Why Depth Matters
Topsoil depth determines how much room your plants' roots have to grow. Different plants root to different depths, and the soil beneath your topsoil (the subsoil) is usually hostile to roots — compacted, poorly aerated, and nutrient-poor.
Get the depth wrong and you'll see the consequences within months: patchy lawns, stunted vegetables, shrubs that blow over in wind because they can't anchor properly, and waterlogging where a thin topsoil layer sits on impermeable clay.
This guide gives specific depth recommendations for every common garden situation.
Depth Recommendations
New Lawns: 100-150mm
Grass roots in a healthy lawn extend 100-150mm into the soil. The absolute minimum for a new lawn is 100mm of topsoil, but 150mm is strongly recommended — especially if your subsoil is heavy clay or compacted.
On clay subsoils, the extra depth creates a more effective drainage buffer. On sandy subsoils, 100mm is usually adequate because roots can penetrate into the free-draining ground below.
For the full preparation process, see our lawn topsoil guide.
Flower Borders: 150-200mm
Most herbaceous perennials root to 150-200mm. Annual bedding plants are shallower (100-150mm), but you're unlikely to plant only annuals in a permanent border.
If you're creating a border from scratch over compacted clay or rubble, aim for 200mm. If you're improving an existing border that already has some reasonable soil, 100mm of fresh topsoil forked into the top of the existing ground is often enough.
Vegetable Patches: 200-300mm
Vegetables are the most demanding garden plants in terms of soil depth:
- Leafy crops (lettuce, spinach, kale): 150mm minimum
- Brassicas (cabbage, broccoli, cauliflower): 200mm
- Root crops (carrots, parsnips, beetroot): 300mm minimum — more for long-rooted varieties
- Potatoes: 250-300mm
- Beans and peas: 200mm
- Courgettes, squash, pumpkins: 200-250mm
For most mixed vegetable growing, 250mm is a good compromise. See our guide on the best topsoil for raised beds if you're building raised vegetable beds.
Raised Beds: Match the Bed Depth
Raised beds should be filled to within 25-50mm of the top. Most commercially available raised beds are 300-450mm deep, which is adequate for almost everything except deep-rooting crops.
For beds over 400mm deep, you don't need to fill the entire depth with premium topsoil. See the layering technique in our raised bed guide.
Trees and Shrubs: Match the Root Ball
When planting trees and shrubs, the topsoil needs to be at least as deep as the root ball. For a typical container-grown garden shrub, that's 200-300mm. For a standard tree, 400-600mm.
Important: do not plant trees deeper than they were in the nursery. The soil mark on the trunk should be at the final soil level. Planting too deep smothers the root flare and can kill the tree over several years.
Hedges: 300-400mm
Hedging plants are typically planted at 300-400mm depth. If you're planting a hedge in a trench through poor soil, excavate to at least 400mm and backfill with quality topsoil. The investment pays off in faster establishment and denser growth.
What Sits Beneath the Topsoil Matters
The depth recommendations above assume your subsoil is reasonably stable but not great for roots. If your subsoil is:
- Heavy, compacted clay: Add 25-50mm to the recommendations above. Break up the top of the clay before adding topsoil to avoid creating a waterlogged layer. See our guide on improving clay soil
- Free-draining sand or gravel: You can use the lower end of the depth ranges. Roots will extend into sandy subsoil more easily than clay
- Rubble, hardcore, or made ground: Use the full depth recommendations. Consider a geotextile membrane between the rubble and topsoil to prevent the two mixing
- Solid rock: You'll need the full depth plus consideration for drainage, as water will sit on the rock surface
The False Economy of Going Shallow
It's tempting to spread a thinner layer over a larger area to save money. This almost always produces poor results.
50mm of topsoil over clay, for example, creates a thin wet layer that roots can't anchor into. It dries out rapidly in summer because there isn't enough depth to hold meaningful moisture. And it mixes with the clay beneath within a season, becoming the same heavy soil you were trying to improve.
If your budget is tight, it's better to do a smaller area properly than a large area poorly. A 20-square-metre lawn with 150mm of quality topsoil will look and perform far better than a 40-square-metre lawn with 75mm.
Calculating Volume from Depth
Once you know the depth, calculating the volume of topsoil you need is straightforward. Our topsoil quantity calculator walks through the full process, but the quick formula is:
Volume (m³) = Area (m²) x Depth (m)
Remember to add 10% for settlement and waste. And when comparing that volume to bulk bags and loose loads, keep in mind that one bulk bag is approximately 0.6m³, not a full cubic metre.