How to use the retaining wall calculator
- Enter the wall length and the exposed height above ground.
- Enter the block height and face length from the product sheet.
- Choose whether to bury the first course, as set out in the installation guide for your block.
- Enter the cap length, or 0 if you are not capping the wall.
- Enter the gravel base trench width and depth.
Check local rules before you build: Decoratoradvice notes that many areas require a permit or an engineer once a wall passes a set height.
Retaining wall formulas
Blocks per course = wall length ÷ block length, rounded up
Courses = exposed height ÷ block height, rounded up, + 1 if burying a course
Wall blocks = blocks per course × courses
Caps = wall length ÷ cap length, rounded up
Base gravel = wall length × trench width × base depth
Worked example
A wall 20 ft long and 3 ft high, 6 × 16 in blocks, one buried course, 12 in caps, base trench 24 in wide and 6 in deep:
240 in ÷ 16 = 15 blocks per course
36 in ÷ 6 = 6 courses, + 1 buried = 7 courses
15 × 7 = 105 blocks
Caps: 240 ÷ 12 = 20
Base: 20 × 2 × 0.5 = 20 cu ft = 0.74 cu yd
When a wall needs an engineer
This calculator is for small garden walls. Taller walls, and walls holding back a slope, driveway, building or poor soil, can require an engineered design and a permit. The height limit varies by block system and by local code, so check the manufacturer's guide and your building department before you start.
Tips
- Add gravel and perforated drain pipe behind the wall if the manufacturer's guide calls for it. Drainage stops water pressure building up behind the blocks.
- Step the base down a slope rather than burying more of the wall.
- Curved walls need more blocks on the outside of a curve; add a few extra.
- For the backfill and soil on top, use the topsoil calculator; for the base and drainage stone by weight, the gravel calculator.
Common mistakes
- Forgetting the buried course. Leaving it out can short you a whole course.
- Using the block depth instead of face length. Count along the face of the wall.
- Skipping the base. A wall on unprepared soil can settle and lean.
Frequently asked questions
How many retaining wall blocks do I need?
Blocks per course × courses. For a 20 ft × 3 ft wall in 6 × 16 in blocks with one buried course, 105 blocks.
Do I need to bury the first course?
Check the installation guide for your block system. Where a buried course is required, leaving it out shortens the wall by a full course.
How much gravel goes behind a retaining wall?
It depends on the wall height and the drainage zone in the manufacturer's guide. Work out the volume as length × zone width × height with the cubic yard calculator.
Do retaining wall blocks need mortar?
Most segmental blocks are dry-stacked with lips or pins and glued caps. Mortared walls use standard blocks; see the concrete block calculator.
Can I build a retaining wall with pavers?
No. Pavers are for flat surfaces; see the paver calculator for a patio beside the wall.
Key takeaways
- Blocks = blocks per course × courses, rounding both up.
- Add a buried course if your block system calls for one.
- Plan the gravel base and drainage at the same time.
- Taller or loaded walls may need engineering and a permit.