Your Result Explained: What These Numbers Mean
Estimating a segmental retaining wall by simply multiplying visible length by visible height misses two critical structural requirements: first, National Concrete Masonry Association (NCMA) standards require burying at least one full bottom course of block below finished grade to prevent the toe of the wall from kicking out; second, every retaining wall requires a 6-inch compacted gravel leveling pad underneath the bottom course plus a 12-inch-wide vertical column of clean #57 drainage stone directly behind the blocks.
Our calculator computes your Above-Grade Courses, adds 1 Buried Base Course (when checked), multiplies by the Blocks per Course along your wall length, and adds your waste factor. It also calculates Top Cap Blocks separately and sums the exact Cubic Yards, Cubic Feet, Short Tons, and 50-lb Bags of gravel required for both the leveling trench and the 12-inch drainage chimney behind the wall.
Worked Example: 20 ft Long × 3 ft Exposed Height Wall Using 6"×16" Blocks
You are building a 20-foot-long backyard retaining wall with 3 feet (36 inches) of exposed height above the lower lawn using 6" H × 16" W × 10" D blocks, including 1 buried course, a 6-inch base pad, 12 inches of drainage backfill, and 5% block cutting waste.
- Above-grade courses: 36 in exposed height ÷ 6 in block height = 6 courses.
- Add 1 buried base course below grade: 6 + 1 = 7 total courses (42 in total wall height).
- Blocks per course: (20 ft × 12 in/ft) ÷ 16 in block width = 240 ÷ 16 = 15 blocks per row.
- Net wall blocks: 7 courses × 15 blocks = 105 blocks; +5% waste = 111 wall blocks.
- Top cap blocks (+5% waste): 15 × 1.05 = 16 cap blocks (127 total blocks including caps).
- Base trench + 12" drainage backfill gravel (+5%): 78.75 cu ft = 2.92 cubic yards (3.94 tons of stone).
Result Summary: Order 111 wall blocks (6"×16"), 16 cap blocks, and 2.92 cubic yards (~4.0 tons) of crushed stone plus a 20-foot roll of 4-inch perforated drain pipe.
How This Is Calculated (Formulas & Measurement Guide)
We calculate wall courses vertically (Exposed Height ÷ Block Height + 1 Buried Course) and blocks per row horizontally (Wall Length ÷ Block Width), then compute the trench and drainage zone aggregate prism behind the wall.
Blocks = Ceiling([(Ceiling(Height ÷ BlockH) + BuriedRow) × Ceiling(Length ÷ BlockW)] × (1 + Waste%)) Calculates exact rows and columns so partial course fractions round up to full structural courses.
Gravel Cu Ft = [Length × TrenchW × BaseDepth + Length × BackfillW × ExposedHeight] × (1 + Waste%) Combines the 6-inch leveling pad trench below the first course with the 12-inch vertical drainage chimney behind the wall.
How to Measure Irregular Areas & Curved Borders
When stepping a retaining wall up a sloped yard, measure the average exposed height across the run, or step the buried base trench up in 6-inch increments using a 4-foot spirit level.
Practical Tips & Common Mistakes to Avoid
Field-Tested Planning Tips
Spend 50% of Your Labor Leveling the First Buried Course
Every course above stacks directly on the rear alignment lip of the course below. If your first buried course is off by even 1/4 inch side-to-side or front-to-back, the error magnifies by course 6. Use a dead-blow rubber mallet, torpedo level, and taut string line on every single base block.
Install a 4-Inch Perforated Drain Pipe (Weep Tile) at the Heel of the Base
Water pressure—not soil weight—is what pushes retaining walls over. Lay a 4-inch perforated PVC or corrugated drain pipe wrapped in a filter sock along the bottom back heel of the first course, pitched to daylight every 30 to 50 feet.
Glue Top Cap Blocks with Exterior Polyurethane Masonry Adhesive
Kids and guests inevitably walk or sit on top of retaining walls. Secure every cap block with two heavy beads of exterior polyurethane construction adhesive after brushing the blocks clean and dry.
Separate Clay Backfill Soil from Drainage Stone with Non-Woven Geotextile
Wrap non-woven drainage filter fabric between the native soil bank and your 12-inch #57 stone column so silt and clay cannot wash into the gravel voids and clog your drain pipe.
Common Ordering & Installation Mistakes
Mistake Backfilling directly behind the wall with native clay dirt instead of #57 gravel
Recommended Fix: Clay soil absorbs rainwater, swells, freezes in winter, and exerts massive hydrostatic pressure against the back of the blocks. Always place at least 12 inches of clean angular #57 drainage stone behind the blocks.
Mistake Setting the bottom row of blocks on bare dirt or uncompacted sand
Recommended Fix: Bare dirt softens when wet, causing the wall to tilt forward. Excavate a trench 6 inches deeper than the buried block and compact 6 inches of road-base crusher run.
Mistake Stacking vertical mortarless joints directly on top of each other
Recommended Fix: Always offset (stagger) vertical joints by half a block width in a running bond pattern; cut a half-block with a masonry saw to start every even-numbered course.
Segmental Retaining Wall (SRW) Block Sizes & Coverage Reference Table
Standard residential and commercial retaining wall block dimensions per NCMA / Allan Block / Versa-Lok specifications:
| Block Type & Size | Face Area | Blocks per Sq Ft | Blocks for 20' × 3' Wall (+Buried Row) | Unit Weight | Max Unreinforced Height |
|---|---|---|---|---|---|
| Small Garden Block (4" H × 12" W × 8" D) | 0.333 sq ft | 3.00 blocks | 200 blocks (10 courses) | ~22 lbs | 2.0 to 2.5 ft |
| Medium Landscape Block (6" H × 16" W × 10" D) | 0.667 sq ft | 1.50 blocks | 105 blocks (7 courses) | ~48 lbs | 3.0 to 3.5 ft |
| Large Structural SRW (8" H × 18" W × 12" D) | 1.000 sq ft | 1.00 block | 84 blocks (6 courses) | ~75 lbs | 3.5 to 4.0 ft |
Reference Source: National Concrete Masonry Association (NCMA) Design Manual for Segmental Retaining Walls & residential SRW manufacturer guides. Clean #57 stone has ~40% void space; granite and trap rock weigh slightly more than porous limestone.
Frequently Asked Questions (Retaining Wall Block Calculator)
How many retaining wall blocks do I need per square foot?
Small 4" × 12" garden blocks require 3.0 blocks per square foot. Medium 6" × 16" landscape blocks require 1.5 blocks per square foot. Large 8" × 18" structural SRW blocks require 1.0 block per square foot.
How deep should the first course of a retaining wall be buried?
NCMA guidelines specify burying at least 1 full block course below grade, or a minimum of 10% (1/10th) of the exposed wall height (minimum 4 to 6 inches buried) on flat ground, and deeper when the ground in front of the wall slopes downhill.
How tall can I build a retaining wall without a permit or geogrid?
In most US municipalities, retaining walls under 3.5 to 4.0 feet in total height (measured from the bottom of the buried footing to the top of the cap) do not require a building permit or geogrid, provided there is no driveway or steep slope surcharge above the wall. Always verify your local city code.
What kind of gravel goes under vs. behind a retaining wall?
Underneath the first course (the 6-inch leveling pad), use dense-graded aggregate (crusher run / road base with fines) because it compacts rock-hard and flat. Behind the wall (the 12-inch drainage zone), use clean washed #57 angular crushed stone (no fines) so water drains freely to the base pipe.
How many retaining wall blocks are on a pallet?
Small 4"×12" blocks (~22 lbs) come 120 to 144 per pallet. Medium 6"×16" blocks (~48 lbs) come 48 to 60 per pallet. Large 8"×18" blocks (~75 lbs) come 32 to 40 per pallet.
How do I calculate retaining wall blocks for a curved wall?
Measure the curved length along the outside face with a flexible tape or garden hose. Notice that tapered trapezoidal wall blocks lose about 1 to 2 inches of back width when you knock off the rear wings for tight inside curves—add 10% waste for curved walls.
Worked Example Project Scenarios (Retaining Wall Block Calculator)
Explore pre-computed material takeoffs for popular residential project dimensions: