How many retaining wall blocks?
Type the wall’s length and height and your block sizes. The retaining wall calculator counts the courses, wall blocks and cap blocks, and the cubic yards of gravel for the base and the drainage zone behind the wall.
- Wall blocks
- 90
You need 90 wall blocks (6 courses of 15), 15 caps and 2.96 yd³ of gravel.
- Courses
- 6
- Blocks per course
- 15
- Cap blocks
- 15
- Wall face
- 60 ft²
- Base gravelyd³
- 0.74
- Drainage gravelyd³
- 2.22
- Gravel in allyd³
- 2.96
- Gravel in all, cubic metresm³
- 2.27
Wall blocks: 90. You need 90 wall blocks (6 courses of 15), 15 caps and 2.96 yd³ of gravel.
How to calculate
Retaining wall calculator: the blocks, courses and cap blocks for a segmental block retaining wall, and the gravel for its base and the drainage zone behind it, with cost.
Example with the default inputs (Wall length 20 ft, Wall height 3 ft, Block length 16 in, Block height 6 in, Cap length 16 in, Base trench width 24 in, Base depth 6 in, Drainage zone 12 in): You need 90 wall blocks (6 courses of 15), 15 caps and 2.96 yd³ of gravel.
Method: Courses = height ÷ block height and blocks per course = length ÷ block length, each rounded up; blocks = courses × blocks per course; caps = length ÷ cap length, rounded up; gravel = length × (base width × base depth + drainage width × height).
- A straight wall of one block thickness, laid in running courses; the wall height includes the buried first course.
- The defaults (16 × 6 in blocks, a 24 in wide, 6 in deep base, 12 in of drainage gravel) are starting points: use your block maker’s sizes and installation guide.
- A tall wall, or one holding back a slope, a drive or a building, may need an engineer’s design; check local rules.
Worked examples
Each example is checked against the calculator on every build.
- Wall length 20 ft, Wall height 3 ft, Block length 16 in, Block height 6 in, Cap length 16 in, Base trench width 24 in, Base depth 6 in, Drainage zone 12 in, Price per block $3.50 gives Courses 6, Blocks per course 15, Wall blocks 90, Cap blocks 15, Base gravel 0.740741, Drainage gravel 2.222222, Block cost $315.00.Source: NIST SP 811 (2008), Appendix B: 1 in = 0.0254 m, 1 yd³ = 27 ft³, https://www.nist.gov/pml/special-publication-811 (retrieved 2026-10-03): 36 ÷ 6 = 6 courses
- Wall length 10 ft, Wall height 2.5 ft, Block length 12 in, Block height 4 in, Cap length 12 in, Base trench width 24 in, Base depth 6 in, Drainage zone 12 in gives Courses 8, Blocks per course 10, Wall blocks 80, Cap blocks 10, Gravel in all 1.296296.Source: NIST SP 811 (2008), Appendix B: 1 in = 0.0254 m, 1 yd³ = 27 ft³, https://www.nist.gov/pml/special-publication-811 (retrieved 2026-10-03): 30 ÷ 4 = 7.5, so 8 courses
- Wall length 19.69 ft, Wall height 3.281 ft, Block length 15.75 in, Block height 7.874 in, Cap length 19.69 in, Base trench width 23.62 in, Base depth 5.906 in, Drainage zone 11.81 in gives Wall blocks 75, Cap blocks 12, Gravel in all, cubic metres 2.34.Source: NIST SP 811 (2008), Appendix B: 1 in = 0.0254 m, 1 yd³ = 27 ft³, https://www.nist.gov/pml/special-publication-811 (retrieved 2026-10-03): 1 ÷ 0.2 = 5 courses × 6 ÷ 0.4 = 15 = 75 blocks
How it works
- Courses = wall height ÷ block height, rounded up.
- Blocks per course = wall length ÷ block length, rounded up.
- Wall blocks = courses × blocks per course.
- Cap blocks = wall length ÷ cap length, rounded up.
- Wall face = wall length × wall height.
- Base gravel = wall length × base trench width × base depth.
- Drainage gravel = wall length × drainage zone width × wall height.
- Gravel in all = base gravel + drainage gravel, in cubic yards (÷ 0.764554857984 m³) and cubic metres.
- Block cost = wall blocks × the price per block, when you type a price.
The defaults are a starting point: 16 × 6 in blocks and 16 in caps, a base trench 24 in wide and 6 in deep, and 12 in of drainage gravel. Use your block maker’s sizes and installation guide.
Rules
Every length is from 1 mm to 1 km. The wall is straight and one block thick.
Output format
Blocks, courses and caps are whole numbers; gravel shows to 2 decimals in cubic yards and cubic metres; the face in ft² (US) or m² (Metric) to 2 decimals; the cost in money. Typed sizes are read back as exact decimals, so 240 in of wall in 16 in blocks is exactly 15 blocks a course, and each count is rounded up once.
Worked examples by hand
A 20 ft wall, 3 ft high, 16 × 6 in blocks, 16 in caps, $3.50 a block. Courses 36 ÷ 6 = 6. Per course 240 ÷ 16 = 15. Blocks 6 × 15 = 90 ($315). Caps 240 ÷ 16 = 15. Base 20 × 2 × 0.5 = 20 ft³ (0.74 yd³); drainage 20 × 1 × 3 = 60 ft³ (2.22 yd³); 80 ft³ = 2.96 yd³ in all.
A 10 ft wall, 30 in high, 12 × 4 in blocks. Courses 30 ÷ 4 = 7.5, so 8. Per course 120 ÷ 12 = 10. Blocks 80; caps 10. Gravel 10 × 2 × 0.5 + 10 × 1 × 2.5 = 10 + 25 = 35 ft³ = 1.30 yd³.
A 6 m wall, 1 m high, 400 × 200 mm blocks, 500 mm caps, base 600 × 150 mm, drainage 300 mm. Courses 5, per course 15, blocks 75; caps 12. Gravel 6 × 0.6 × 0.15 + 6 × 0.3 × 1 = 0.54 + 1.8 = 2.34 m³.
Other questions people ask
How many blocks do I need for a retaining wall?
Divide the wall height by the block height for the courses, and the wall length by the block length for the blocks in each course, rounding each up, then multiply. A 20 ft wall 3 ft high in 16 × 6 in blocks is 6 courses × 15 blocks = 90 blocks.
Does the wall height include the buried course?
Yes. Type the full height of all the courses, including the first course set below ground, but not the caps.
How many cap blocks do I need?
Divide the wall length by the face length of one cap and round up. 20 ft of wall with 16 in caps takes 15 caps.
How much gravel does a retaining wall need?
Gravel for the base trench (length × trench width × base depth) plus gravel behind the wall for drainage (length × zone width × wall height). A 20 ft wall, 3 ft high, with a 24 × 6 in base and 12 in of drainage gravel needs 20 + 60 = 80 ft³, about 2.96 yd³.
What if the height is not a whole number of blocks?
The courses round up, so a 30 in wall in 4 in blocks takes 8 courses (32 in). Bury the extra or cut the top course.
When does a retaining wall need an engineer?
A tall wall, or one holding back a slope, a driveway or a building, may need an engineered design and a permit. Check local rules and the block maker’s height limits.