700L builders bag.

How many litres are in a tonne of topsoil?

A tonne of topsoil is about 700L, going by our 850kg bags, which hold about 600L. The figure moves with how wet the soil is, which is why this calculator works in litres.

Is my bulk bag of topsoil the volume I paid for?

A bulk bag of topsoil sold by the litre is a volume, so it can be checked with a tape. Multiply the length, the width and the height of the soil in metres, then multiply by 1,000 for litres.

Do I need topsoil before laying turf?

Topsoil before turf is only needed where the existing soil is thin, poor or below the level you want. Decent garden soil that has been loosened to 13 to 15cm and raked level will take turf as it is.

Does leftover topsoil go off?

Leftover topsoil does not go off, and a heap kept for a year is still usable. Weeds are the problem, so I would keep it on a sheet and covered over.