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Retaining Wall Calculator

Count the blocks, cap stones and gravel you need for a block retaining wall, from its length, height and block size.

Level: Grades 6–7

  • Common Core standard 7.G.B.6Solve real-world and mathematical problems involving area, volume and surface area of two- and three-dimensional objects composed of triangles, quadrilaterals, polygons, cubes, and right prisms.
Printable worksheet

Retaining Wall Calculator

Inputs

Height above the finished ground in front. Taller walls often need an engineered design and a permit; check locally.

Burying the first course is common practice. Follow the block maker's guide.

Enter 0 for no caps.

The base trench is the block depth plus this extra width.

Crushed stone is roughly 1.3–1.5 US tons per cubic yard. Ask your supplier.

Result

Wall blocks

111

Order about 111 blocks, 21 caps and 3.33 yd³ (4.7 tons) of gravel.

Cap stones
21
Courses (rows), including buried
7
Blocks per course
15
Gravel (base + drainage)
3.33 yd³
Gravel weight
4.67 tons
Drainage gravel
70 ft³
Base gravel
20 ft³

See the step-by-step solution ↓

Step-by-step solution

  1. Blocks per course: wall length ÷ the 16 in block, rounded up, since you can't buy part of a block

    240 in16 in=15=rounded up:15

  2. Courses (rows): height above ground ÷ the 6 in block height, rounded up so the wall is tall enough, plus 1 buried

    6+1=7

  3. Blocks: per course × courses, plus 5% extra for cuts and breakage, rounded up

    15×7×1.05=110.25=rounded up:111

  4. Caps: wall length ÷ cap length, plus the same extra, rounded up

    24012×1.05=rounded up:21

  5. Drainage gravel behind the wall: length × built height × zone width (all in feet)

    20×3.5×1=70 ft³

  6. Base gravel under the wall: length × trench width × base depth

    20×2×0.5=20 ft³

  7. Add them up. Divide by 27 for cubic yards, then × 1.4 for tons, the ways suppliers sell it

    (70+20)27=3.333 yd³=4.667 tons

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

What this calculator does

Find out how many blocks, cap stones and tons of gravel to order for a block retaining wall. A segmental retaining wall is built from concrete blocks stacked without mortar. They hold back a slope with their weight and the lips that lock each row to the one below.

Planning one means counting three things. You need the blocks in every course (row), the cap stones that finish the top, and the gravel. Gravel goes underneath as a level base and behind the wall to let water drain.

The calculator divides the wall length by the block's face length to get the blocks in one course. It divides the height above ground by the block height to get the number of courses, then adds any buried courses. It adds an allowance for cut and broken blocks and counts caps along the top. Then it works out the gravel for the base trench and the drainage zone, in cubic feet, cubic yards and tons.

Where it's used

Why this math matters outside the classroom:

  • Garden terraces. Homeowners building a 20 ft wall from 16 in blocks divide 240 in by 16 in to get 15 blocks in each course.
  • Gravel orders. Landscapers turn 90 ft³ of base and drainage gravel into 90 ÷ 27 = 3.33 cubic yards to order from the quarry.
  • Course counts. A 30 in high wall of 4 in blocks needs 7.5 courses, which rounds up to 8 so the wall reaches full height.
  • Volume practice. Students find the drainage zone as a rectangular prism: 20 ft long, 3.5 ft high and 1 ft wide, or 70 ft³.
  • Project budgets. Contractors multiply 111 blocks by the price per block and add the gravel tonnage to quote a small wall.

Grade level and standards: Grades 6–7; Common Core State Standards for Mathematics 7.G.B.6. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.

How to use it

  1. Enter the wall's length and the height you want above the ground in front of it.
  2. Enter the block's face length, height and depth from the product sheet. Then enter how many courses will be buried.
  3. Enter the cap length (or 0 for none) and the drainage zone width. Then enter the base depth and how much wider than the block the trench will be.
  4. Set the gravel weight and the extra percentage for blocks. Then read the counts and the gravel to order.

What the inputs mean

Exposed wall height
Height above the finished ground in front. Taller walls often need an engineered design and a permit; check locally.
Courses below ground
Block makers usually call for at least one course below ground level. Taller walls and walls on a slope need more. The installation guide for your block gives the exact rule.
Cap length
Enter 0 for no caps.
Gravel drainage zone behind the wall
Clean crushed stone behind the wall lets water drain away instead of building up pressure. A zone about 12 in wide is common for small walls, often with a drain pipe full of holes at the bottom.
Compacted gravel base depth
A packed-down base of crushed stone, commonly about 6 in deep, keeps the first course level and stable.
Trench width beyond the block depth
The base trench is the block depth plus this extra width.
Gravel weight
Suppliers sell gravel by the ton or by the cubic yard. The weight per yard varies with the stone and how wet it is, so ask for their figure.

Formula and method

blocks per course = wall length ÷ block length, rounded up
courses = exposed height ÷ block height, rounded up, + buried courses
blocks = blocks per course × courses × (1 + extra ÷ 100), rounded up
caps = wall length ÷ cap length × (1 + extra ÷ 100), rounded up
gravel (ft³) = length × built height × drainage width + length × trench width × base depth;  yd³ = ft³ ÷ 27

Exposed height is the part of the wall above ground. Built height adds the buried courses. Extra is the percentage you add for cuts and breakage. ft³ is cubic feet and yd³ is cubic yards, and one cubic yard holds 27 cubic feet.

Lengths are turned into inches to match the block sizes. Each course is rounded up to whole blocks, because a partial block still needs a full one to cut from. The courses are rounded up too, so the wall reaches at least the height you want.

The drainage zone runs the full built height, buried courses included. The base trench is the block depth plus some extra working room, dug to the base depth.

Worked example

A 20 ft garden wall, 3 ft high, built from 16 × 6 × 12 in blocks with one buried course. It has 12 in caps, a 12 in drainage zone and a 6 in base in a trench 12 in wider than the block. Gravel weighs 1.4 tons per yard, and you allow 5% extra blocks.

  1. Per course: 240 in ÷ 16 in = 15 blocks.
  2. Courses: 36 in ÷ 6 in = 6, plus 1 buried = 7.
  3. Blocks: 15 × 7 × 1.05 = 110.25, rounded up to 111. Caps: 240 ÷ 12 × 1.05 = 21.
  4. Drainage gravel: 20 × 3.5 × 1 = 70 ft³. Base: 20 × 2 × 0.5 = 20 ft³.
  5. Total gravel: 90 ft³ ÷ 27 = 3.33 yd³, about 4.67 tons.

Result: Order 111 blocks, 21 caps and about 3.33 cubic yards (4.67 tons) of gravel.

Understanding the result

The block count is the number to order. Corners and curves need extra cuts, so raise the allowance for walls with lots of them.

Order a bit more gravel than the figure. Packing it down shrinks its volume, and running short halfway through a base is a nuisance.

Assumptions and limitations

  • The wall is straight and the same height along its length. It's built from one size of block, with each row's joints staggered over the row below (running bond).
  • The drainage zone and base are rectangular when seen from the end.
  • Steps in the base on sloping ground, curves, corners and stairs all change the counts.
  • It doesn't size geogrid (plastic mesh that ties taller walls into the soil), drain pipe, filter fabric or the soil behind the wall.
  • Wall height limits, how far each row steps back, and engineering rules come from the block maker and local codes. Taller walls usually need a design by a qualified professional.

Frequently asked questions

Why bury the first course?

A buried course stops the bottom of the wall from sliding forward. It also protects the base gravel from washing away. Many guides call for burying about 1 in of block for every 8 in of wall height. They also want at least one course, or half a block, below ground.

Do I need gravel behind a small retaining wall?

Drainage is what keeps most block walls standing. Wet soil is heavy and pushes hard. So even a short wall benefits from a zone of clean crushed stone behind it.

How many caps do I need?

Divide the wall length by the cap length and add a little for cuts. Caps are usually glued to the top course with a masonry adhesive.

How tall can I build a block retaining wall myself?

Each block system lists the tallest wall you can build without reinforcement. Many local codes require a permit or an engineered design above about 3 to 4 ft. They may also require one when the wall holds back a driveway or building. Check both before you start.

How this calculator is checked

Method: Running-bond block count by length and course height with whole-unit rounding; gravel as rectangular prism volumes converted at 27 ft³ per yd³. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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