Count the courses, then the blocks in each course. Bury 1 in of block for every foot of wall, add it to the height and divide by the block height; divide the length by the block length. A 20 ft wall 3 ft high in 8 × 18 in blocks is 5 courses of 14: 70 blocks and 14 caps. Allan Block sets it on 4 in of compacted rock in an 18 in trench and fills 12 in behind it with rock, about 4 tons in all. Over 4 ft, the model code wants an engineer.
How to measure for a retaining wall
- Length: along the front of the wall, end to end. For a curve, measure along the curve with a tape or a hose laid on the ground.
- Height: the tallest part that will show above the ground in front, from the finished grade to the top. A wall that steps down a slope is several walls: run each height on its own.
- Block: the face height and length from the maker’s sheet. The three Allan Block units here come from its residential manual; for any other block, pick Another block and type its size.
The math, with your numbers
These lines follow whatever you typed into the calculator above.
Allan Block’s own estimating chart gives 69 blocks and 14 caps for a wall 20 ft long and five courses high, and its caps add 4 in to the height. The calculator fills the height with block and puts the caps on top, so a capped wall ends up a cap’s thickness taller. Wall rock is weighed at 1.4 tons a cubic yard, the loose-gravel figure from the gravel calculator; Allan Block also fills the hollow cores of its blocks with rock, which this total doesn’t include.
Block sizes and how much wall each covers
| Allan Block unit | Height × depth × length | Face per block | Weight |
|---|---|---|---|
| AB Classic | 8 × 12 × 18 in | 1 sq ft | about 75 lb |
| AB Lite Stone | 4 × 12 × 18 in | 0.5 sq ft | about 35 lb |
| AB Jumbo Jr | 8 × 9.5 × 9 in | 0.5 sq ft | about 35 lb |
Source: Allan Block, Residential Installation Manual. Other makers’ blocks differ: use the face size on their sheet.
So a wall of AB Classic takes one block for each square foot of face you see, plus the buried course and the ends you have to cut. 70 blocks of 75 lb is over 2½ tons of block, so ask about delivery to the spot where the wall goes.
The base trench and the rock behind the wall
| Wall height | Trench width | Trench depth | Rock behind |
|---|---|---|---|
| Under 4 ft | 18 in | 4 in of base + 1 in per ft of wall | 12 in |
| 4 ft and over | 24 in | 6 in of base + 1 in per ft of wall | 12 in |
Source: Allan Block, Residential Installation Manual. The base is wall rock compacted with at least two passes of a plate compactor; heavy clay, wet soil or previously dug ground is replaced with granular material first.
The rock goes in the hollow cores and in a band at least 12 in deep behind the blocks, from the base to the top course. It lets water out of the soil before it can push on the wall; retaining wall drainage covers the gravel, the pipe and the outlets. Allan Block adds a drain pipe at the back of the base for walls over 4 ft, reinforced walls, poorly drained soil, slopes or paving above the wall, and vents it to daylight at least every 50 ft.
How tall can a retaining wall be?
Two limits apply: what the code allows without a design, and what the block can hold on its own.
- The code: the International Residential Code doesn’t require a permit for a retaining wall up to 4 ft, measured from the bottom of the footing to the top, unless it supports a surcharge such as a driveway (R105.2). Walls retaining more than 48 in of soil, or over 24 in with other loads on them, must be designed to resist sliding and overturning (R404.4). Your town may set its own limits.
- The block: Allan Block’s chart gives the tallest gravity wall, with no reinforcement, for its 6° blocks: 3 ft 2 in in clay and 4 ft 7 in in clean sand with level ground above; 2 ft 4 in and 4 ft 1 in under a 3:1 slope; 1 ft 4 in and 3 ft 1 in with a patio or drive above. Taller walls need geogrid, and the maker says final designs belong to a local professional engineer.
Have the buried lines marked before you dig the trench. Allan Block’s manual says to call the local utility companies first: a line can sit right where the wall needs to go.
Questions people ask
How many retaining wall blocks do I need?
Courses × blocks per course. Add 1 in of buried block per foot of wall to the height, divide by the block height and round up; divide the length by the block length and round up. A 20 ft wall 3 ft high in 8 × 18 in blocks needs 5 × 14 = 70.
How deep should a retaining wall base be?
Allan Block digs 6 in plus 1 in for every foot of wall height, and 4 in plus the buried block for walls under 4 ft, then compacts wall rock in the trench.
How much gravel goes behind a retaining wall?
A band at least 12 in deep from the base to the top, plus the block cores. For a 20 ft wall with 40 in of block that’s about 67 cu ft, 3.5 tons, before the cores.
Do I need a permit for a retaining wall?
Under the model code, not for a wall up to 4 ft from the bottom of the footing that doesn’t support a surcharge (IRC R105.2). Many towns set their own rules, so check.
Does a retaining wall need a drain pipe?
Allan Block puts one in walls over 4 ft, reinforced walls, poorly drained sites and walls with slopes or paving above, vented to daylight at least every 50 ft.
Sources
- Allan Block, Residential Installation Manual for Allan Block Retaining Walls (walls under 6 ft)
- International Code Council, 2021 IRC chapter 1 (R105.2, work exempt from permit) and chapter 4 (R404.4, retaining walls)
- FEMA, Appeal decision, Road Repair, Appanoose County, Iowa (1930-DR-IA) (1.4 tons per cubic yard of uncompacted gravel)
Checked October 12, 2026. Metric values are converted from feet, inches and US tons.