How it works
Check local code; consult a professional for walls over your local height limit. This calculator counts materials. It does not design a wall, choose a base or check drainage or stability.
Wall blocks = ceil(wall length × wall height ÷ block face area × (1 + waste ÷ 100) − 1e-9). Block face area is the face width × face height you enter. Brands vary, so no brand size is assumed.
Cap blocks = ceil(wall length ÷ cap length). Base gravel volume = wall length × base width × base depth ÷ 27, the same calculation as the Cubic Yard Calculator. Base gravel is optional: tick “Add base gravel”, then enter a base width and depth from your block maker's installation guide or local code. No base size is assumed.
Worked example
A 30 ft wall, 3 ft high, has 90 sq ft of face. An 18 × 6 in block face is 1.5 × 0.5 ft = 0.75 sq ft.
90 ÷ 0.75 = 120 blocks with no waste. With an 18 in cap: 30 ft ÷ 1.5 ft = 20 caps.
Tips and common mistakes
- Enter the visible face size from the block maker's specification, not the block depth.
- Include every course in the height, including any course that sits below the ground.
- If a face size looks like it was entered in the wrong unit (under 2 in or over 36 in), the calculator asks you to check it.
- Waste starts at 0% so the worked example holds. Add your own allowance for cuts, corners and curves.
- Need block counts for a mortared wall instead? Use the Concrete Block Calculator.
Questions about this calculator
How do I calculate a retaining wall?
For materials, multiply wall length × height to get the wall face area. Divide by the face area of one block (its face width × height, from the maker's spec sheet). Then add waste.
Cap blocks are the wall length ÷ the cap length, rounded up.
This page works out quantities only, not wall design.
How many retaining wall blocks do I need?
Example: a wall 30 ft long and 3 ft high has a face of 90 sq ft. With blocks whose face is 18 × 6 in (0.75 sq ft), 90 ÷ 0.75 = 120 blocks before waste.
Brands differ, so use your block's own face size.
How do I build a block retaining wall?
Follow the block manufacturer's installation guide and your local building code. Check local code, and consult a professional for walls over your local height limit. This calculator only works out the blocks, caps and base gravel to buy.
Sources
Values marked with a link come from the linked source, which opens in a new tab. Unit conversions and our own defaults are listed separately. Every value the site uses is in the methodology source register.
Where the numbers come from 1
- Meters per international foot
- 0.3048 m/ft
- NIST, international foot conversion (opens in a new tab)nist.gov
Unit conversions and our defaults13
- Inches per foot
- 12 in/ft
- Unit definition: one foot contains twelve inches
- Square inches per square foot
- 144 sq in/sq ft
- Geometry: 12 × 12 inches
- Cubic feet per cubic yard
- 27 cu ft/cu yd
- Geometry: 3 × 3 × 3 feet
- Centimeters per meter
- 100 cm/m
- Metric prefix definition: centi is one hundredth
- Percent denominator
- 100 %
- Definition of percent: parts per hundred
- Retaining wall starting dimensions
- 30, 3 ft length, ft height
- Owner worked example; editable starting measurements, not a recommended wall height
- Retaining wall starting block face
- 18, 6 in width, in height
- Owner worked example; brands vary, so enter your block face
- Retaining wall starting cap length
- 18 in
- Build choice: matches the starting face width; enter your cap length
- Block face unit-check range
- 2 – 36 in
- Build choice: face sizes outside this range prompt a unit check; not a product range
- Retaining wall waste default
- 0 %
- Owner build choice preserves the worked example; editable
- Count rounding tolerance
- 1e-9 unitless
- Owner specification: ceil(x − 1e-9)
- Volume and weight display precision
- 2 decimal places
- Owner specification: display volume and tons to two decimals
- Cubic yard depth unit-check threshold
- 24 in
- Owner specified unit-check warning, not a recommended depth