Retaining Wall Block Calculator
This free retaining wall block calculator tells you how many blocks to buy for your wall. Enter the wall length and height, the exposed face size of your block, and a waste allowance — it instantly estimates the block count (rounded up), the count with waste, and the number of courses.
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Results update as you type.
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Enter your wall dimensions to see the estimate.
How to use this calculator
Measure your wall's length and height in feet and enter them. Include the buried base course in the height — the first course of a gravity retaining wall sits below grade (usually about 6 in), and those blocks still need to be bought.
Enter the exposed face size of your block in inches (the default is the common 16 in × 8 in face). Then keep the default 10% waste allowance for end/corner cuts and breakage — raise it to 12–15% for curved walls. Results update instantly as you type.
How it works
Each block's exposed face area is computed in square feet, then the wall's face area is divided by it and rounded up to a whole block:
face area per block = (width ÷ 12) × (height ÷ 12)
blocks = ceil(wall length × wall height ÷ face area per block)
The waste allowance is then applied to that count:
blocks to buy = ceil(blocks × (1 + waste % ÷ 100))
For example, a 20 ft by 3 ft wall is 60 sq ft. A 16 in × 8 in block exposes 0.8889 sq ft, so 60 ÷ 0.8889 = 67.5, rounded up to 68 blocks. Add 10% waste: ceil(68 × 1.10) = 75 blocks to buy.
This is a planning estimate for standard dry-stacked segmental retaining wall (SRW) blocks. It assumes the wall face is a simple rectangle and does not account for the block's interlock lip, cap stones for the top course, the gravel base, drainage backfill, or geogrid reinforcement that taller walls may require. Walls over about 3–4 ft tall often need an engineered design and a permit — confirm quantities and structural requirements with your supplier and local building code.
Frequently asked questions
How many retaining wall blocks do I need?
Divide the wall's face area by the face area of one block, round up, then add a waste allowance. For example, a 20 ft by 3 ft wall is 60 sq ft. A 16 by 8 in block exposes 0.8889 sq ft (16/12 × 8/12), so 60 ÷ 0.8889 = 67.5, which rounds up to 68 blocks. Add 10% waste and you buy 75 blocks. Enter your wall size in the calculator above and it does this math instantly.
What is the standard size of a retaining wall block?
Most segmental retaining wall (SRW) blocks expose a face of 12 to 16 in wide by 6 to 8 in tall, with the 16 by 8 in face being the most common default. The depth (front to back) varies by manufacturer and does not change the block count. Measure the exposed face of the exact block you plan to buy and enter it in the calculator, since a few inches of difference changes the total.
Do I need to bury the first course of a retaining wall?
Yes. Gravity retaining walls should have at least one full course buried below grade — typically 6 in, or about one-tenth of the wall height — to keep the base from sliding. Include that buried course in the wall height you enter in the calculator, or add one course worth of blocks to your total. Setting the base course on 4 to 6 in of compacted gravel is just as important as the blocks themselves.
Should I add extra blocks for waste?
Yes — add at least 10%. Cut blocks at wall ends and corners, breakage during transport, and a few spares for future repairs all consume blocks. For curved walls or walls with steps, use 12 to 15% waste instead. The calculator's default is 10% but you can adjust it. Cap stones for the top course are usually counted separately from wall blocks.
Do retaining wall blocks need mortar?
No. Most modern segmental retaining wall blocks are dry-stacked without mortar — they lock together with a lip, pin, or interlocking system. What they do need is a compacted gravel base (4 to 6 in), drainage gravel and filter fabric behind the wall, and backfilling in layers. Also note that walls taller than about 3 to 4 ft often need an engineered design and a permit, so check your local building code before you start.