The plan
The one quantity on this page that comes from an engine is the soil. Filling a 6 m by 1.2 m border to a settled depth of 300 mm is 7.2 m² at the RHS’s own figure for a bed most plants will grow in, and the topsoil calculator turns that into the tonnes and bags below. Everything else here — how thickly to sow grass or wildflower seed, how deep a bulb goes, how far apart a hedge is planted, and which months to do it in — is a horticultural figure from the RHS, in its own table further down, because no calculator on this site produces it.
The reason the two are kept apart is that they are different kinds of number. The soil take-off is arithmetic on measurements you give it, so it is exact. A seed rate or a spacing is a rule of thumb from a named authority, so it is a guide — and where the authority gives a range rather than a single figure, the range is what you get here, labelled as a range.
Doing it, in order
- Fill the border to the depth the crop needs before you plant anything: 300 mm here, which the RHS rates as enough for salad leaves, dwarf bulbs and strawberries. Most plants want 450 mm or more.
- Let the soil settle and top it up. Loose topsoil drops by roughly a tenth once it is watered — the calculator builds that allowance in, which is why you order 2.38 m³ for a 2.16 m³ bed.
- Sow or plant in the window the figure gives you, not when the weather looks nice for a weekend. A lawn sown outside its window is a lawn that needs watering every day.
- Set out the spacing before you dig: run a line, mark the stations, and plant to the marks. Spacing decides whether the hedge closes or the border looks thin in three years, and it is far easier to get right on the first day than to fix later.
- Plant to the depth, not to the pot. A bulb goes two to three times its own height down; a bare-root hedge goes in at the same depth it was lifted from.
- Water in, then mulch. The figures cover what to sow and how far apart — they do not cover the aftercare, which is what the first summer is made of.
Don’t forget
- Seed rates are per square metre, not per pack The RHS rates here — 35 g/m² for a lawn, 4–5 g/m² for a wildflower-and-grass mix, 1–3 g/m² for pure wildflower — are for the area you are sowing. Check the packet as well, because a mix can differ from the standard rate.
- A range is a range Where the RHS gives 30–60 cm for hedge spacing, this page prints 30–60 cm. It does not pick 45 cm and present it as the figure, and species differ within it — box about 25 cm, hawthorn 30 cm, beech or hornbeam 60 cm.
- What this page does not size It does not tell you how many plants to buy for a hedge, or how many grass seeds are in a gram, and it does not size the soil for a hedge trench or a lawn. Those need your own area and your own figure, which is what the calculators take.
- The date each figure was read The RHS pages were read on 25 September 2026 and the date is recorded with each source below, so a figure can be checked against the source as it stands.
Material take-off for this plan
Worked out by the site’s own Topsoil for this plan’s dimensions, not typed in by hand.
The working
| Working | Value |
|---|---|
| Area to fill | 7.2 m² 6.00 m × 1.20 m |
| Depth of topsoil | 300 mm The depth you want once it has settled, not the depth you tip in |
| Volume before settlement | 2.16 m³ 7.2 m² × 300 mm |
| Settlement allowance | 10% +0.22 m³ to allow for slumping |
| Order | 2.38 m³ ≈ 2,376 litres at 1.5 t/m³ |
| Bulk bags (850 kg) | 5 bags Rounded up to whole bags |
| 25 kg bags | 143 bags Rounded up. Only sensible for small beds |
What to order
| Material | Quantity | Notes |
|---|---|---|
| Screened topsoil (loose, by weight) | 3.56 tonnes | Loose loads are cheapest per tonne but need a tip point a wheelbarrow can reach. |
| Bulk bags (850 kg) | 5 bags | Delivered on a pallet — you need a hard, flat surface and somewhere dry to keep them. |
| 25 kg bags | 143 bags | Roughly 3,564 kg in total. Fine for a couple of raised beds. |
| Soil conditioner or compost (optional) | 1 part to 3 parts soilestimate | For raised beds and borders, mixing in compost improves structure and drainage. |
Settlement is the thing that catches people out: loose topsoil is full of air, and once it is watered and walked on it drops by roughly a tenth. That is why the calculator adds a settlement allowance on top of the finished depth you asked for. Fill the bed a little proud and let the rain do the rest.
Planting and sowing figures — from a named source, not from an engine
No calculator on this site produces a seed rate, a spacing or a sowing window, so these figures are not engine output. Each one is from the RHS, read on the date recorded with the source underneath. Where the source gives a range, the range is printed.
| Item | Quantity | Where the figure comes from |
|---|---|---|
| Grass seed, new lawn | 35 g per m² | The RHS rate for a new lawn (1¼ oz per square yard). Check your seed packet as well — a mix can carry a different rate. |
| Wildflower seed, wildflower-and-grass mix | 4–5 g per m² | The RHS rate for a mix that includes grass. It is a range, so the quantity you sow depends on the mix you bought. |
| Wildflower seed, pure wildflower (no grass) | 1–3 g per m² | The RHS rate where the mix is all wildflower. Much lower than a grass mix, because there is no grass seed in it. |
| Bulb planting depth | 2–3 × the bulb’s own height | The RHS rule for most bulbs. It is proportional, not a fixed depth: a 50 mm bulb goes 100–150 mm down. |
| Bulb spacing | at least 2 × the bulb’s own width | The RHS spacing. Closer than this gives a fuller first season at the cost of size the following year. |
| Soil depth, raised bed or border | 300 mm shallow crops; 450 mm or more for most plants | The RHS rates 30 cm as enough for salad leaves, dwarf bulbs and strawberries, and 45 cm or more for most plants including fruit bushes, shrubs and small trees. |
| Hedge spacing, bare-root | 300–600 mm apart; species differ | The RHS range for most bare-root hedging: box at about 250 mm, hawthorn 300 mm, beech or hornbeam 600 mm. A staggered double row uses 450 mm between rows and 900 mm between plants. |
| Lawn sowing window | September–November or March–April | The RHS windows for a new lawn: early autumn or mid-spring, when the soil is warm and moisture is reliable. |
Source: RHS — Starting a lawn from seed — Read 25 September 2026. The 35 g per m² sowing rate, and the September–November and March–April sowing windows.
Source: RHS — How to sow a wildflower patch — Read 25 September 2026. The 4–5 g per m² wildflower-and-grass rate and the 1–3 g per m² pure-wildflower rate, and the advice to mix seed with sand for an even scatter.
Source: RHS — Bulbs: planting tips — Read 25 September 2026. The depth of two to three times the bulb’s height and the spacing of at least twice its width.
Source: RHS — How to make a raised bed — Read 25 September 2026. The 30 cm and 45 cm-plus soil depths, and that salad leaves and strawberries crop in the shallower bed while most plants need the deeper one.
Source: RHS — Hedges: planting guide — Read 25 September 2026. The 30–60 cm bare-root hedging range, the per-species figures, and the staggered double-row spacings.
Every horticultural figure on this page is a named source, read on the date shown, and where the source gives a range the range is printed rather than one number chosen from it. None of them is engine output: the take-off above is the only engine figure here, and it is the soil for the border, not the planting. Where a figure could not be sourced it has been left out rather than filled in.
Planting and sowing, planned out FAQs
What rate do I sow grass seed at?
35 g per square metre for a new lawn, the RHS figure. Check your own seed packet as well, because a mix can carry a different rate.
How deep should I plant bulbs?
Two to three times the bulb’s own height, the RHS rule of thumb, and space them at least twice their own width apart. Individual varieties can differ, so the packet governs.
How deep should a raised bed or border be?
The RHS rates 30 cm as enough for salad leaves, dwarf bulbs and strawberries, and says most plants — fruit bushes, shrubs and small trees — need 45 cm or more. This plan fills to 300 mm, so add a course for anything deeper-rooted.
When should I sow a new lawn?
Early autumn, September to November, or mid-spring, March to April — the RHS windows, when the soil is warm and moisture is reliable. Sowing in summer is possible but needs daily watering.
How far apart do I plant a hedge?
30 to 60 cm for most bare-root hedging, the RHS range; box goes closer at about 25 cm and beech or hornbeam wider at 60 cm. A staggered double row uses 45 cm between the rows and 90 cm between plants.