How many blocks do I need for a 30 ft retaining wall?

At 2 ft exposed, 80 blocks — 4 courses of 20 — or 84 with the 5% the manufacturer recommends ordering. Raise it to 3 ft exposed and the same wall takes 120 blocks, because height adds whole courses of 20 at a time while length adds one block per 18 in.

How deep should the first course of a retaining wall be buried?

At least 6 in, or 1 in for every foot of wall height if that is more, per Allan Block's installation manual. Below that sits 6 in of compacted wall rock, so the trench for a short wall is 12 in deep in total. The buried course is what stops the wall from being pushed out at its base.

How high can I build a retaining wall without a permit?

The model code exempts walls not over 4 ft measured from the bottom of the footing to the top of the wall, and only when nothing surcharges them. Jurisdictions amend that text and some set a lower bar, so your building department is the authority here, not this page. Note that the measurement includes the buried part.

Does this calculator handle geogrid reinforcement?

No. It counts a gravity wall — blocks holding soil back by their own weight and setback. Taller walls, walls on a slope and walls carrying a surcharge need geogrid layers embedded in the backfill at spacings that come from the soil and the loads. That is an engineered design, and the quantities come out of it rather than out of a calculator.

Does the base rock figure include the drainage behind the wall?

No. The tonnage covers the levelling pad only: a 6 in compacted course under the blocks, one block depth wide. Drainage aggregate, the perforated pipe at the foot of the wall and the backfill above it are separate orders.

Retaining Wall Calculator

A segmental wall is counted in courses, and the first one is the course people forget: part of the wall is buried before anything shows above grade. This calculator works out the buried depth your height requires, counts the blocks from there, and sizes the base rock under them.

How it works

Block dimensions are Allan Block's AB Classic: 8 in high, 12 in deep, 18 in long, covering about a square foot of wall face. Courses come from the height, blocks per course from the length, each rounded up on its own — a 30 ft wall is 20 blocks across whether it stands two courses or seven.

The buried course is the manufacturer's rule: at least 6 in, or 1 in for every foot of wall height, whichever is greater. Height there means the whole wall including the buried part, so the calculator solves for it rather than guessing — and the 6 in minimum governs everything up to about 5-1/2 ft exposed, which covers most garden walls. Blocks to order then add the 5% Allan Block recommends for breakage and cuts.

Base rock is a trench one block deep wide and 6 in thick along the wall, converted to tons at 139 lb/ft³, the FHWA figure for compacted aggregate base. Total excavation is that 6 in plus the buried block: 12 in below grade on a short wall.

Formula: Buried in = greater of 6 in and 1 in per ft of total wall height · Courses = (exposed height in + buried in) ÷ 8 in, rounded up · Blocks per course = wall length in ÷ 18 in, rounded up · Base rock ft³ = length ft × 1 ft trench width × 0.5 ft deep

Measuring on site

Measure the exposed height you want above finished grade, not the total wall. The buried course is added for you.
Measure along the face, following the curve if it curves. Blocks on a curve sit at a slight angle to each other, so a curved run takes a little more than the straight-line distance between its ends.
Step a wall that runs downhill instead of averaging its height. Each step is its own wall for counting, with its own buried course.
Check what sits behind the wall before you order. A slope above it, a driveway or a pool deck is a surcharge, and a surcharge turns the wall from something you estimate into something an engineer designs.

Waste and what it covers

The 5% order margin is the manufacturer's, and the only allowance here with a source behind it. What this calculator does not size is the drainage behind the wall — the wall rock chimney, the perforated pipe and the filter fabric that keep water from loading its back. Those belong in every correct installation, and their quantities depend on how deep the excavation behind the wall runs.

Blocks and base rock, AB Classic 8 × 12 × 18 in, including the 5% order margin
Wall	Buried	Courses	Blocks	Order	Base rock
20 ft × 2 ft exposed	6 in	4	56	59	0.37 yd³ / 0.70 t
30 ft × 2 ft exposed	6 in	4	80	84	0.56 yd³ / 1.04 t
30 ft × 3 ft exposed	6 in	6	120	126	0.56 yd³ / 1.04 t
40 ft × 3 ft exposed	6 in	6	162	171	0.74 yd³ / 1.39 t

Questions

How many blocks do I need for a 30 ft retaining wall?: At 2 ft exposed, 80 blocks — 4 courses of 20 — or 84 with the 5% the manufacturer recommends ordering. Raise it to 3 ft exposed and the same wall takes 120 blocks, because height adds whole courses of 20 at a time while length adds one block per 18 in.
How deep should the first course of a retaining wall be buried?: At least 6 in, or 1 in for every foot of wall height if that is more, per Allan Block's installation manual. Below that sits 6 in of compacted wall rock, so the trench for a short wall is 12 in deep in total. The buried course is what stops the wall from being pushed out at its base.
How high can I build a retaining wall without a permit?: The model code exempts walls not over 4 ft measured from the bottom of the footing to the top of the wall, and only when nothing surcharges them. Jurisdictions amend that text and some set a lower bar, so your building department is the authority here, not this page. Note that the measurement includes the buried part.
Does this calculator handle geogrid reinforcement?: No. It counts a gravity wall — blocks holding soil back by their own weight and setback. Taller walls, walls on a slope and walls carrying a surcharge need geogrid layers embedded in the backfill at spacings that come from the soil and the loads. That is an engineered design, and the quantities come out of it rather than out of a calculator.
Does the base rock figure include the drainage behind the wall?: No. The tonnage covers the levelling pad only: a 6 in compacted course under the blocks, one block depth wide. Drainage aggregate, the perforated pipe at the foot of the wall and the backfill above it are separate orders.

Sources

Allan Block Corporation — Commercial Installation Manual for Allan Block Retaining Walls“8 in H x 12 in D x 18 in L ... 1 sq ft approx ... [Material Estimation Worksheet, notes:] * Wall height needs to have the amount of buried block included. Buried block should be a minimum of 6 in (150 mm) or 1 in (25 mm) for each 1 ft (0.3 m) of wall height. See the approved plans to find out what the final height including the buried block will be. ** Extra blocks will need to be included if walls have step ups and/or stairs. Ordering an extra 5% is always recommended to account for any problems during construction.”
Federal Highway Administration, Central Federal Lands Highway Division — Application Rates and Weights for Estimating Purposes“Aggregate Base | 2225 kg/m3 (139.0 lb/ft3)”
International Code Council — 2024 International Building Code §105.2 — Work exempt from permit“Retaining walls that are not over 4 feet (1219 mm) in height measured from the bottom of the footing to the top of the wall, unless supporting a surcharge or impounding Class I, II or IIIA liquids.”

Figures and sources last checked by The BuilderTally Team in August 2026.