How to calculate retaining wall blocks
A 16 × 6 in block face = 0.667 sq ft → 1.5 blocks per sq ft
Example: a wall 30 ft long and 2 ft high has a 60 sq ft face. With 16 × 6 in blocks that's 90 blocks, plus 5% extra ≈ 95 blocks.
Remember the first course is usually buried 1 block deep (or about 10% of the wall height), so include that buried row in the height you enter.
Drainage gravel
Most block walls need about 12 inches of clean drainage stone (like #57) behind the blocks for the full height, often with a perforated drain pipe at the bottom. Water pressure behind a wall is the most common reason walls fail. The calculator estimates this gravel for you.
Base
Blocks sit on a compacted base, typically 6 inches of crushed stone in a trench about 2 ft wide. Use our crushed stone calculator with the trench length and width.
Caps
Cap blocks are counted by length, not area: divide the wall length (in inches) by the cap length. For example, 30 ft = 360 in ÷ 16 in caps ≈ 23 caps.
Frequently asked questions
How many retaining wall blocks do I need per square foot?
It depends on the face size: 3 for 12 × 4 in blocks, 1.5 for 16 × 6 in, about 1.33 for 18 × 6 in and 1.125 for 16 × 8 in.
How tall can a block retaining wall be without a permit?
Many US areas allow up to about 3–4 ft without an engineered design, but rules vary by city and county — check locally.
How much gravel goes behind a retaining wall?
Typically a 12-inch-wide column of drainage stone for the full wall height: about 1 cubic yard for every 27 sq ft of wall face.
Should the first row be buried?
Yes. Most manufacturers recommend burying the first course, usually about 1 inch for every 8 inches of wall height, with a minimum of about 6 inches.
Related calculators
These figures are estimates for planning only. Material weights and coverage vary by supplier, moisture and compaction. Always confirm quantities and prices with your supplier before ordering. See our terms & disclaimer.