Retaining Wall Block Calculator
Count the blocks, caps, base gravel and drainage stone for a retaining wall, and check whether your block is made to stand as tall as you want.
The blocks are the easy part to count. What people run short on is everything around them: a row of blocks that goes below ground, the gravel pad under that row, and a foot of drainage stone behind the whole wall, which often weighs more than the blocks. This counts all of it from the block makers' own estimating method, and checks the height against what your block is built for.
The wall
Most garden walls are 15 to 30 feet long and 1 to 3 feet tall.
Wall length
Height above the ground
Block size
Pick the face size closest to the block you're looking at. Bigger, deeper blocks can hold back a taller wall.
Your soil
Dig down a foot, grab a handful and squeeze. Most yards are clay.
Standard 6-inch blocks to buy
109 blocks
Includes one row below ground and a few spares
Blocks are the wall's face divided by one block's face, plus a row below ground and 5 percent spare. The gravel under the wall and the stone behind it follow the block makers' own guides. Height limits assume flat ground above and below the wall and nothing heavy near the top, like a driveway. 2026 figures.
💡About this calculator▼
Counting retaining wall blocks looks like simple division, and that's how people end up a pallet short. The wall has a row nobody sees, buried below the grass to keep the bottom from kicking out. It sits on a compacted gravel pad. And behind the whole face goes a foot of clean drainage stone, which on a 3-foot wall weighs more than the blocks do.
This counts all of it, using the block makers' own estimating method: blocks with the buried row and 5 percent for cuts, caps and the adhesive to hold them, the base gravel, the drainage stone, and the drain pipe when your soil or height calls for one.
It also tells you if the wall is too tall for the block. Every dry-stacked block has a height it's built for, lower for small garden blocks and lower in clay. If yours is past it, the result says so and what to do instead.
Blocks come from the face of the wall. Length times height gives the face in square feet, and each block covers its own face: about a third of a square foot for a small 4-inch garden block, about three-quarters for a standard 6 by 17½-inch block, and about one square foot for a large 8 by 18-inch block. Add one full row below ground, the length of the wall divided by one block's length, then 5 percent for cuts and breakage.
The base is a compacted gravel pad under that buried row. For the smaller blocks the trench is twice as wide as the block is deep, with 3 inches of compacted base for walls up to 1½ feet, 4 inches at 2 feet and 6 inches at 3 feet. For the large block it's an 18-inch trench with 4 inches of base, or 24 inches wide and 6 deep once the wall reaches 4 feet. A quarter is added for compaction.
Drainage stone fills 12 inches behind the whole face, plus a quarter for compaction. Blocks with open cores take 15 percent more of both, since the cores get filled too.
Caps run the length of the wall, plus 5 percent, glued down with about 3 ounces of exterior concrete adhesive per foot.
Worked example. A 30-foot wall, 2 feet tall, in standard 6-inch blocks: 60 square feet of face is 83 blocks, the buried row is 21 more, and with 5 percent that's 109 blocks. The small wedge caps come to 89, with 9 tubes of adhesive. The base is a 4-inch pad in a 14-inch trench, about 1.2 tons, and the drainage stone behind is about 2.8 cubic yards, roughly 3.8 tons. In clay, add 30 feet of perforated drain pipe.
📐How it's calculated▼
Blocks = (length × height ÷ one block's face + length ÷ one block's length) × 1.05, rounded up. Block faces: 4 × 11.6 in (small), 6 × 17.4 in (standard), 8 × 18 in (large).
Base = (trench width × base depth × length + block depth × block height × length ÷ 2) × 1.25, × 1.15 for cored blocks; tons at about 1.6 per cubic yard.
Drainage stone = length × height × 12 in × 1.25, × 1.15 for cored blocks; tons at about 1.35 per cubic yard.
Caps = length ÷ cap length × 1.05. Adhesive = length × 3 oz ÷ 10-oz tubes.
Height limits on level ground: small block 1½ ft; standard block 3 ft; large block 3 ft 2 in in clay, 4 ft 7 in in sandy soil. Over 4 ft, every block needs an engineered design and a drain pipe.
Default (30 × 2 ft, standard, clay): 109 blocks, 89 caps, 9 tubes, 1.2 tons of base, 3.8 tons of drainage stone, 30 ft of pipe.
Large block, 40 × 3 ft, clay: 154 blocks, 28 caps, 12 tubes, 2.9 tons of base, 8.6 tons of stone, 40 ft of pipe; 3 ft is inside the 3 ft 2 in limit.
📎Sources:Allan Block — Residential Landscape Retaining Wall Guide (gravity wall height chart, base trench, drainage),Belgard — Standard Wall Quick Reference Install Guide (leveling pad, buried course, 12 inches of drainage aggregate),Pavestone — RockWall Large product cut sheet (block dimensions and pallet coverage),Penn State Center for Dirt and Gravel Road Studies — Driving Surface Aggregate Handbook (tons of crushed aggregate per area)
🔍Finding your inputs▼
Wall length: Along where the wall will sit, in feet. For an L-shaped wall, add the legs together.
Height above the ground: How tall the wall will stand once built, caps included. The buried row is added for you, so don't count it. If the ground slopes along the wall, use the average height.
Block size: The face size closest to what you're buying. Small garden blocks (about 4 by 12 inches) are for edging and low beds. Standard wall blocks (about 6 by 17½) are the common choice for garden walls up to 3 feet. Large wall blocks (8 by 18, a foot deep) are for taller walls and slopes.
Your soil: Dig down a foot, take a handful and squeeze. If it holds together in a ball it's clay, which holds water and pushes harder on a wall; if it crumbles it's sandy or gravelly. Most yards are clay. Clay adds a drain pipe and, for the large block, lowers the height it can reach without reinforcement.
⚠️Special situations▼
How many retaining wall blocks do I need for a 30-foot wall?
In standard 6-inch blocks, about 109 for a 2-foot wall and 152 for a 3-foot wall, including the row that goes below ground and 5 percent for cuts. In large 8-by-18-inch blocks it's about 84 at 2 feet and 116 at 3 feet. The buried row alone is 21 standard blocks on a 30-foot wall, which is the part most counts leave out.
There's a driveway or a slope above my wall.
Then the height limits here are too generous. The block makers' limits assume level ground and no load near the top. Allan Block's own chart for its standard blocks in clay drops from 3 feet 2 inches on level ground to 2 feet 4 inches with a 3-to-1 slope above and 1 foot 4 inches with a driveway or patio behind. Keep the wall lower, add geogrid to the maker's chart, or get a design.
Can I use the small garden blocks for a 2-foot wall?
Not as a retaining wall. The small 4-inch blocks are made for walls up to about 1½ feet; above that, the soil behind them can push the wall out. For 2 feet, step up to a standard 6-inch block, which is built for up to 3 feet, or build two 1-foot walls stepped back from each other.
Why does the drainage stone weigh so much?
Because it's a foot-thick layer behind the whole face. A 30-foot wall 3 feet tall has 90 square feet of face, so the stone behind it is about 4.2 cubic yards, roughly 5.6 tons, while the 152 standard blocks themselves weigh about 3 tons. Skipping it is the most common reason block walls bulge: without it, water builds up behind the wall and pushes.
❓Common questions▼
How do I calculate how many retaining wall blocks I need?
Multiply the wall's length by its height above ground for the face in square feet, and divide by the face of one block: about 0.73 square feet for a 6 by 17½-inch block. Then add one row for below ground, the wall length divided by one block's length, and 5 percent for cuts. A 30-foot wall 2 feet tall in 6-inch blocks is 83 + 21 = 104, and 109 with the extra.
How much gravel do I need for a retaining wall base?
A compacted pad 3 to 6 inches deep in a trench about twice as wide as the block is deep, 3 inches for walls up to 1½ feet, 4 inches at 2 feet and 6 inches at 3 feet, plus a quarter extra for compaction. For a 30-foot wall 2 feet tall in standard 6-inch blocks that's about 1.2 tons of crushed base with fines. That's separate from the clean drainage stone that goes behind the wall.
How tall can a retaining wall be without reinforcement?
It depends on the block and the soil. Small 4-inch garden blocks are made for about 1½ feet and standard 6-inch blocks for about 3 feet. A large 8-by-18-inch block a foot deep can go about 3 feet 2 inches in clay and 4 feet 7 inches in sandy soil on level ground. Over 4 feet, the block makers call for a drain pipe and an engineered design, and a building permit may be required.
Do I need a drain pipe behind my retaining wall?
Yes if the wall is over 4 feet or your soil drains poorly, which includes most clay. The pipe is 4-inch perforated pipe laid along the back of the bottom row in the drainage stone and run out to daylight, with an outlet every 50 feet or so on a long wall. In sandy or gravelly soil under 4 feet, the foot of drainage stone behind the blocks is usually enough.
Should I bury the first row of retaining wall blocks?
Yes. The block makers all bury the first row, which keeps the bottom of the wall from sliding out. It sits on the compacted base pad, below the finished grade in front of the wall. That row is why a wall needs more blocks than its visible face suggests: on a 30-foot wall of standard blocks it's 21 extra.
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