Retaining Wall Block Calculator
Estimate exactly how many blocks, cap blocks, and gravel base material you need for small landscape retaining walls.
Last reviewed: June 2026
Remember to include the height of the buried base course (usually 1 block).
Usually twice the block depth from front to back.
Commonly 4 to 6 inches of compacted crushed stone base.
Results update automatically as you enter measurements.
Your Material List
For this project, buy/order approximately
For this project, use these quantities as your shopping or delivery list.
Formula and Assumptions
Formula used
The formula takes the total square footage of the wall face ($Length imes Height$) and divides it by the square footage of a single block face. To find courses, height is divided by block height. Cap blocks are found by dividing total wall length by block length. Base material volume is calculated as $Trench Length imes Width imes Depth$. Waste percentages are applied to account for cutting and breakage.
Unit conversions
Inches are converted to feet before volume is calculated. Cubic feet are converted to cubic yards by dividing by 27. Bag counts are rounded up because stores do not sell partial bags.
Waste factor explanation
Waste factor helps account for uneven surfaces, cuts, spills, compaction, settling, and measurement differences. The right buffer depends on your project and material.
Material Assumptions and Disclaimer
Bag yield or density assumptions
- Calculations assume standard running bond patterns.
- Wall height includes the buried courses, if any. Wall blocks and caps are distinct items depending on the product line.
Estimate disclaimer
These results are planning estimates based on the measurements and assumptions shown here. Confirm quantities with your supplier, product label, local code requirements, or a qualified professional before purchasing.
When to be careful
- Taller retaining walls face immense lateral earth pressure. Consult a structural engineer if building higher than 3-4 feet or if the wall is load-bearing.
Example Calculation
A 20-ft long, 2-ft high wall using 12x4 inch blocks. Base trench is 6 inches deep and 24 inches wide. 10% waste:
- 1. Wall Area: $20 imes 2 = 40$ sq ft
- 2. Block Area: $1 imes 0.333 = 0.333$ sq ft
- 3. Raw Blocks: $40 div 0.333 = 120$
- 4. Add 10% waste: $120 imes 1.10 = 132$ blocks
- 5. Raw Caps: $20 ext{ ft} div (12 ext{ in} / 12) = 20$
- 6. Add waste to Caps: $20 imes 1.10 = 22$ cap blocks
- 7. Base Volume: $20 imes (24/12) imes (6/12) = 20$ cu ft (plus 10% = 22 cu ft)
You need 132 wall blocks, 22 cap blocks, and 22 cubic feet of crushed stone base.
Frequently Asked Questions
Do I need to bury the first row of blocks?
Yes. For structural stability, the rule of thumb is to bury one inch of block for every one foot of exposed wall height, or bury one entire block course minimum.
What kind of gravel do I need for the base and backfill?
For the base, use a compactable crushed stone like Class 2 aggregate or Item 4. For the backfill behind the wall, use a clean, free-draining angular stone like 3/4-inch crushed washed rock. Never backfill entirely with dirt!
Why do walls fail?
Poor drainage is the #1 reason retaining walls fail. Hydrostatic pressure builds up behind the wall when water cannot escape. Always install a perforated drain pipe and clean stone backfill.
Do I need a building permit?
Most municipalities require a permit and sometimes engineered drawings for any retaining wall over 3 or 4 feet in height, or any wall that supports a surcharge like a driveway or slope.
Do I need geogrid?
Geogrid is a synthetic mesh laid between block layers and extending into the hillside to tie the wall and soil together into one unified mass. It is generally required on taller walls or walls with challenging soil loads.
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