Retaining Wall Calculator
Enter your wall dimensions to estimate blocks, cap stones, and the drainage gravel every retaining wall needs.
Runs entirely in your browser — your numbers never leave this page.
How it works
A retaining wall is a drainage structure that happens to hold dirt. Block count is simple; drainage is the craft:
Blocks = (Length × Height) ÷ 0.89 sq ft per block
- Bury the first course: 6 inches of block below grade plus 6 inches of compacted gravel — walls fail from the bottom.
- Batter the wall: lean it back ~1 inch per foot of height into the hill. Gravity is your mortar.
- Drainage: 12 inches of clean gravel behind the wall plus a perforated pipe at the base daylighting out the ends. Water pressure topples more walls than soil weight.
- Geogrid: walls over 3 ft in clay soils need geogrid mesh layered back into the slope — check the block manufacturer's charts.
- Permits: most jurisdictions require engineering above 4 ft of exposed height. Build it right or build it twice.
Frequently asked questions
How many blocks do I need for a retaining wall?
Divide wall face area by 0.89 (standard 16×8 in block face), add 10% for cuts and breakage. A 30×3 ft wall is 90 sq ft → ~112 blocks + 20 cap blocks.
Do I need a permit for a retaining wall?
Usually above 4 ft of exposed height, most areas require an engineered design and permit. Some HOAs restrict even small walls — check both.
Why do retaining walls fail?
Water, almost always — hydrostatic pressure behind the wall. Proper gravel backfill and a drain pipe prevent nearly all failures. The second cause is no buried base course.
How deep should the base be?
6 inches of compacted gravel, 24 inches wide, with the first block course buried 6 inches below grade. The buried course is structural, not cosmetic.
What is batter / setback?
Leaning each course back ~1 inch per foot of height. Segmental block systems build this in via their lip or pins — it turns soil pressure into downward force.
Do I need landscape fabric behind the wall?
Yes — between the gravel backfill and the soil, to keep soil from migrating into and clogging your drainage gravel over the years.
What are cap blocks?
Finished top stones (~18 in long) that crown the wall, shed water, and give a clean edge to sit on. Do not skip them — an uncapped wall looks unfinished and lets water in.
Can I build a retaining wall with wood?
Pressure-treated timber walls work for 2–3 ft heights and last 10–20 years. For anything taller or permanent, segmental block outlasts wood several times over.
How do curves affect block count?
Gentle curves use slightly more blocks (cutting and tighter joints) — the 10% waste covers most curves. Tight radiuses need many cuts; bump waste to 15%.
Should the wall be level or follow the slope?
Step it: keep each section level and step down the slope in full-block increments. A wall that follows the slope looks amateur and drains poorly.