How the retaining wall calculator works
A segmental block wall is a stack of courses on a compacted gravel pad, with the first course buried and clean stone behind it to drain water away. The calculator counts blocks per course and courses, adds the buried course, and works out the base and drainage stone from Allan Block's and Basalite's installation guides.
Blocks per course = wall length ÷ block face length, rounded up
Courses = height above ground ÷ block height, rounded up, + buried courses
Buried = 1 in per ft of wall height, at least one full course
Blocks = blocks per course × courses × (1 + extra %)
Base = length × trench width × 6 in, at 2.16 tons per compacted cu yd
Trench width = block depth + 12 in, and at least 18 in (24 in for walls 4 ft and up)
Drainage stone = length × 12 in × height of block, at 1.4 tons per cu yd
Caps = wall length ÷ cap length × (1 + extra %); adhesive = 2 beads × length ÷ 13.6 ft per tube
Worked example
A garden wall is 20 ft long and 2 ft tall, built from 12 by 4 by 7 inch blocks. Each course takes 240 ÷ 12 = 20 blocks. The wall shows 24 ÷ 4 = 6 courses, and one more is buried, so 7 courses: 140 blocks, or 147 blocks with 5% extra.
The trench is 7 + 12 = 19 inches wide and gets 6 inches of compacted base: 20 × 19/12 × 0.5 = 15.8 cu ft, or 0.59 cubic yards, about 1.3 tons. Behind the 28 inches of block go 20 × 1 × 28/12 = 46.7 cu ft of drainage stone, 1.73 cubic yards or 2.4 tons. Add 21 caps, three 10 oz tubes of adhesive and two 10 ft lengths of drain pipe.
Common block sizes
| Block | Face (L x H) | Depth | Blocks per sq ft of face |
|---|---|---|---|
| Small garden block | 12 x 4 in | About 7 in | 3 |
| Medium block | 16 x 6 in | 10 to 12 in | 1.5 |
| Large block (Allan Block size) | 18 x 8 in | About 12 in | 1 |
Sizes vary by brand, and many are a fraction under the round number (11 3/4 in, say). Use the numbers on your block's label.
How tall can it be?
Every block has a maximum height as a gravity wall, without geogrid reinforcement. Basalite limits its small Bayfield and Brisa walls to 3 ft including the buried course. Larger blocks go higher. The International Residential Code requires walls that hold back more than 48 inches of soil to be engineered, and a slope, driveway, pool or building above the wall can lower that limit. Check with your building department before you dig.
Building it
- Call 811 before digging so buried lines are marked.
- Compact the base with at least two passes of a plate compactor, then level the first course block by block.
- Fill 12 in behind the block with clean drainage stone as each course goes on, and compact it every course or two. Native soil goes only behind the stone.
- Lay the drain pipe at the back of the base and run it out to daylight. Allan Block requires one for walls over 4 ft and wherever drainage is poor.
- Glue the caps with a 3/8 in bead about 2 in in from the front and back.
Sources
- Allan Block: basic retaining wall installation, for the trench width, base depth, buried block, drainage stone and drain pipe
- Basalite: typical retaining wall installation instructions (PDF), for the base pad 6 in past the block and the 3 ft height limit for small blocks
- Loctite PL 500 Landscape Block Adhesive technical data sheet (PDF), for 13.6 ft per 10 oz tube at a 3/8 in bead
- City of Grand Prairie, Texas: residential retaining walls under the 2021 International Residential Code, for the 48 inch engineering threshold
- Penn State Center for Dirt and Gravel Roads (PDF), for the weight of compacted dense-graded stone
Common questions
How many retaining wall blocks do I need?
Divide the wall length by the block's face length for blocks per course, and the height by the block height for courses, then add one buried course. A 20 ft wall 2 ft tall in 12 by 4 inch blocks is 20 blocks by 7 courses, or 147 blocks with 5% extra.
How deep should the first course of a retaining wall be buried?
Allan Block recommends burying 1 inch of block for every foot of wall height, on top of at least 6 inches of compacted base. In practice, most small walls bury the whole first course.
How much gravel do I need behind a retaining wall?
Allan Block fills 12 inches behind the block with clean drainage stone for the full height. For a 20 ft wall with 28 inches of block, that is about 1.7 cubic yards, or 2.4 tons.
How tall can I build a retaining wall without an engineer?
The International Residential Code requires an engineered design for walls holding back more than 4 ft of soil, and many small DIY blocks are rated only to about 3 ft without geogrid. A slope, driveway or building above the wall lowers that limit. Check your block's chart and local rules.
Do retaining wall caps need glue?
Yes. Glue caps down with a landscape block adhesive. Loctite recommends a 3/8 inch bead outdoors, about 2 inches in from the front and back of each cap, and a 10 oz tube runs about 13.6 ft.