Formula
Cubic feet = (main length × main width + wing length × wing width) × depth (in) ÷ 12; cubic yards = cubic feet ÷ 27. Example: an 8×4 ft main section plus a 4×4 ft wing at 12 in deep needs (32 + 16) × 12 ÷ 12 = 48 cubic feet, or 1.78 cubic yards.
How to use it, step by step
Split the L into two rectangles: the main rectangle, and the wing that sticks out beyond it. Measure the wing only as the part beyond the main rectangle — if you measure the wing's full length including the corner overlap, you will double-count that corner and over-order soil. Enter depth in inches, then use the soil mix calculator to split the total into topsoil, compost and aeration material.
Worked example in detail
Take a bed shaped like an L: main section 8 ft × 4 ft, wing 4 ft × 4 ft beyond the main section, depth 12 inches. Main area = 32 sq ft, wing area = 16 sq ft, total 48 sq ft. At 12 inches deep, volume = 48 cubic feet. If you had measured the wing as 8 ft long (including the 4 ft corner overlap), you would have calculated 64 sq ft and 64 cubic feet — 16 cubic feet too much, or about 8 extra bags.
What affects real soil needs
The same settling rule as any raised bed applies: fresh soil settles after watering, so add about 10% or top up after the first watering. Corners are harder to fill evenly than straight sections, so check corners when filling and top them up separately. An L-shaped bed also has more edge than a rectangle of the same area, so it dries out faster at the edges — mulch helps, especially in the wing.
Frequently asked questions
What if my bed is U-shaped?
Split it into three rectangles — two wings plus the base — and add their areas, using the same no-overlap rule for each join.
Can I use this for a rectangular bed?
Yes — set the wing to zero. But the rectangular soil volume calculator is simpler for that job.
Should I build an L-shaped bed at all?
L-shaped beds fit corners and awkward spaces well, and give more edge access than a long rectangle. They are harder to mow around and to cover with a single cold frame, so weigh fit against those practical points.