Retaining wall calculator: blocks, caps and gravel

Quick answer

A wall 20 ft long and 2 ft high built from blocks with an 18 × 8 in face needs 14 blocks per course, 3 courses above the ground and 1 buried: 56 blocks, or 59 with 5 percent extra, plus 14 caps. Under it goes a gravel pad 24 in wide and 6 in thick, and behind it 12 in of drainage gravel, together about 2.7 cubic yards or 4.4 tons.

What this calculator does

It counts segmental wall blocks, the dry-stacked concrete blocks sold for garden walls, retaining walls and fire pits. From the face size of one block it works out the blocks per course and the number of courses, adds the courses buried below the ground, and applies an allowance. It also counts cap blocks and the gravel for the levelling pad under the wall and the drainage zone behind it.

In fire pit mode it counts a ring of tapered blocks from the inside diameter, and shows the inside diameter at which the courses close with whole blocks.

It does not design the wall. Whether a wall stands depends on the soil, the slope above and below it, water, and loads such as a driveway or a fence. See the safety notes below before you build anything that holds back soil.

Height limits and when you need an engineer

Block makers publish the height to which their blocks can be stacked without reinforcement. Allan Block, for its AB Classic block with level ground above, gives 3 ft 2 in in clay and 4 ft 7 in in silty sand, and much less with a slope or a load above the wall; Versa-Lok says its standard units are stable to about 4 ft in good soil with level backfill. Above those heights the wall needs geogrid reinforcement designed for the site.

Building rules set limits too. In the United States, the International Residential Code (section R404.4, 2024 edition) requires an engineered design for a wall that is not supported at the top and retains more than 48 in of soil, or more than 24 in when it also carries other loads; walls up to 4 ft, measured from the bottom of the footing, are usually exempt from a permit unless they support a surcharge (R105.2). In Queensland, a wall up to 1 m with no load above it and at least 1.5 m from a building needs no approval; other states set their own rules. The UK government advises expert help for any wall that retains soil.

The calculator shows a note on straight walls from about 3 ft (0.9 m). That threshold is this site's choice, near the lowest of the limits above. Check your block maker's chart and your local rules, and have tall walls designed by an engineer.

How to use it

1. Choose metric or US units. In US units you can type feet and inches, such as 20' 6".
2. Choose a straight wall or a round fire pit. For a wall, enter its length; for a fire pit, the inside diameter.
3. Enter the height above the finished ground and the number of courses you bury. The examples bury one course of a garden wall and none of a fire pit.
4. Enter the face length, height and depth of one block from the product sheet. For a fire pit, use the outer face of the tapered block.
5. Choose whether you use caps and enter the cap length, then the allowance.
6. Check the gravel pad and drainage zone and the weight of the gravel your supplier sells, then select Calculate.

How the result is calculated

1. Courses above the ground = height ÷ block height, rounded up. Courses = courses above the ground + buried courses.
2. Length of a course: for a straight wall, the wall length; for a fire pit, π × (inside diameter + 2 × block depth), the circumference at the outer faces of the blocks.
3. Blocks per course = length of a course ÷ block face length, rounded up. Blocks = blocks per course × courses × (1 + allowance ÷ 100), rounded up.
4. Caps = length of a course ÷ cap length × (1 + allowance ÷ 100), rounded up.
5. Gravel pad = length along the middle of the blocks × pad width × pad thickness. Drainage gravel (straight walls) = wall length × drainage width × the height of all courses.
6. Weight of gravel = (pad + drainage) × weight of the gravel.
7. Fire pit: inside diameter for whole blocks = blocks per course × face length ÷ π − 2 × block depth.

Tapered fire pit blocks touch along their outer faces, so the course is measured there. We checked this with a Pavestone fire pit kit of 45.8 in outside diameter: the outer faces of one course add up to 144.0 in, and π × 45.8 = 143.9 in.

Worked example

US units: a garden wall 20 ft long and 2 ft high, of blocks with an 18 in by 8 in face, 12 in deep. Blocks per course: 20 × 12 ÷ 18 = 13.3, so 14. Courses above the ground: 24 ÷ 8 = 3, plus 1 buried, 4 courses. Blocks: 14 × 4 = 56, with 5 percent extra 58.8, so 59. Caps of 18 in: 13.3 × 1.05 = 14. Face above the ground: 20 × 2 = 40 sq ft.

Gravel pad 24 in wide and 6 in thick: 20 × 2 × 0.5 = 20 cu ft, 0.75 cubic yards rounded up. Drainage 12 in wide behind 4 courses of 8 in: 20 × 1 × 2.67 = 53.3 cu ft, 1.98 cubic yards. Together 73.3 cu ft at 120 lb per cu ft: 8,800 lb, 4.4 tons.

Metric: a wall 6 m long and 0.6 m high of blocks with a 460 × 200 mm face, 300 mm deep: 6 ÷ 0.46 = 13.04, so 14 per course; 0.6 ÷ 0.2 = 3 courses plus 1 buried; 56 blocks, 59 with 5 percent; 14 caps; face 3.6 m². Pad 6 × 0.6 × 0.15 = 0.54 m³; drainage 6 × 0.3 × 0.8 = 1.44 m³; 1.98 m³ at 1923 kg/m³ is 3.81 t.

Fire pit: an inside diameter of 33 in with tapered blocks of 10 1/4 in face, 7 in deep and 3 1/2 in high. The outer faces lie on a circle of 33 + 14 = 47 in: π × 47 = 147.7 in ÷ 10.25 = 14.4, so 15 blocks per course. At 12 in high, 4 courses and 60 blocks, 63 with 5 percent. With 15 blocks the courses close at an inside diameter of 15 × 10.25 ÷ π − 14 = 34.9 in.

Fire pit rings by diameter

Blocks per course for the example tapered block (10 1/4 in outer face, 7 in deep, 3 1/2 in high), four courses, without allowance. Adjust the diameter to the one in the fourth column so the ring closes with whole blocks.

Concrete wall blocks are not made for direct fire. Unilock says its wall stones are not fire rated and a fire pit should be lined with fire brick, and Belgard recommends a metal fire ring or insert. Belgard also advises placing a fire pit 10 to 20 ft from the house; check local fire rules.

Base, burial and drainage

Allan Block digs a trench 24 in wide and 6 in deep for the gravel base, plus 1 in for every foot of wall height for the buried block; for walls under 4 ft it allows 18 in wide and 4 in deep. Versa-Lok asks for a levelling pad at least 6 in thick and 24 in wide and buries one tenth of the exposed height. The concrete masonry association NCMA gives a pad at least 6 in thick reaching 6 in in front of and behind the blocks, and a burial of at least 6 in.

Behind a retaining wall, Allan Block, Versa-Lok and NCMA all place at least 12 in of clean crushed stone, and a perforated drain pipe for taller walls or poor drainage: Versa-Lok above 3 ft, Allan Block for gravity walls above 4 ft. The calculator fills the drainage zone to the full height of the blocks; many builders finish the top few inches with soil instead.

The gravel weight preset, 120 lb per cu ft (1923 kg/m³), is the unit weight Allan Block uses for wall rock in its estimating guide. Ask your supplier for the weight of the stone you buy. The 5 percent allowance for blocks is this site's starting value for breakage and cut blocks at the ends; it is not a figure from a block maker.

Common mistakes

• Forgetting the buried course. The first course sits below the ground and counts as blocks to buy.
• Using a rounded size instead of the face size on the product sheet. Pavestone's small RockWall block, for example, has a face 11.63 in long and 4 in high.
• Counting a fire pit from the inside diameter with the outer face length. That undercounts; the outer faces lie on a larger circle.
• Skipping the drainage gravel. Water pressure behind a wall is a common reason for failure.
• Building above the height in the block maker's chart without geogrid or a design, or ignoring a slope, driveway or fence above the wall.