How to use the retaining wall calculator
Enter the wall's length and how tall it will stand above the finished ground, then choose the block size. The calculator adds the buried part of the wall, works out the courses and blocks, and sizes the gravel base and the drainage rock that goes behind the blocks, following the Allan Block installation guide. The section drawing shows every layer.
This is for segmental (dry-stacked) concrete block walls. For a mortared wall of standard 8 × 8 × 16 blocks, use the concrete block calculator.
The formulas
buried = 1 in per ft of wall height
courses = (height + buried) ÷ block height, rounded up
blocks = courses × (length ÷ block length) × (1 + extra)
base = length × trench width × base depth (18 in × 4 in under 4 ft; 24 in × 6 in above)
drainage rock = length × 12 in × total height
Worked example: a 30 ft wall, 3 ft high, 16 × 6 in blocks
- Buried: 3 in. Total height: 36 + 3 = 39 in.
- Courses: 39 ÷ 6 = 6.5, so 7 courses.
- Blocks per course: 360 in ÷ 16 = 22.5, so 23.
- Blocks: 7 × 23 = 161, plus 5% = 170 blocks, and 23 caps.
- Base: 30 × 1.5 ft × 0.33 ft = 15 ft³, about 0.8 tons of crushed base.
- Drainage rock: 30 × 1 ft × 3.25 ft = 97.5 ft³, about 4.9 tons of clean stone.
Building it right
- The base is everything. A firm, level, compacted base course is what keeps the wall straight. Take your time on the first course.
- Drainage, not soil, behind the wall. Water pressure topples more walls than soil does. Fill the cores and the 12 in behind the blocks with clean crushed rock, and backfill with soil only beyond that.
- Drain pipe. Required for gravity walls over 4 ft and on wet sites; a good idea on any wall. Vent it to daylight.
- Compact in lifts of 8 in or less behind the wall as you go up.
- Fabric. Landscape fabric between the drainage rock and the soil keeps fines from clogging the rock.
When to get an engineer
Gravity block walls are usually limited to roughly 3 to 4 ft, depending on the block and site. Anything taller, any wall with a slope, driveway, pool or building above it, and any wall near a property line generally needs geogrid reinforcement, an engineered design and a permit. Check with your local building department before you dig.
Common mistakes
- Forgetting the buried course. It's often a whole extra course of block.
- Backfilling with clay. It holds water against the wall.
- Not stepping the base on slopes. On sloping ground, the base trench steps up in block-height increments.
Questions people ask
How many retaining wall blocks do I need?
Divide the wall length by the block length to get blocks per course, and the total height (including the part buried below grade) by the block height to get courses. Multiply them and add about 5% for cuts. A 30 ft wall, 3 ft high, with 16 × 6 in blocks takes about 170 blocks.
How deep should the first course of a retaining wall be buried?
Allan Block's installation guide buries 1 inch of block for every foot of wall height, on top of a compacted gravel base: 4 inches of base for walls under 4 ft, 6 inches for taller walls.
How much gravel goes behind a retaining wall?
Manufacturers call for about 12 inches of clean crushed drainage rock directly behind the blocks, for the full height of the wall, with the block cores filled too. That's roughly 1 cubic foot of rock per square foot of wall face.
How tall can a retaining wall be without reinforcement?
Gravity walls of typical landscape block are usually limited to around 3 to 4 ft, depending on the block, setback and soil. Taller walls need geogrid reinforcement and often an engineered design and permit. Many local codes require a permit for walls over 4 ft.
Do I need a drain pipe behind a retaining wall?
Allan Block requires one for any reinforced wall, any gravity wall over 4 ft and any site with poor drainage. A 4 in perforated pipe at the back of the base, vented to daylight, is cheap insurance on most walls.
Sources and assumptions
- Allan Block basic installation guide: base trench width and depth, buried block, wall rock 12 in behind the block, drain pipe rules
- Rock densities from Vulcan Materials: base 1.45 tons/yd³, clean drainage stone 1.35 tons/yd³
- Block sizes vary by manufacturer. Enter your block's face size for an exact count. Walls over 4 ft, or holding back a slope, driveway or building, need an engineered design.
Last reviewed 27 October 2026. Every formula on this page is checked against the worked example above in our automated tests. Found a problem? Tell us.