How many retaining wall blocks do I need?

Divide the exposed wall height by the block face height to get the exposed courses, add the buried course, and multiply by the blocks per course (wall length ÷ block length, rounded up). A 20 ft wall, 3 ft high, in 16 × 6 in blocks is 6 + 1 = 7 courses of 15, or 105 blocks, and 116 with 10% extra for cuts and breakage. The table on this page lists other sizes.

How many courses should be buried?

Typically the first course goes below grade, or about 1 in for every 8 in of wall height, whichever your block maker’s installation guide gives; these are typical figures, so check the guide for your block. The calculator offers one buried course (the default), 1 in per 8 in of height in whole courses, or none for a wall that starts on a slab.

How do I work out cap blocks?

Divide the wall length by the length of one cap and round up. At 12 in per cap, a 20 ft wall takes 20; with 10% extra for cuts that is 22. Caps sit on top of the last course and are usually glued with masonry adhesive, so they are not counted in the exposed height.

How much gravel goes under and behind the wall?

Under the blocks, a typical base is a trench about 24 in wide with 6 in of compacted crushed stone: 0.81 cu yd (≈ 1.14 tons) for the 20 ft wall, with 10% extra. Behind the wall, a typical drainage column of clean crushed stone is about 12 in wide and as tall as the wall: 2.44 cu yd (≈ 3.42 tons). Tons use about 1.4 tons per cubic yard, the figure the gravel calculator uses; your supplier’s own figure wins. These widths and depths are estimates: your block’s installation guide sets the real ones.

Do I need a permit or an engineer for a retaining wall?

Often, once a wall is taller than about 4 ft, and sometimes at lower heights or when the wall holds up a slope, a driveway, a building or a neighbor’s land. Many walls that tall are also built with geogrid reinforcement. The height and the rules differ from place to place, so check local rules with your building department before you buy blocks. This calculator counts blocks and stone; it does not design a wall.

Why are the block sizes only nominal?

Block makers publish face sizes such as 12 × 4 in, 16 × 6 in and 18 × 6 in, but the real size differs a little by brand and by how the block sits in the wall. Use the size printed on the block you are buying, or pick custom and enter it.

Does the calculator include drain pipe, fabric and backfill?

No. It counts the blocks, the cap, the base gravel and the drainage stone. Drain pipe, landscape fabric behind the stone, geogrid, backfill soil and adhesive depend on the wall and the soil. See the landscape fabric and gravel calculators for those materials.

Retaining Wall Block Calculator: How Many Blocks Do I Need?

A 20 ft long retaining wall, 3 ft tall above grade, built from 16 × 6 in blocks with one buried course needs 116 blocks (7 courses of 15, with 10% extra), 22 cap blocks, 0.81 cu yd of base gravel and 2.44 cu yd of drainage stone. Blocks = (wall height ÷ block height + buried courses) × (wall length ÷ block length). Walls over about 4 ft usually need engineering, permits and geogrid: check local rules.

Figures based on the sources listed belowUpdated 2026-10-04How we calculate

Wall length

Exposed wall height

Extra blocks

Standard: cuts and breakage (10%)Minimal: straight wall (5%)Exact (0%)Custom

Block face sizeFace length × height as installed, nominal: check the block you are buying, because sizes vary by brand.

Buried first courseTypical rules from block makers’ installation guides: bury the first course, or about 1 in for every 8 in of wall height. Follow the guide for your block and your local rules.

Cap block length (in)

Length of one cap along the wall; 12 in is typical. Caps sit on top of the last course and are not part of the exposed height.

Base trench width (in)

Width of the gravel base under the blocks; 24 in is typical, more for taller walls.

Base gravel depth (compacted) (in)

Compacted depth of the base; 6 in is typical. Poor soil may call for more.

Drainage stone width behind the wall (in)

Width of the clean crushed stone column behind the blocks; 12 in is typical. It is counted up the exposed height.

Price per block (optional)

Buy 116 blocks

20 linear ft + 10% waste = 22 linear ft

Courses of block: 7 courses, 6 exposed + 1 buried
Blocks per course: 15 blocks
Cap blocks: 22 caps
Base gravel (compacted): 0.81 cu yd (≈ 1.14 tons)
Drainage stone behind the wall: 2.44 cu yd (≈ 3.42 tons)

Low

$348.00

Expected

$580.00

High

$1,044.00

Includes 2 linear ft waste

How this number is calculated

20 linear ft + 10% waste = 22 linear ft ÷ 0.19 linear ft per block = 116 blocks

Shopping list

Retaining wall block adhesive (construction adhesive)
Hand tamper
4 ft level
Rubber mallet
Masonry chisel and brick set
Perforated drain pipe (4 in)

Show diagram

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Blocks needed by wall length and height

16 × 6 in face blocks, one buried course, 10% extra, worked by this calculator at build time. Height is above grade, cap not included; add cap blocks, base gravel and drainage stone separately. Walls over about 4 ft usually need engineering: check local rules.
Wall length	2 ft high	3 ft high	4 ft high
10 ft	44 (5 courses)	62 (7 courses)	80 (9 courses)
20 ft	83 (5 courses)	116 (7 courses)	149 (9 courses)
50 ft	209 (5 courses)	293 (7 courses)	377 (9 courses)

Methodology & assumptions

The wall is treated as one straight run. Exposed courses = exposed height ÷ block face height, rounded up; buried courses are one by default (the first course goes below grade), or 1 in per 8 in of wall height in whole courses, or none for a wall that starts on a slab. Blocks per course = wall length ÷ block face length, rounded up; total blocks = courses × blocks per course × (1 + extra), rounded up once. Cap blocks = wall length ÷ cap length, with the same extra, and the cap is not part of the exposed height. The base gravel is the wall length × trench width (24 in) × compacted depth (6 in); the drainage stone is the wall length × column width (12 in) × exposed height. Both are volumes in cubic yards with the same extra, and tons use the 1.4 tons per cubic yard the gravel calculator uses (about 1.4 for crushed stone: your yard’s figure wins). The buried course, the 24 × 6 in trench and the 12 in drainage column are typical figures, not a design: they are estimates, so follow the installation guide for your block and your local rules. Curves, corners, steps, geogrid, drain pipe and fill are not counted; face sizes are nominal and vary by brand. Walls taller than about 4 ft (or whatever height your local rules set) usually need engineering, a permit and geogrid reinforcement; check local rules before you buy. The metric toggle shows lengths in m and volumes in m³.

Versa-Lok Retaining Wall Systems, Standard Unit installation guide (buried base course, compacted base, drainage zone behind the wall)
Allan Block, Retaining Wall Installation Manual (base trench, buried first course, drainage aggregate)
ICC-ES Evaluation Report ESR-1784, Versa-Lok Retaining Wall Systems (compacted crushed stone leveling pad)
National Concrete Masonry Association (NCMA), Design Manual for Segmental Retaining Walls (engineering of taller and reinforced walls)
Gravel density: 1.4 tons per cubic yard, as in this site’s gravel calculator (estimate; use your supplier’s figure)
Typical 24 × 6 in base trench, 12 in drainage column and 12 in cap length: rules of thumb (estimates), not from a single document

How to calculate retaining wall blocks

Measure the wall. Measure the wall length and the exposed height above the finished grade at the lowest point. Over about 4 ft, check local rules first.
Count the courses. Divide the exposed height by the block face height and round up, then add the buried first course.
Count blocks per course. Divide the wall length by the block face length and round up.
Add waste. Multiply courses by blocks per course and add about 10% for cuts and breakage.
Add caps and stone. Cap blocks = wall length ÷ cap length. Base gravel = length × trench width × compacted depth; drainage stone = length × column width × height, in cubic yards.

Worked examples

Project	Result
20 ft wall, 3 ft high, 16 × 6 in blocks, one buried course	20 linear ft + 10% extra → 116 blocks, 22 caps
10 ft wall, 2 ft high, 12 × 4 in blocks, one buried course, no extra	10 linear ft + 0% extra → 70 blocks, 10 caps
50 ft wall, 4 ft high, 18 × 6 in blocks, one buried course, 5% extra	50 linear ft + 5% extra → 322 blocks, 53 caps

Frequently asked questions

How many retaining wall blocks do I need?: Divide the exposed wall height by the block face height to get the exposed courses, add the buried course, and multiply by the blocks per course (wall length ÷ block length, rounded up). A 20 ft wall, 3 ft high, in 16 × 6 in blocks is 6 + 1 = 7 courses of 15, or 105 blocks, and 116 with 10% extra for cuts and breakage. The table on this page lists other sizes.
How many courses should be buried?: Typically the first course goes below grade, or about 1 in for every 8 in of wall height, whichever your block maker’s installation guide gives; these are typical figures, so check the guide for your block. The calculator offers one buried course (the default), 1 in per 8 in of height in whole courses, or none for a wall that starts on a slab.
How do I work out cap blocks?: Divide the wall length by the length of one cap and round up. At 12 in per cap, a 20 ft wall takes 20; with 10% extra for cuts that is 22. Caps sit on top of the last course and are usually glued with masonry adhesive, so they are not counted in the exposed height.
How much gravel goes under and behind the wall?: Under the blocks, a typical base is a trench about 24 in wide with 6 in of compacted crushed stone: 0.81 cu yd (≈ 1.14 tons) for the 20 ft wall, with 10% extra. Behind the wall, a typical drainage column of clean crushed stone is about 12 in wide and as tall as the wall: 2.44 cu yd (≈ 3.42 tons). Tons use about 1.4 tons per cubic yard, the figure the gravel calculator uses; your supplier’s own figure wins. These widths and depths are estimates: your block’s installation guide sets the real ones.
Do I need a permit or an engineer for a retaining wall?: Often, once a wall is taller than about 4 ft, and sometimes at lower heights or when the wall holds up a slope, a driveway, a building or a neighbor’s land. Many walls that tall are also built with geogrid reinforcement. The height and the rules differ from place to place, so check local rules with your building department before you buy blocks. This calculator counts blocks and stone; it does not design a wall.
Why are the block sizes only nominal?: Block makers publish face sizes such as 12 × 4 in, 16 × 6 in and 18 × 6 in, but the real size differs a little by brand and by how the block sits in the wall. Use the size printed on the block you are buying, or pick custom and enter it.
Does the calculator include drain pipe, fabric and backfill?: No. It counts the blocks, the cap, the base gravel and the drainage stone. Drain pipe, landscape fabric behind the stone, geogrid, backfill soil and adhesive depend on the wall and the soil. See the landscape fabric and gravel calculators for those materials.

Handy tools

Slope Calculator: Check the grade along the wall, or the slope of the ground behind it, from rise and run.