How to use the retaining wall calculator
A segmental block retaining wall is a kit of parts: wall blocks stacked in courses, a row of caps on top, a compacted gravel base under the first course, a zone of clean drainage stone behind the blocks and usually a perforated pipe at the bottom. This calculator turns the wall length and height into a quantity for each of them, so you can price the wall, order materials and check whether it is tall enough to need an engineer.
- Choose units. US mode uses feet for the wall and inches for the blocks and returns cubic yards and US tons. Metric mode uses metres and millimetres and returns cubic metres and tonnes.
- Enter the wall length and exposed height, which is the visible height of the block courses above finished ground, not counting the cap.
- Pick a block or type the face height, face length and depth from the product sheet. Set the cap length if the caps differ from the blocks.
- Check the base and drainage settings. The defaults of a 6 in (150 mm) base and a 12 in (300 mm) drainage zone are common manufacturer guidance.
- Add extra for cut blocks and for gravel lost to compaction, then read the results.
Retaining wall formulas
Buried depth = max(6 in, exposed height ÷ 8), rounded up to whole courses
Courses = ⌈exposed height ÷ block height⌉ + buried courses
Blocks = ⌈length ÷ block length⌉ × courses × (1 + extra %)
Caps = ⌈length ÷ cap length⌉ × (1 + extra %)
Base gravel = length × (block depth + 12 in) × base depth
Drainage stone = length × drainage width × (total block height − 6 in)
Trench = length × base width × (base depth + buried depth)
Weights use about 1.8 t/m³ for compacted crushed base and 1.6 t/m³ for clean drainage stone. These are estimates; ask the supplier for the density of the product they sell.
| Typical block (face height × length × depth) | Face area per block | Blocks per ft² / m² of face |
|---|---|---|
| Small garden block, 4 × 12 × 7 in | 0.33 ft² | 3.0 per ft² |
| Standard wall block, 6 × 16 × 12 in | 0.67 ft² | 1.5 per ft² |
| Large wall block, 8 × 18 × 12 in | 1.0 ft² | 1.0 per ft² |
| Metric block, 150 × 400 × 300 mm | 0.06 m² | 16.7 per m² |
| Metric block, 200 × 450 × 300 mm | 0.09 m² | 11.1 per m² |
Worked examples
A 3 ft garden wall
A wall 40 ft long with 3 ft exposed, built from 8 × 18 × 12 in blocks. The buried depth is the larger of 6 in and 36 ÷ 8 = 4.5 in, so one 8 in course goes below grade. Five courses give 40 in exposed, so there are six courses in all. Each course takes ⌈480 ÷ 18⌉ = 27 blocks: 162 blocks, or 171 with 5% extra, plus 29 caps. The base is 40 × 2 × 0.5 = 40 ft³ (1.48 yd³, about 2.25 US tons). The drainage stone is 40 × 1 × 3.5 = 140 ft³ (5.19 yd³, about 7 US tons), with 40 ft of perforated pipe. Add 10% extra to the gravel when ordering.
A low metric planter wall
A 12 m wall, 0.6 m exposed, in 150 × 400 × 300 mm blocks. The buried depth is 150 mm, one course, and four courses make the 600 mm exposed height, so five courses in total. That is 30 blocks per course and 150 blocks, 158 with 5% extra. The base is 12 × 0.6 × 0.15 = 1.08 m³, and the drainage stone is 12 × 0.3 × 0.6 = 2.16 m³, roughly 1.9 and 3.5 tonnes.
Who this calculator helps
- Landscape architects and garden designers sizing terraces, raised beds and level changes on a site plan.
- Architects putting early quantities on site walls around driveways, patios and lower-ground courtyards.
- Contractors and landscapers turning a site measure into a block and aggregate order.
- Homeowners pricing a garden wall and checking when it crosses into permit or engineering territory.
- Students learning how embedment, drainage and courses fit together in a gravity wall.
Practical tips
- Level and compact the base carefully; an error in the first course shows all the way up the wall.
- Fill the cores of hollow blocks and the space between them with drainage stone as you lay each course.
- Wrap the drainage stone in filter fabric where the retained soil is fine, so it doesn't clog.
- Work out the dig with the excavation calculator and order extra stone with the gravel calculator. Check the grade above and below the wall with the slope calculator.
- Treat any wall over about 4 ft, or one holding back a driveway, slope or building, as an engineering job.
Want to see the terrace before you build it? Try the wall and planting on a photo of the garden with Landscape.
