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Retaining Wall Block Calculator

Enter the wall length and exposed height, the block size and how many courses are buried. The calculator estimates blocks, cap units, the drainage gravel behind the wall and the length of drain pipe. A 6 m wall 600 mm high with 400 × 150 mm blocks and one buried course needs 75 blocks and 15 caps before waste. Walls taller than 1 m (4 ft) are flagged for an engineer.

Estimates only. Structural elements such as beams, footings, stairs, retaining walls over 1 m (4 ft) and rebar design must be designed or approved by a licensed engineer and comply with your local building code and permits. Results are "within typical limits", never "approved" — verify with your building authority.

Measurements

Units

Height above the finished ground in front of the wall.

Leave as is if you won't use caps; set to 0 for none.

Courses below ground level. Follow the block manufacturer's installation guide.

Width of free-draining gravel directly behind the blocks.

Results

Blocks

105 pcs

Caps
20 pcs
Courses (including buried)
5
Drainage gravel
1.85 yd³
Drain pipe
20 ft
  • Within limit: exposed height up to 4 ft (above this, get an engineer's design)
Show the working
  1. Courses⌈2 ft ÷ 6 in⌉ + 1 = 5
  2. Blocks per course⌈20 ft ÷ 12 in⌉ = 20
  3. Blocks with waste⌈5 × 20 × (1 + 5%)⌉ = 105
  4. Gravel behind the wall20 ft × 2.5 ft × 12 in = 1.85 yd³

Material takeoff

ItemQuantityUnit priceCost
Wall blocks105 pcs–
Cap blocks20 pcs–
Drainage gravel1.85 yd³–
Perforated drain pipe20 ft–
Estimated total–

Enter your local prices to estimate the cost. Prices aren't stored anywhere.

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How to use the retaining wall calculator

  1. Choose Metric or Imperial.
  2. Enter the wall length and the exposed height above the finished ground in front of the wall.
  3. Enter the block length and block height from the product sheet, and the cap length if you will use caps.
  4. Set the number of buried courses recommended by the block manufacturer.
  5. Enter the width of drainage gravel behind the wall and a waste allowance.
  6. Price the takeoff, or add it to your project materials list.

How the estimate works

Courses = ⌈exposed height ÷ block height⌉ + buried courses. Blocks per course = ⌈wall length ÷ block length⌉. Blocks = courses × blocks per course × (1 + waste), rounded up. Caps = ⌈wall length ÷ cap length⌉. Drainage gravel = wall length × (exposed height + buried depth) × gravel width. Drain pipe = wall length.

Worked examples

Garden wall. 6 m long, 600 mm exposed, 400 × 150 mm blocks, one buried course: 4 + 1 = 5 courses of 15 blocks = 75 blocks, 15 caps, and 6 × 0.75 × 0.3 = 1.35 m³ of drainage gravel.

US planter wall. 20 ft long, 2 ft exposed, 12 × 6 in blocks, one buried course: 4 + 1 = 5 courses of 20 blocks = 100 blocks, plus 20 caps.

How to read the check

The height check compares the exposed height with 1 m (4 ft). Above that, the calculator shows a warning: get an engineer’s design and check whether you need a permit.

Tips and common mistakes

  • Start with a level, compacted base as the block manufacturer specifies; the first course decides how straight the wall will be.
  • Step the base on sloping ground instead of tilting the blocks.
  • Use clean, free-draining gravel behind the wall, not the soil you dug out.
  • Lead the drain pipe to daylight or a drain so water has somewhere to go.
  • Allow for curves and corners, which use more cut blocks.

Assumptions and limitations

The wall is treated as a straight run of constant height. For stepped or curved walls, run the calculator per section. Geogrid reinforcement, base material and surcharge loads aren’t included. Retaining walls are structural: have them designed or checked by a qualified professional where required.

Frequently asked questions

How many retaining wall blocks do I need?

Divide the wall height by the block height and round up, add the buried courses, then multiply by the number of blocks per course (wall length ÷ block length, rounded up). Add a few percent for cuts and breakage.

How many courses should be buried?

Most block manufacturers specify at least one buried course, and more for taller walls. Follow the installation guide for your block system.

Why do I need gravel behind a retaining wall?

Free-draining gravel lets water escape instead of building up pressure behind the wall. A perforated drain pipe at the base carries the water away.

When does a retaining wall need an engineer?

Many building authorities require an engineer’s design and a permit for retaining walls above about 1 m (4 ft), or for walls that support a driveway, building or slope. Check your local rules before you start.

Does the calculator design the wall?

No. It estimates materials only. It doesn’t check stability, soil pressure, reinforcement or drainage design.

Do I need caps on a retaining wall?

Caps finish the top course and protect it from weather. Many block systems recommend gluing them with a masonry adhesive. Set the cap length to 0 if you won’t use them.

Sources