Retaining Wall Block Calculator

What this calculator works out

Six quantities, and the basis for each.

Block count and cap count from your wall and your unit. Embedment, meaning how far the bottom course sits below grade. Leveling pad volume. Gravel backfill volume. Drain pipe length. And the two thresholds that decide whether you should be building this yourself at all.

Two of those six are governing checks rather than single numbers. Embedment and gravel width each have more than one published minimum that applies at the same time, and the larger one wins. The calculator reports which rule governed, in the same way the block fill calculator reports which basis produced its volume.

Enter your own block size, not a brand name

Concrete masonry units are standardized. A nominal 8 by 8 by 16 inch CMU is the same unit whoever makes it, because ASTM C90 fixes it.

Segmental retaining wall units are not. No standard fixes their face height, face length or depth. Every producer sets its own, and the same block class varies by several inches between yards. A calculator that asks you to pick a brand from a dropdown is guessing at dimensions it cannot know, and presenting the guess as precision.

So this one asks for the three measurements that drive every result: face height, face length and depth. The presets fill those fields for the four common classes and stay editable. Measure the block, or read the pallet label, and type what it says.

Depth matters more than people expect, because it drives the leveling pad width and the gravel zone width, not just the look of the wall.

How many retaining wall blocks do you need?

Blocks per course is the wall length divided by the block face length, rounded up. Courses is the total wall height divided by the block face height, rounded up. Multiply the two, then add your waste allowance.

The part that gets missed is that total height is not the height you want to see. It is the exposed height plus the buried course. A wall you want standing 3 feet proud of the ground is a 3 foot 6 inch wall to order for, and every course calculation has to use that larger number.

Caps come off the top of that height before the courses are counted, not after. A 4 inch cap on a 42 inch wall leaves 38 inches of units to divide, not 42.

How deep does the first course go?

This is where published calculators diverge, and in the range most people build, they diverge in an unsafe direction.

Three bases are in circulation for embedment.

| Basis | Rule | 3 ft exposed wall | |---|---|---| | CMHA and NCMA Best Practices | Greater of 6 in, or H'/20 on level ground | 6 in | | Manufacturer best practice | Greater of 6 in, or 1 in per foot of height | 6 in | | Widely published rule of thumb | 10 percent of wall height | 3.6 in |

The first two are published by the industry association and by a unit producer. They disagree about the percentage and agree about the floor: 6 inches, whatever the arithmetic says.

The third carries no source we could find, and across the whole do it yourself range it lands below that floor. At 2 feet it gives 2.4 inches, at 3 feet 3.6 inches, at 4 feet 4.8 inches. It only catches up to 6 inches at a 5 foot wall, which is already past the point where a permit and an engineer are involved.

The percentage rules only start to matter on tall walls. At 10 feet exposed, the CMHA rule gives 6 inches and the per foot rule gives 10. This calculator computes both, takes the larger, and names the one that governed.

Front slope changes it again. CMHA's figure gives H'/20 on level ground, H'/10 on a 3H:1V slope below the wall, and H'/7 on 2H:1V. Ground that falls away in front of the wall needs more block buried, not less.

Schematic section, not to scale. Ratios from the CMHA and NCMA Best Practices Guide, Figure 1.3-1.

Base pad and gravel backfill

The leveling pad is at least 6 inches thick and extends 6 inches past both the toe and the heel of the bottom unit, per CMHA. That makes the pad width your block depth plus 12 inches, which is a different figure from the doubled block depth that circulates, and it is the number this calculator uses. The material is crushed stone or unreinforced concrete.

One thing worth knowing before you dig: a segmental retaining wall is a flexible structure, and CMHA states its footing does not need to be placed below the frost line, provided the foundation has enough bearing capacity. That is the opposite of the mortared CMU wall covered in the retaining wall cinder blocks guide, which does need frost depth. Carrying the CMU rule across to an SRW is a common and expensive mistake.

Gravel backfill is the second governing check. For walls up to 10 feet, two minimums apply together: at least 12 inches measured from the back of the unit, and at least 24 inches measured from the wall face.

| Block depth | 12 in behind | 24 in from face | Governs | |---|---|---|---| | 8 in | 12 in | 16 in | From face | | 10 in | 12 in | 14 in | From face | | 12 in | 12 in | 12 in | Equal | | 16 in | 12 in | 8 in | Behind unit |

Anything shallower than a 12 inch unit needs more than 12 inches of gravel behind it. A calculator that applies a flat 12 inches to every wall under orders the gravel on every shallow unit, which is most garden and seat wall block.

Above 10 feet the band widens to 24 inches behind and 36 from the face, and at that height the wall is an engineered design anyway.

Schematic section, not to scale.

Worked example

A 20 foot wall, 3 feet exposed, on level ground, built from a structural unit measuring 8 inches face height by 18 inches face length by 12 inches deep, with caps, 5 percent waste.

Embedment is the greater of 6 inches and 36 divided by 20, which is 1.8. So 6 inches, and the floor governs.

Total height is 36 plus 6, which is 42 inches. Caps take 4, leaving 38 inches of units. At 8 inches per course that is 4.75, rounded up to 5 courses, so the finished wall stands slightly above the 3 feet entered.

Blocks per course is 240 inches divided by 18, which is 13.3, rounded up to 14. Five courses gives 70 units, and 5 percent waste gives 74. Caps are 14, plus waste, so 15.

Leveling pad width is 12 plus 12, which is 24 inches. At 6 inches thick over 20 feet that is 20 cubic feet, or 0.74 cubic yards before compaction allowance.

Gravel is the greater of 12 inches and 24 minus 12, both of which are 12 here, so 12 inches. Over 42 inches of height and 20 feet of length that is 70 cubic feet, or 2.6 cubic yards.

Total height is 42 inches, which is 3.5 feet, under the 4 foot permit line. Three inches more exposed height and it would not be.

When this calculator stops and an engineer starts

CMHA names four conditions that put a segmental retaining wall in the hands of a qualified engineer: the wall carries a surcharge load, the wall carries live loads, the wall is founded on poor soil, or the design conditions require special consideration.

On top of that, IBC Section 105.2 exempts earth retaining structures from a permit only up to 4 feet of total height, and total height includes the buried course. Gravity walls, which hold soil by weight alone, typically top out at 3 to 4 feet, and lower where soil is poor or a surcharge is present.

The calculator flags all of these from your inputs. It does not override them.

What this calculator does not cover

Geogrid reinforced design. Once a wall passes the gravity ceiling for its unit, the geogrid length, spacing and tensile strength are an engineered calculation, not an estimate.

Global and sliding stability, bearing capacity, seismic loading, terraced wall interaction, curves and corners, and any wall carrying a structure, a driveway or a slope above it.

It also does not price anything. Material prices vary by region and by week, and we do not publish numbers we cannot source.

For unit counts on a mortared block wall rather than an SRW, use the cinder block calculator. For why published calculators disagree on figures like these, see why masonry calculators disagree.

Frequently asked questions

How many blocks do I need for a retaining wall? Divide wall length by block face length and round up for blocks per course, then divide total wall height by block face height and round up for courses, then multiply. Total height includes the buried bottom course, not just the part you see.

How deep should the first course of a retaining wall be buried? At least 6 inches, which is the minimum in the CMHA and NCMA Best Practices Guide. On level ground the percentage rule of H'/20 only exceeds 6 inches on walls above 10 feet exposed. Steeper ground in front of the wall requires more.

How much gravel goes behind a retaining wall? For walls to 10 feet, at least 12 inches measured from the back of the unit and at least 24 inches from the wall face, whichever is greater. For a 10 inch deep unit that works out to 14 inches, not 12.

How thick should the base of a retaining wall be? At least 6 inches of crushed stone or unreinforced concrete, extending 6 inches past the toe and the heel of the bottom unit. That makes the pad width your block depth plus 12 inches.

Does a retaining wall footing need to be below the frost line? Not for a segmental retaining wall. CMHA states these are flexible structures and the footing need not go below frost depth where bearing capacity is adequate. A mortared CMU retaining wall is a different case.

How high can I build before I need a permit? Over 4 feet of total height triggers a permit under IBC 105.2, and height is measured including the buried course. A shorter wall carrying a surcharge can still need one.

What size are retaining wall blocks? There is no standard. Face heights run from about 4 inches for garden units to 8 inches for structural units, with 24 inch bin blocks above that. Measure the unit you are buying rather than assuming.

Sources

• CMHA SRW-TEC-005, Guide to Segmental Retaining Walls. Leveling pad thickness and extension, gravel fill minimum behind the units, frost line, and the four conditions requiring a qualified engineer.
• CMHA and NCMA, Segmental Retaining Walls Best Practices Guide. Minimum embedment and the embedment ratios by front slope, Table 1.3-1 and Figure 1.3-1. Gravel fill widths by wall height, Section 3.5 and Table 1.3-2. Heel drain gravel volume, Section 4.1.2.1.
• CMHA SRW-FAQ-001, Segmental Retaining Wall FAQ. Permit threshold under IBC 105.2, gravity wall height limits, and how wall height is measured.
• CMHA SRW-GSP-001, SRW Sample Specifications. Drain pipe diameter and slope.
• Allan Block, Best Practices for SRW Walls, 2.6(a). The alternative embedment basis of 1 inch per foot of wall height with a 6 inch floor.
• How we source and correct figures and every constant on this site.