Retaining Wall Calculator

How to calculate a block retaining wall

Segmental retaining wall blocks are dry-stacked in courses. Count the courses from the height, the blocks per course from the length, then add the buried course and a little extra.

Rounding the courses up means the wall comes out a little taller than you asked for, never shorter. Keep that in mind near a height limit. The calculator’s height warnings use this built height: the 3 ft example builds to 40 in (3.33 ft), so it gets the over-3-ft message about geogrid.

Example: 20 ft long, 3 ft high, 8 × 18 in blocks

1. Courses above ground: 36 in ÷ 8 in = 4.50 → 5 courses (40 in of wall showing)
2. Plus 1 buried course = 6 courses
3. Blocks per course: 240 in ÷ 18 in = 13.33 → 14
4. Blocks: 14 × 6 = 84; + 5% = 88.2 → 89 blocks
5. Caps, if they are 18 in long: 240 ÷ 18 = 13.33 → 14 caps
6. Base: 20 ft × 2 ft × 0.5 ft = 20 cu ft compacted; × 1.4 = 28 cu ft = 1.04 cu yd of compactable crushed stone (1.3–1.7 tons)
7. Drainage stone: 20 ft × 1 ft × 4.00 ft (all 6 courses) = 80 cu ft compacted; × 1.14 = 91 cu ft = 3.38 cu yd of #57 stone (4.2–4.9 tons)

At 75 lb each (AB Classic), 89 blocks weigh about 6,675 lb, and at 85 lb (Rockwood Classic 8) about 7,565 lb, before caps.^[4][5] Plan on delivery.

Example: 40 ft long, 2 ft high, 6 × 16 in blocks

1. Courses: 24 ÷ 6 = 4 above ground + 1 buried = 5
2. Blocks per course: 480 ÷ 16 = 30
3. Blocks: 30 × 5 = 150; + 5% = 157.5 → 158 blocks, plus 30 caps
4. Base: 40 cu ft × 1.4 = 2.07 cu yd; drainage stone: 40 × 1 × 2.5 = 100 cu ft; × 1.14 = 4.22 cu yd

Block sizes

What matters for the count is the size of the block’s face: height × length. Three of the blocks below have an 8 × 18 in face and cover about 1 sq ft each; 6 in high blocks cover less, so you need more of them.

Face areas are length × height ÷ 144 and match the makers’ figures: about 1 sq ft (Allan Block, Rockwood), 0.667 sq ft (VERSA-LOK) and 34.9 sq ft per 48-block pallet (Pavestone). Belgard’s is our calculation from its listed size. Smaller wall blocks are sold too; measure the face of the block you buy and enter it as the closest size, or check the count by hand with the formula above.

The buried course

The first course goes below ground. A DIY guide hosted by The Home Depot says to bury the whole base course; Allan Block asks for at least 6 in of buried block, and VERSA-LOK for about one tenth of the exposed height.^[3][9][10] For a 3 ft wall that is 3.6 in by the VERSA-LOK rule, so a full buried course of 6 or 8 in meets all three. The calculator adds one buried course by default; add more if your block maker’s tables call for it.

Base and drainage stone

The wall sits on a compacted gravel pad in a trench. Allan Block and VERSA-LOK both describe a base about 24 in wide and at least 6 in deep; Allan Block also gives a smaller 18 in wide, 4 in deep option for walls under 4 ft.^[11][10][12] The trench depth is the base plus the buried course: 14 in for 8 in blocks on a 6 in base.

The calculator figures the base as compactable crushed stone (crusher run or road base) compacted to 6 in, weighs it at 1.30–1.60 tons per loose cubic yard, and orders 1.4 times the compacted volume. A compacted base takes about 1.33 to 1.5 times its volume in loose stone: 1.33 in Penn State’s 8 in to 6 in rule, about 1.4 from Tennessee DOT’s weights and 1.34–1.52 in Wisconsin DOT’s tests; FHWA reports a volume loss of 30% or more on gravel roads.^{[13][14][15][16]} Allan Block fills its base with the same “wall rock” it uses behind the wall.^[11] If your base is clean crushed stone such as #57, it barely compacts, so order about the trench volume plus 10–15% instead of the calculator’s base figure.^[14]

Behind the wall, all three guides call for a zone of drainage stone 12 in deep from the back of the block, and VERSA-LOK’s specification asks for at least 1 cu ft of drainage stone per square foot of exposed face.^{[11][10][12][3]} The calculator fills that 12 in zone the full height of the blocks. In the first example that is 80 cu ft against a VERSA-LOK minimum of 67 cu ft. The guides compact this stone, so the calculator orders 1.14 times the zone’s volume, Tennessee DOT’s factor for compacted clean stone. Weights use loose #57 stone at 1.25–1.45 tons per cubic yard.^[14][17] Allan Block adds a drain pipe for gravity walls over 4 ft and compacts the backfill in layers no more than 8 in thick.^[11]

How high can you build without an engineer?

The Allan Block heights are exposed height including the cap, without earthquake loads. We left out one row of that chart (level sand and gravel) because it shows the same height as clay, which looks like an error. In clay, or with a slope or load above the wall, the chart’s heights run from 1 ft 4 in to 3 ft 2 in.

Watch how the permit exemption is measured. The first example shows 40 in above ground, but from the bottom of the 6 in base to the top of the blocks it is 6 × 8 + 6 = 54 in (4 ft 6 in), over 4 ft before the caps go on. The 2 ft example is 36 in (3 ft). The calculator measures the same way and, once a wall passes 48 in from the bottom of its base, warns that the exemption may not apply. Ask your building department how they measure and whether your city or county has amended these rules.

Before you order

• Add extra for cuts and breakage: 5% for a straight wall, and another 10% for a curved one, per the Home Depot-hosted guide.^[3]
• Cap blocks are not always the same length as wall blocks. The calculator assumes caps as long as the wall blocks; divide the wall length by your cap’s length if it is different.
• Corners, steps and returns need extra blocks or corner units. Count them from the maker’s layout guide.

Related calculators

• Gravel calculatorBase and drainage stone in yards, tons or bags
• Crushed stone calculator#57 and crusher run with compaction
• Dirt calculatorFill and backfill by the cubic yard
• Project materials listPrint everything you added from each calculator

Prepared by the Yard & Build editors. The formulas behind this calculator are checked against hand-worked answers every time the site is built. Block sizes and code values checked September 26, 2026.