Plant Spacing Calculator

How the Plant Spacing Calculator Works

The calculator works out how many plants fit into your bed based on the spacing between each plant and the layout pattern. In a grid (square) layout, plants sit in straight rows and columns — the number of plants equals (rows + 1) × (columns + 1), where rows and columns are determined by dividing the bed dimensions by the spacing. In a staggered (triangular) layout, alternate rows are offset by half the spacing, and the row-to-row distance shrinks to spacing × √3 ÷ 2 (roughly 86.6% of the plant spacing).

Staggered planting fits about 15% more plants per square metre than grid planting at the same spacing because it uses the space between rows more efficiently. This makes staggered layouts the better choice for ground cover, where fast canopy closure is the goal. For linear boundary plantings rather than area beds, the hedge spacing tool handles single- and double-row calculations.

A quick way to sanity-check the count is plants per square metre. In an open square grid this is roughly one divided by the spacing in metres, squared, with a staggered layout adding about 15% on top:

• 150mm spacing: about 44 plants per m² square, 51 staggered
• 200mm spacing: about 25 square, 29 staggered
• 250mm spacing: about 16 square, 18 staggered
• 300mm spacing: about 11 square, 13 staggered
• 450mm spacing: about 5 square, 6 staggered
• 600mm spacing: about 3 square, 3 staggered

These are open-ground figures based on the standard density method published by Iowa State University Extension. The calculator's own figures run a little higher than this table: every edge gets a full row of plants rather than tapering off, and the total includes the extra-plants percentage you set.

The extra plants percentage covers losses from transplant failure, poor soil spots, and replacements in the first growing season. A 10% buffer is standard for most planting projects.

What the Numbers Mean

Three outputs appear after calculation: planting area (bed footprint), total plants needed (including extras), and density in plants per square metre. The density figure is the one to compare against published planting rates. As a border starting point, the RHS suggests roughly five herbaceous perennials, three small shrubs, or one large shrub per square metre. The spacing-to-quantity logic mirrors how fabric repeat calculations work for interior projects.

If the density the calculator returns is higher than the published rate for your species, widen the spacing; if it is lower, tighten it for faster coverage. The total plants figure is the number to order from the nursery, and it already includes the extra percentage for losses.

For a large scheme, work from density rather than counting rows. Multiply your target plants per square metre by the total area: one hectare is 10,000 square metres, so a two-hectare planting at 11 plants per m² (300mm square spacing) needs around 220,000 plants. This calculator covers beds up to 100 × 50m (half a hectare); for anything larger, read the density off a small sample area here and multiply by your full plot size.

Getting It Right: Planting Advice

Ground cover plants at 200–300mm spacing give the fastest coverage, typically one to two growing seasons to form a solid mat in good soil. Popular UK ground covers spread at different rates, so treat spacing as a range: Geranium macrorrhizum at 300–450mm (its mature spread reaches 0.5–1m), Vinca minor anywhere from 200mm for quick cover up to 450mm over larger areas, and Ajuga reptans at 150–225mm. Mark your planting positions before digging any holes, using a measured stick cut to the spacing length, and offset every second row by half the spacing for staggered layouts.

Summer bedding is spaced by its mature size rather than a single rule. As a rough guide for popular UK bedding plants:

• Lobelia: 100–150mm
• Petunia and bedding begonia: 150–200mm
• French marigold, sweet alyssum, and bedding salvia: 150–250mm
• Busy Lizzie (Impatiens): 200–300mm
• African marigold and snapdragon (Antirrhinum): 250–300mm
• Bedding geranium (Pelargonium): 300–450mm

These are typical ranges that vary by cultivar, so check the plant label or the RHS Plant Finder for your variety before ordering.

Water each plant in well after planting and mulch the gaps between plants with bark or compost to suppress weeds during the establishment period. Weed control is critical in the first year before the canopy closes, since unchecked weeds compete for light and moisture and can overwhelm young plants.

For larger borders, buy plug plants or bare-root stock rather than pot-grown plants. Plugs cost a fraction of pot-grown perennials and establish quickly when planted in autumn or early spring. A 15 m² bed at 300mm staggered spacing needs 225 plants: at £0.50–£1.50 per plug that is £112–£337, compared with £3–£6 per pot for the same species.

When to Adjust These Numbers

Wider spacing (450–600mm) suits shrubs and taller perennials that spread to fill the gaps within two to three seasons. Closer spacing (150–200mm) creates instant impact but costs more in plant numbers. Staggered layouts work best for mass planting and ground cover because the offset rows eliminate the bare diagonal lines visible in grid layouts.

A reliable starting point from the RHS is to space at roughly 75 to 80% of a plant's mature spread, so a perennial that grows 60cm wide goes in at about 45 to 50cm centres, close enough to knit together without crowding. For a formal parterre or geometric bed, grid spacing gives the cleanest visual lines.

On heavy clay, roots establish and spread more slowly, so plants take longer to knit together. Lean toward the closer end of a plant's spacing range rather than the wider end so the bed fills in a reasonable time. On a slope, denser planting helps bind the soil and limit surface erosion while plants establish, so again favour the closer end of the range and choose species with fibrous, spreading roots. If planting into raised beds, follow a dedicated soil mix approach for better root development.

For a circular bed, base the count on the true circle area (π × radius²) rather than treating it as a square, which overstates the area by about a quarter (27%). Work out that area and enter a square bed of the same size: a 3m-diameter circle is about 7.1 square metres, so enter 2.66m for both.