Why area comes first
Almost every other number in a garden plan depends on area. Plant quantity is area divided by the space each plant needs. Soil and mulch are area multiplied by depth. Water is area multiplied by how many millimetres your crops use in a week. Get the square metres right and the rest of the plan falls into place. Get them wrong and you end up with half a bed of bare soil, or a car boot full of compost with nowhere to go.
How to measure awkward spaces
Use a long tape measure and two pegs or canes. Measure along the soil surface rather than across the top of a wall, because edging, kerbs and paving eat into planting space more than you expect.
- Rectangles and squares: length × width. If the two long sides differ, measure both and average them.
- Circles: measure straight through the centre to get the diameter. The calculator halves it and applies π for you.
- Corner beds: for a right-angled triangle, the two short sides are the base and height. Area is half of base × height.
- L-shapes and irregular borders: split the space into rectangles and triangles, calculate each one, then add them together. A quick sketch with each piece labelled saves a lot of re-measuring.
For curving borders, lay a hosepipe along the edge and then measure the hose. That gives you the edging length, which is the figure you need when buying lawn edging, border stone or timber for a surround.
Choosing raised-bed dimensions
Width matters most. If you can reach the bed from both sides, around 1.2 m (4 ft) lets most adults reach the middle without stepping on the soil, which keeps it loose and airy. Against a wall or fence, where you can only reach from one side, 60–90 cm is far more comfortable.
Length is flexible, but long beds make you walk further around them. Many gardeners break runs of more than about three metres with a cross path. Longer timber sides also tend to bow outwards once filled, so a brace or stake halfway along is worth adding.
Depth depends on what you grow. Shallow-rooted salads and herbs cope with 15–20 cm of good soil over loosened ground. Carrots, parsnips and potatoes appreciate 30 cm or more. Beds built to 45 cm and higher reduce bending, which makes a real difference if you garden with back or knee pain, or from a seat.
Leave paths of at least 45 cm between beds, and make one route wide enough for a wheelbarrow.
Raised-bed materials compared
| Material | Relative lifespan | Relative cost | Best for |
|---|---|---|---|
| Untreated softwood boards | Shorter | Low | Trial beds and budget builds |
| Rot-resistant timber (larch, cedar) | Long | Moderate to high | Natural look with good durability |
| Galvanised or coated steel | Long | Moderate | Tall, slim-walled beds that keep more growing space |
| Recycled plastic or composite | Long | Moderate to high | Low maintenance, no rot |
| Brick, block or stone | Very long | High, plus labour | Permanent beds and seating edges |
Raised beds versus in-ground planting
Raised beds: pros
- Soil warms and drains faster in spring
- You control the soil mix completely
- Less bending and a tidy, defined edge
- Works over poor, stony or compacted ground
Raised beds: cons
- Up-front cost of materials and soil
- Dries out faster in hot weather
- Timber eventually needs replacing
- Slugs love the sheltered edges
How the board count works
The raised-bed calculator divides bed depth by the height of one board to find how many courses, or layers, you need, rounding up. It multiplies the perimeter by that number of courses and by the number of beds to give the total running length of timber or panel. Add around ten percent for cuts and mistakes. Because it uses internal dimensions, chunky sleepers make the outside run a little longer than shown, so round up when ordering.