A 20-ft long, 2-ft tall retaining wall is one of the most satisfying weekend projects in the landscape — it solves a drainage problem and looks finished the same day. The math for block count is simple. The math for whether you need an engineer is more important. Here's both.
Estimate only. This guide and the calculator cover unreinforced gravity walls under 4 ft of exposed face. Anything taller, anything holding back a slope steeper than 1:3, or anything supporting a structure above needs engineered design and likely a permit. Verify your local code before building.
The block count formula
Block walls are stacked in courses (rows). The math is:
blocks_per_course = wall_length_in ÷ block_length_in courses = wall_height_in ÷ block_height_in total_blocks = blocks_per_course × courses
Example: a 20 ft long × 2 ft tall wall using standard 16×6×10 in retaining wall blocks. (20 × 12) ÷ 16 = 15 blocks per course. (2 × 12) ÷ 6 = 4 courses. 15 × 4 = 60 blocks total. Plus 15 cap blocks for the top course finish. Round up to a full pallet from the supplier.
The retaining wall calculator handles three common block sizes and adds the cap count and base gravel automatically.
Block sizes — what your home center actually stocks
Retaining wall blocks come in three rough size classes. Heavier blocks build taller stable walls but each one is a wheelbarrow trip:
- Small landscape (12×4×8 in, ~25 lb): Pavestone Anchor Highland Stone and similar. Good for walls up to ~2 ft. Easy to handle solo. Uses more pieces per ft of wall.
- Standard (16×6×10 in, ~60 lb): Allan Block, Anchor Diamond, and most home-center generic SKUs. Good for walls up to 4 ft. Two-handed lift.
- Large (18×6×12 in, ~85 lb): Built for engineered walls 4-8 ft with geogrid reinforcement. One block per wheelbarrow trip. Order delivered to the site, not to the garage.
Cap blocks are a separate SKU at every size — typically 4 in tall with a flat top. Don't skip them: they finish the wall visually and shed water away from the joint between the top two courses.
Why 4 ft is the magic number
The IRC (International Residential Code) section R404.5 lets homeowners build retaining walls up to 4 ft of exposed face without an engineer or permit in most jurisdictions. Above 4 ft, three things change:
- The lateral earth pressure at the base roughly doubles vs a 2 ft wall — gravity blocks alone won't hold
- You need geogrid soil reinforcement extending back into the slope (typically 6-8 ft of geogrid for an 8 ft wall)
- A licensed engineer needs to size the geogrid layout and stamp the design
Some jurisdictions (California, parts of New England) drop the threshold to 3 ft or even 2 ft. Call your local building department before you buy a single block. The fee for a permit is $150-400. The fee for tearing out an unpermitted wall after a code complaint is the cost of the wall plus the engineering you should have done.
Base prep — the step that prevents the wall from leaning
A retaining wall that leans isn't failing at the blocks — it's failing at the base. The standard build:
- Dig a trench 6 in deep × 12 in wider than the block (so 22 in wide for a 10 in deep block)
- Add 6 in of compacted gravel (3/4 in minus or paver base) — compact in 2 in lifts
- Bury the first course at least 1 in below grade — every additional ft of wall height adds another inch buried
- Backfill the back of each course with drainage gravel (3/4 in clean stone), not the original soil
- Run a drain pipe at the base behind the bottom course on any wall over 2 ft — perforated 4 in PVC daylit at one end
Skipping the buried first course is the #1 reason DIY walls lean forward. The wall has to push back against the soil below grade to resist the soil pushing forward above grade. Without that buried anchor, the only thing holding it up is friction between the blocks.
What pros do differently
Order 5% extra blocks.Block edges chip in transit and during install. A 5% pad covers the ones you have to flip or set aside. Most yards take returns on full pallets only — order one extra pallet if you're close.
Use a string line, not a level, for the long axis. A 4-ft level on a 20 ft wall accumulates error. Drive stakes at each end, run mason's string at the top course height, and check every block against the string.
Sweep concrete adhesive between caps. Caps are heavy enough to stay put under foot traffic but kids and lawnmowers will knock them off. A bead of polyurethane construction adhesive between the cap and the top course locks them in.
Quick FAQ
Do I need to set the blocks back as I go up? Most modern segmental blocks have a built-in setback lip on the back face — the wall self-batters about 1 in per course. That's why they hold a slope better than vertical brick walls.
Can I use mortar between courses? No. Gravity retaining wall blocks are designed to flex slightly — mortar cracks under the same loads the dry-stack joints absorb. Caps get adhesive, not mortar.
How long does a wall like this last? Properly built with a compacted gravel base and back drainage, 30-50 years. Failures are almost always traced back to base prep or missing drainage, not the blocks themselves.
Estimate only — not engineered design. The retaining wall calculator handles unreinforced gravity walls under 4 ft. For taller walls, walls supporting structures, or walls in soft/expansive soil, get a licensed engineer to design the reinforcement and stamp the plan.