How to calculate retaining wall blocks
A segmental retaining wall is built from dry-stacked concrete blocks set on a compacted gravel base, with a buried first course, clean drainage stone behind the blocks and a perforated pipe at the bottom. To estimate it, count blocks by course, then figure the stone by volume.
Courses = exposed height ÷ block face height, rounded up, + buried courses
Wall blocks = blocks per course × courses × (1 + waste %)
Cap blocks = wall length (in) ÷ cap length (in) × (1 + waste %)
Base gravel (cu yd) = length (ft) × trench width (ft) × base depth (ft) ÷ 27
Drainage stone (cu yd) = length (ft) × stone width (ft) × total block height (ft) ÷ 27
Buried depth follows a common manufacturer rule of about 1 in. for every 8 in. of total wall height, with a minimum of one full course. Because the buried part counts toward total height, that works out to about 1/7 of the exposed height. Your manufacturer's guide may differ, so check it.
Worked example: a 20 ft wall, 3 ft tall, with 16 × 6 in. blocks
- Blocks per course: 20 ft = 240 in., and 240 ÷ 16 = 15 blocks.
- Courses above grade: 36 ÷ 6 = 6 courses.
- Buried course: 36 ÷ 7 = 5.1 in., which rounds up to 1 full 6 in. course. Total: 7 courses.
- Wall blocks: 15 × 7 = 105. Add 5% waste: 105 × 1.05 = 110.25, round up to 111 blocks.
- Caps at 12 in.: 240 ÷ 12 = 20. Add 5%: 21 caps.
- Base, 24 in. wide × 6 in. deep: 20 × 2 × 0.5 = 20 cu ft. Add 10%: 22 cu ft, or 0.81 cu yd (about 1.2 tons).
- Drainage stone, 12 in. wide × 42 in. of block height: 20 × 1 × 3.5 = 70 cu ft. Add 10%: 77 cu ft, or 2.85 cu yd (about 3.9 tons).
- Drain pipe: about 20 ft, plus whatever it takes to reach a daylight outlet.
The drainage stone is often the biggest surprise. Behind a 3 ft wall it takes more than three times the volume of the base, so price both before you order.
Retaining wall block sizes
Most retaining wall blocks sold at US home centers and landscape yards fall into a few face sizes. Face size is what matters for the count; block depth (front to back) matters for the trench width and the wall's stability. Many blocks are made slightly under their nominal size, so if the spec sheet lists an exact face like 11.5 × 4 in., enter it as a custom size.
| Face size | Face area | Blocks per sq ft of wall face | Blocks per 10 ft course | Courses per ft of height |
|---|---|---|---|---|
| 12 × 4 in. (garden wall) | 0.33 sq ft | 3 | 10 | 3 |
| 16 × 6 in. | 0.67 sq ft | 1.5 | 8 (7.5) | 2 |
| 18 × 6 in. | 0.75 sq ft | 1.33 | 7 (6.67) | 2 |
| 18 × 8 in. | 1 sq ft | 1 | 7 (6.67) | 1.5 |
| 12 × 8 in. | 0.67 sq ft | 1.5 | 10 | 1.5 |
Small garden wall blocks are light and easy to handle, but they are generally meant for planters, tree rings and low walls. Larger blocks are much heavier, and many have a rear lip or pins that set each course back slightly. That setback leans the wall into the hill. It doesn't change the block count in any meaningful way.
Base, buried course and drainage
- Trench: dig deep enough for the gravel base plus the buried course. With a 6 in. base and one 6 in. buried course, that's 12 in. below the finished grade in front of the wall. Make it wide enough for the block depth plus about 6 in. on each side.
- Base: use crushed stone with fines (often sold as crusher run, road base, paver base or 3/4 in. minus), compacted in thin layers with a plate compactor. A 6 in. compacted base is a common minimum; some manufacturers allow less for short garden walls. Level the first course carefully. Every course above it follows that first course.
- Drainage stone: fill the zone directly behind the blocks with clean crushed stone (such as #57 stone, roughly 3/4 in. with no fines), usually 12 in. wide and the full height of the wall. If your blocks have hollow cores, fill them with the same stone; that adds to the total.
- Drain pipe: run 4 in. perforated pipe along the back of the bottom course and slope it to an outlet where water can drain away, at one or both ends of the wall. Many installers wrap the stone in landscape fabric to keep soil from clogging it.
The calculator prices stone by weight using about 1.5 tons per cubic yard for base gravel and 1.35 tons per cubic yard for clean crushed stone, the same figures our gravel calculator uses. Weights vary with the stone and moisture, so confirm with your supplier if they sell by the ton.
How tall can you build it yourself?
Height limits depend on the block, the soil and what sits above the wall. Walls up to about 3 ft are common DIY projects with most block systems. Taller walls usually need layers of geogrid reinforcement extending back into the compacted soil, and many jurisdictions require a permit, an engineered design or both at around 4 ft. Codes based on the IRC and IBC often measure that 4 ft from the bottom of the footing, not from the ground in front of the wall. A slope, driveway, patio or building above the wall adds load and can trigger engineering on a shorter wall. Check local code and the block manufacturer's maximum height before you buy.
If the job calls for terracing, two shorter walls set well apart are often simpler than one tall wall, but the upper wall still loads the lower one if it's too close. The manufacturer's guide gives the minimum spacing.
Buying tips
- Buy all wall blocks and caps at the same time. Color varies from batch to batch.
- Caps are usually glued down with masonry or landscape block adhesive. One tube typically covers a few linear feet of cap, so check the coverage on the tube.
- For a curved wall, the inside (concave) curve needs a little extra cutting and the outside curve can open gaps between blocks. Add 5% or more waste.
- Planning the area in front of the wall? Use the paver calculator for a patio, the topsoil calculator for backfill and planting beds, or the cubic yards calculator for any other fill.
Frequently asked questions
How many retaining wall blocks do I need?
Divide the wall length in inches by the block face width to get blocks per course, then multiply by the number of courses, including the buried one. A 20 ft wall that is 3 ft tall, built with 16 × 6 in. blocks, needs 15 blocks per course × 7 courses = 105 blocks, or 111 with 5% waste.
How tall can a retaining wall be without a permit?
It depends on where you live. Many codes based on the IRC and IBC exempt walls up to 4 ft, measured from the bottom of the footing to the top of the wall, unless the wall holds back a slope, driveway or other added load. Some cities set lower limits, so check with your local building department.
How much gravel do I need behind a retaining wall?
A typical drainage zone is 12 in. wide and runs the full height of the wall. For every 10 ft of a wall with 42 in. of total block height, that is 10 × 1 × 3.5 = 35 cu ft, or about 1.3 cu yd of clean crushed stone. The base gravel under the wall is a separate amount.
How deep should a retaining wall base be?
A compacted crushed-stone base of about 6 in. is a common minimum, with the first course of blocks buried below grade on top of it. Many manufacturers call for burying about 1 in. per 8 in. of wall height, with at least one full course. Follow your block manufacturer's guide.
Does a retaining wall need a drain pipe?
Most do. Water that builds up behind a wall pushes on it and is a leading cause of bowing and failure. A 4 in. perforated pipe at the bottom of the drainage stone, sloped to an outlet, lets that water escape. Very short garden walls in sandy, well-drained soil are sometimes built without one, but the pipe is cheap insurance.
What is the best gravel for behind a retaining wall?
Use clean, angular crushed stone without fines, such as #57 stone. It drains freely and locks in place. Avoid pea gravel and anything with sand or dust, which hold water. Use crushed stone with fines for the compacted base under the blocks instead.
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