EngineeringFormulaCivil & Structural Engineering
Retaining Wall Calculator
Calculate how many blocks you need to build a retaining wall, with optional waste factor and cost estimate.
Planning a retaining wall? This calculator tells you exactly how many blocks to buy. Enter your wall's length and height, plus the dimensions of your chosen block, and the calculator works out the number of rows (courses), blocks per row, and total block count. Add a waste factor - typically 10% for cuts, breakage, and corners - and a price per block to get a material cost estimate before you head to the hardware store.
Wall Dimensions
L · Total horizontal run of the wall
H · Exposed height from base to top
Block Dimensions
ℓ · Face length of one block along the wall
h · Height of one course (single block)
Options
W% · 10% for straight walls; 15–20% for curves or corners
$ · Optional - leave blank to skip cost estimate
Solution
Enter wall and block dimensions to calculate your block count.
N = ⌈Rows × BPR × (1 + W%)⌉
Formula Sheet
- NTotal Blocks
- LWall Length
- HWall Height
- ℓBlock Length
- hBlock Height
- WWaste Factor
Variables & Units
| Symbol | Variable | Description | Common Units |
|---|---|---|---|
| N | Total Blocks | Total number of blocks to purchase, rounded up and including the waste allowance. | |
| L | Wall Length | The total horizontal run of the retaining wall. | ft, m, in |
| H | Wall Height | The total exposed height of the retaining wall from the base to the top. | ft, m, in |
| ℓ | Block Length | The face length of a single block along the wall run. | in, cm, mm |
| h | Block Height | The height of a single block (one course). | in, cm, mm |
| W | Waste Factor | A decimal allowance for cut blocks, breakage, and extras (e.g. 0.10 = 10%). |
How to Use This Calculator
- 01Measure or plan the wall length and height in your preferred unit (feet, metres, or inches).
- 02Enter the block length and block height from the manufacturer spec sheet or the block itself. Use the outer dimensions, not the face dimensions.
- 03Set the Waste Factor (%) to account for cut blocks at corners, steps, and breakage. A standard allowance is 10% for a straight wall; increase to 15–20% for walls with curves, steps, or many corners.
- 04Optionally enter the price per block to calculate your total material cost.
- 05The calculator instantly shows the number of rows, blocks per row, and the total block count rounded up to the nearest whole block.
- 06Use the block count as the starting shopping list only. For a real segmental retaining wall, also plan for a compacted gravel base, drainage stone, perforated pipe, filter fabric, cap units, and any geogrid required by the wall system.
How the Formula Works
The calculator first determines how many horizontal courses (rows) are needed by dividing the wall height by the block height, then rounding up to the nearest whole number - you can't use a fraction of a block.
It then finds how many blocks fit along the length of each row by dividing the wall length by the block length, again rounding up.
Total blocks is the product of rows and blocks per row. The waste factor adds a percentage buffer before a final round-up, ensuring you order enough material to cover off-cuts and breakage.
Cost is simply the total block count multiplied by the price per block.
Worked Example 01
Standard backyard retaining wall
Known
- Wall Length (L): 20 ft
- Wall Height (H): 3 ft
- Block Length (ℓ): 16 in
- Block Height (h): 6 in
- Waste Factor (W): 10%
Formula
Rows = ⌈H / h⌉
Substitution
Rows = ⌈3 ft / 6 in⌉ = 6, BPR = ⌈20 ft / 16 in⌉ = 15, N = ⌈6 × 15 × 1.10⌉
Result
99 blocks
A 20-foot long, 3-foot high wall using 16×6-inch blocks needs 6 rows of 15 blocks each - 90 base blocks. Adding a 10% waste buffer gives 99 blocks to order.
Worked Example 02
Metric garden raised bed
Known
- Wall Length (L): 4 m
- Wall Height (H): 0.6 m
- Block Length (ℓ): 40 cm
- Block Height (h): 20 cm
- Waste Factor (W): 0%
Formula
Rows = ⌈H / h⌉
Substitution
Rows = ⌈0.6 / 0.2⌉ = 3, BPR = ⌈4 / 0.4⌉ = 10, N = 3 × 10
Result
30 blocks
A clean 4 m × 0.6 m raised garden bed using standard 40 cm × 20 cm blocks requires exactly 30 blocks with no waste, since both wall dimensions are exact multiples of the block.
Applications
- 01Planning the number of blocks and material cost for a garden, yard, or driveway retaining wall
- 02Estimating materials for a raised garden bed or planter box
- 03Quick quantity takeoff for contractor bidding or homeowner budgeting
Assumptions
- 01The wall is straight. Curved walls require more cuts and a higher waste factor.
- 02All blocks are the same size throughout the wall.
- 03The base course is at or just below grade - buried base courses should be included in your wall height.
- 04Cap blocks and corner/end blocks are not separately calculated. Add cap blocks based on wall length; typical cap blocks run one per linear foot.
Where This Model Stops
- 01Does not calculate drainage gravel, geogrid reinforcement, or footing materials - all critical for walls over 3 ft (0.9 m). Consult a structural engineer for walls above this height.
- 02Does not account for stepped or terraced walls - calculate each tier separately.
- 03Does not calculate mortar - most modern segmental retaining wall blocks are dry-stacked; confirm with your block supplier.
- 04Local building codes typically require a permit and engineering review for retaining walls over 4 ft high. Always check before building.
- 05Does not check global stability, sliding, overturning, bearing pressure, soil type, surcharge loads, water pressure, or frost depth. Those are design checks, not block-count checks.
References
- [1]
Segmental Retaining Walls - NCMA Design Manual
National Concrete Masonry Association
Standard design guidance for segmental retaining wall blocks including block dimensions and waste factors.
Frequently Asked Questions
What waste factor should I use?
For a straight wall with no curves, 10% is a reliable standard. Add 5–10% more for each of the following: wall curves, steps or terraces, many inside/outside corners, or a pattern that requires lots of half-cuts. When in doubt, round up - returning unused blocks is easier than a second hardware store trip.
Should I include the buried base course in my height?
Yes. Retaining walls should start below grade for stability. A common rule is to bury at least one-tenth of the exposed wall height, with a minimum of one full block course. Measure the total wall height from the bottom of the buried course to the finished top.
Does this calculator work for Versa-Lok, Allan Block, or other specific brands?
Yes, as long as you enter the correct block length and height from the manufacturer's datasheet. Each brand's blocks differ slightly - always verify with the spec sheet rather than measuring a single block, as slight manufacturing tolerances can add up over a long wall.
Do I need a permit to build a retaining wall?
In most jurisdictions, walls over 3–4 feet (0.9–1.2 m) in height require a building permit and, often, engineer-stamped plans. Some areas also restrict walls near property lines regardless of height. Contact your local building department before starting any retaining wall project.
Why does drainage matter if this calculator is only counting blocks?
Water pressure behind a retaining wall can be more damaging than the soil weight itself. Even for small landscape walls, a gravel drainage zone, filter fabric, and an outlet path help prevent hydrostatic pressure from building up behind the blocks.
How is this different from the Concrete or Gravel Calculator?
This calculator estimates block count from wall face dimensions. Use the Gravel Calculator for base and drainage aggregate volume, and the Concrete Calculator only if your wall uses poured footings or concrete components.