Retaining wall calculator: blocks, caps and gravel
Enter the wall length and height and pick a block. You get the block and cap count, the gravel for the base and the drainage zone, and a rough materials cost you can edit to your supplier's prices.
Visible height, not counting the cap.
Extra for cuts and breakage
Gravel
20 ft long × 2 ft high = 40 sq ft of wall face
147 wall blocks
20 per course × 7 courses (1 buried), plus waste
21 cap blocks
Gravel
- Base gravel (3/4 in minus, compacted): 0.7 cu yd · 1 ton
- Drainage stone behind the wall (clean 3/4 in): 1.7 cu yd · 2.3 tons
- 4 in perforated drain pipe: 20 ft along the back of the base course
- Landscape fabric between drainage stone and soil: about 67 sq ft
Rough materials cost
$677
- Blocks: $441
- Caps: $84
- Gravel: $152
Edit the prices to match your supplier. Excludes delivery, adhesive, drain pipe, tools and labor.
Height check
Walls under about 3 ft are usually a DIY job with no permit. Over roughly 3 to 4 ft, or any wall holding up a driveway, slope or building, most areas require a permit and an engineered design with geogrid. General guidance only: check your local code.
Assumes 1.4 tons per cubic yard for base gravel and 1.35 tons (2,700 lb) for clean drainage stone. Both vary by supplier and moisture.
How the retaining wall calculator works
A segmental block wall is a grid of courses. Divide the wall length in inches by the face width of one block and round up: that is blocks per course. Divide the height above ground in inches by the face height of one block and round up: that is the number of visible courses. Most block makers want the first course buried, so the calculator adds one course below grade by default. Blocks per course × courses, plus a waste allowance for cut blocks and breakage, is your block count.
Caps run along the top in a single row. Wall length in inches divided by the cap length, plus the same waste, gives the cap count. The calculator defaults the cap length to the block face width, but caps are often a different size, so check and edit it.
Gravel comes in two parts. The base is a trench of compactable 3/4 inch minus gravel, typically 24 inches wide and 6 inches deep, under the first course. The drainage zone is clean crushed stone, at least 12 inches wide, filled behind the wall for its full height. For each, length × width × depth in feet ÷ 27 gives cubic yards. Tons use 1.4 per cubic yard for base gravel and 1.35 (about 2,700 lb) for clean stone. Both vary by quarry, so treat them as estimates and ask your supplier.
- Blocks per course = wall length (in) ÷ block face width (in), rounded up.
- Courses = wall height (in) ÷ block face height (in), rounded up, plus 1 buried.
- Caps = wall length (in) ÷ cap length (in), plus waste.
- Gravel (cu yd) = length × width × depth (all in ft) ÷ 27.
Worked example
A wall 24 feet long and 2.5 feet high, built with 16 × 6 inch blocks. Blocks per course: 288 inches ÷ 16 = 18. Visible courses: 30 inches ÷ 6 = 5, plus one buried course makes 6. That is 108 blocks, and with 5 percent waste, 114. Caps at 16 inches long: 18, plus waste, 19.
Base gravel, 24 inches wide and 6 inches deep: 24 × 2 × 0.5 = 24 cubic feet, or 0.89 cubic yards, about 1.2 tons. Drainage stone, 12 inches wide for the full 3 foot wall including the buried course: 24 × 1 × 3 = 72 cubic feet, 2.67 cubic yards, 3.6 tons. At $5 a block, $4 a cap and $45 a ton for gravel, the materials come to about $864 before delivery, pipe and adhesive.
Block sizes and what they are for
Small 12 × 4 inch garden wall blocks are sold at every home center and are fine for bed edging and walls up to about 2 feet. The 16 × 6 inch size is the standard landscape wall block: heavier, with a lip or pin system that sets the backward lean (setback) of each course. 18 × 6 inch blocks are the larger commercial-style units used for taller or longer walls. Bigger blocks mean fewer pieces and faster building, but each one is heavier to carry. The face sizes in the calculator are nominal; enter the exact dimensions from the product sheet if they differ.
| Block face | Blocks per sq ft of wall | Typical use | Default price in calculator |
|---|---|---|---|
| 12 × 4 in | 3.0 | Garden walls, raised beds, edging | $3 |
| 16 × 6 in | 1.5 | Standard landscape walls | $5 |
| 18 × 6 in | 1.33 | Larger, longer walls | $7 |
When a wall needs an engineer
As general guidance, walls under about 3 feet of exposed height are DIY territory in most places. Once a wall gets past roughly 3 to 4 feet, or holds back a driveway, a slope, a pool or a building, most jurisdictions require a permit and an engineered design, usually with geogrid reinforcement layered into the backfill. Rules vary a lot by city and county, so check your local building code before you buy block. The calculator flags any wall at 3 feet or more.
Whatever the height, drainage is what makes walls fail. Water trapped behind a wall adds pressure every time it rains and freezes. Clean stone behind the blocks, a perforated pipe at the base that daylights or drains to a lower point, and fabric between the stone and the soil are cheap compared with rebuilding a bowed wall.
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Redesign my yard freeFrequently asked questions
How many retaining wall blocks do I need?⌄
Divide the wall length in inches by the block face width for blocks per course, and the height in inches by the block face height for the number of courses. Add one buried course, multiply, and add about 5 percent. A 20 foot long, 2 foot high wall in 12 × 4 inch blocks needs 147.
How much gravel do I need behind a retaining wall?⌄
Plan for a zone at least 12 inches wide behind the blocks for the full wall height. For a 20 foot wall that is 28 inches tall including the buried course, that is about 1.7 cubic yards, roughly 2.3 tons of clean 3/4 inch stone.
How deep should the base be for a retaining wall?⌄
A common spec is 6 inches of compacted 3/4 inch minus gravel in a trench about twice as wide as the block depth, typically 24 inches, with the first course buried. Taller walls get deeper bases; follow the block maker's installation guide.
Do I need a permit for a retaining wall?⌄
Often not for walls under about 3 feet. Many jurisdictions require a permit and an engineered design once a wall passes roughly 3 to 4 feet, or if it supports a load such as a driveway or a slope. This is general guidance; check your local building code.
How much does a retaining wall cost per square foot?⌄
Mostly it depends on the block. In the worked example above (16 × 6 inch block at $5), materials come to about $14 per square foot of visible wall face, and premium textured block can double that. Enter your supplier's prices for a real number. Hiring a crew typically adds at least as much again in labor.