Choose a sensible spacing source, understand centre-to-centre measurements, and translate that spacing into square, staggered or row-based plant numbers without pretending every species needs the same distance.
For perennials, RHS advice is to check the predicted spread or width on the label or in a reliable plant reference. That mature spread is a sensible starting point for centre-to-centre spacing. RHS gives the example that a plant expected to spread to about 60 cm might be planted at around 50 cm if the intention is for neighbouring plants to knit together.
That example is useful because it shows why a spacing table cannot be a universal botanical database. The same 50 cm geometric spacing could be sensible for one plant, too close for another and unnecessarily wide for a smaller species. Use the geometry below only after choosing the plant-specific spacing.
Plant spacing normally means the distance from the centre of one plant to the centre of the next. It is not the pot diameter and it is not the empty gap between two leaves. Marking centres with canes, sand or pots before planting makes the pattern easier to see and helps prevent the rows drifting.
Plant centres line up in both directions. If the spacing is s metres, theoretical density is 1 ÷ s² plants per m².
Alternate rows are offset by half a spacing. The perpendicular row distance is about 0.866 × the centre spacing, increasing theoretical density by about 15.5% compared with a square grid.
Useful when the distance along a row is different from the distance between rows. This is a different geometry, so enter both values rather than forcing a square spacing.
RHS bedding guidance uses a row-by-column approach for simple square or rectangular blocks, but advises a scaled visual plan for more complicated shapes. That is a good practical dividing line: use the calculator for repeated geometry, and use a sketch for a mixed or highly irregular design.
This table is mathematics, not a species recommendation. It shows the theoretical density if a whole area were filled at the same centre spacing. Real bed counts can be lower because edges, paths, access gaps and mixed planting remove positions.
| Centre spacing | Square grid plants/m² | Staggered plants/m² |
|---|---|---|
| 10 cm | 100.00 | 115.47 |
| 15 cm | 44.44 | 51.32 |
| 20 cm | 25.00 | 28.87 |
| 25 cm | 16.00 | 18.48 |
| 30 cm | 11.11 | 12.83 |
| 40 cm | 6.25 | 7.22 |
| 50 cm | 4.00 | 4.62 |
| 60 cm | 2.78 | 3.21 |
| 75 cm | 1.78 | 2.05 |
| 100 cm | 1.00 | 1.15 |
Staggered density uses a triangular-grid cell area of spacing² × √3/2. Figures are rounded to two decimal places.
Suppose a bed is 4 m × 2 m and the spacing is 50 cm. The theoretical square-grid density is 4 plants/m², so 8 m² × 4 = 32 plants. If you use a half-spacing boundary inset, the actual grid also gives 8 positions along the 4 m side and 4 across the 2 m side: 32 plants.
If instead you deliberately put plant centres on both edges, the same bed can contain 9 positions by 5 positions: 45. The spacing number has not changed — the edge rule has. This is why a useful calculator must expose its boundary assumption rather than hiding it.
A 5 m × 1.8 m rectangular border has an area of 9 m². The chosen perennial is supplied with a recommended centre spacing of 45 cm. If a square grid and half-spacing inset are used, the layout can be counted row by row. If a more natural staggered drift is wanted, the rows can be offset and the total recalculated.
A mixed border rarely deserves one plants-per-m² figure. Shrubs, herbaceous perennials, bulbs and ground cover can occupy different layers and have different mature spreads. Measure the area, but divide the design into planting groups rather than treating every square metre as identical. For complex shapes, a scaled sketch is often the safest way to avoid buying too many plants.
If the physical shape itself is difficult to measure, use the Garden Area Calculator first.
RHS perennial planting guidance covers predicted spread and spacing. RHS bedding guidance explains row/column counting and recommends scaled planning for complex shapes. RHS border planning also emphasises measuring the area and planning spacings once plants are selected.
Use the calculator with your chosen centre spacing and compare a square grid, staggered grid or separate row spacing.
Move from bed measurements into soil quantities, hedge runs and related planning tools.