Blocks Are the Smallest Part of the System
Count-the-blocks calculators lie by omission. The blocks are the visible 40 percent of a retaining wall; the base, backfill and drainage are the invisible 60 percent that decides whether the wall stands for decades or leans by spring. This estimate prices the whole system, because a wall built of perfect blocks on a bad base is just an expensive future demolition.
The physics is simple and unforgiving: soil plus water weighs about 110 pounds per cubic foot, and it pushes sideways with a force that grows with the square of the wall height. Double the height, quadruple the push. Everything below — the compacted base that spreads the load, the drainage stone that keeps water from adding its weight, the pipe that carries water away — exists to manage that force.
Under 3 feet, a careful DIYer with segmental blocks can build a wall that lasts. Over 3 to 4 feet, the forces, the permits and the consequences all change character — that is where the calculator starts flagging geogrid and where a structural engineer earns their fee.
Courses, Setback, and the Batter
Segmental walls lean back into the hill as they rise — about 1 inch of setback per course, called the batter. The batter is structural: it moves the wall's weight over its own base, resisting overturning. It also means each course is slightly shorter than the one below, which is why the calculator shrinks every course by the setback instead of multiplying one course count.
Course count is wall height divided by block face height, rounded up. A 2-foot wall with 8-inch faces needs exactly 3 courses; a 30-inch wall needs 4, and that fourth course is mostly buried or capped — which is fine, because the first course is always buried. Bury at least 6 inches (one full course for 8-inch block) below grade: buried courses do not count toward the pretty exposed height, but they anchor everything.
Cap blocks finish the top: they overhang slightly, shed water, and give the wall its finished look. Order caps for the full wall length — the calculator uses 12-inch caps — plus a couple of spares, because caps are also the pieces most likely to chip during handling.
The Base: 6 Inches of Crushed Stone
The base is 6 inches of compacted crushed stone (3/4-inch minus / crusher run), extending 6 inches beyond the block faces front and back. Not pea gravel, not sand, not “packed dirt” — angular crushed stone that locks together under compaction. The calculator's base volume uses the block depth plus 12 inches of width; on a 20-foot wall with 12-inch-deep blocks that is about 3/4 of a cubic yard.
Compaction is the whole game: compact the trench bottom, add stone in 2-inch lifts, compact each lift with a plate compactor or hand tamper. Then screed the top perfectly level — the first course of blocks sits directly on this surface, and every sixteenth of an inch of error multiplies up the wall. Spend an hour here and save a day of shimming.
Never set blocks on undisturbed topsoil or, worse, on fill that has not been compacted. Topsoil compresses, fill settles, and the wall follows. Dig to firm subgrade — if the trench bottom is soft, dig deeper and add more stone.
Backfill and the Drainage Sandwich
Behind the wall goes 12 inches of clean drainage stone (3/4-inch clear, no fines), from the base all the way to within a few inches of the top. Water moving through soil hits this stone column and drops straight down instead of pushing on the wall — this single detail prevents more wall failures than everything else combined. The calculator figures a full 12-inch column behind the full wall height.
At the bottom of that stone column sits a perforated 4-inch drain pipe, holes down, sloping to daylight or a storm drain. “Daylight” means the pipe must actually exit somewhere lower than the wall base — a pipe to nowhere is decoration. Wrap the pipe and stone in filter fabric to keep soil from migrating in and clogging the system over the years.
Above the stone, fold the fabric over and finish with soil and sod. Never backfill with clay directly against the blocks — clay holds water like a bathtub against exactly the surface you are trying to keep dry.
When Geogrid Enters the Chat
Over about 3 feet of exposed height, gravity walls want help: layers of geogrid — synthetic mesh — sandwiched between courses and extending back into the backfill. The grid turns the wall and a wedge of soil behind it into one coherent mass that cannot tip or slide. It is the difference between stacking blocks and building a structure.
Grid layers typically go every 2 courses, extending back 60 to 100 percent of the wall height, and they must be the right strength for the height — this is specified by engineers or the block manufacturer's engineering tables, not guessed. The calculator flags the requirement; the specification comes from the tables.
Walls over 4 feet in most jurisdictions need a permit and engineered drawings, and many places count the buried course and the slope above in the height. Check before digging — “I didn't know” does not survive the conversation with the inspector or the neighbor downhill.
Curves, Corners, and Corners That Aren't
Gentle curves are free with segmental block — just fan the blocks, keeping the front faces tight and letting the backs gap, filling gaps with stone. Tight curves need blocks cut or special corner units; never force a radius so tight the faces gap open, because the interlock disappears with the contact.
Inside corners (wall turning into the hill) are easy: interleave the courses. Outside corners need corner blocks or careful miter cuts — they are the slowest part of any wall, so budget time, not just material. Steps in the wall (stepped top following a slope) each need their own cap treatment; plan cap quantities per level.
Where the wall meets a house foundation or existing structure, leave a small expansion gap filled with backer rod and flexible sealant. Rigid walls bonded to structures crack at the joint within a few freeze-thaw cycles — the gap is cheaper than the repair.
Soil You Can't See Matters Most
Before ordering a single block, dig a test hole where the wall will stand. Clay swells when wet and pushes relentlessly — clay backfill is the number-one killer of DIY walls, which is why the drainage column matters doubly there. Sand drains beautifully but needs the fabric wrap to keep it out of your stone. Fill of unknown history must be treated as guilty until proven innocent: compact it in lifts or dig through it.
Water sources uphill — downspouts, sump discharges, neighbor's grading — must be rerouted around the wall, never behind it. A downspout dumping behind a retaining wall is a demolition plan with a 5-year timeline. Extend downspouts past the wall's ends or pipe them through.
If the slope above the wall continues upward (a surcharge), the effective push increases — steeper surcharge, bigger push. Terraced walls (two short walls instead of one tall one) handle surcharges far better than single tall walls, and each terrace under 3 feet stays in friendly DIY territory.
Ordering and Delivery Day
Order blocks by the pallet when you can — pallets run 40 to 60 blocks depending on size, and the per-block price drops. The calculator's block count plus 5 percent waste, rounded up to pallets, is the order. Caps usually come in smaller cubes; stone and base come by the cubic yard or by the ton (about 1.4 tons per cubic yard of crushed stone).
Have the delivery placed as close to the wall line as possible — every block weighs 30 to 80 pounds, and carrying them 100 feet each way across a yard is how backs and schedules break. A pallet near the work beats a pallet in the driveway.
Stage the work: day one dig and base, day two first courses and drainage, then courses and backfill together — backfill as you rise, compacting the stone every course or two. Never stack the full wall and backfill afterward; you cannot compact stone behind a finished wall, and uncompacted backfill settles and drags the top courses with it.
What Failure Looks Like (So You Can Avoid It)
Walls rarely collapse dramatically; they lean, bulge and slide over seasons. A forward lean at the top means overturning pressure — usually waterlogged backfill with no drainage. A belly in the middle means the base settled or the backfill was never compacted. Stepped cracking through the courses means differential settlement below.
Catch it early and the fix is drainage: expose the back, add the stone column and pipe that should have been there, and rebuild the top courses. Caught late, the fix is rebuilding. The materials for doing it right the first time cost a fraction of doing it twice — drainage stone is the cheapest insurance in landscaping.
Walk your wall every spring. Sight along the top cap: it should be dead straight. Any new lean, any fresh gap at a corner, any water seeping through the face after rain — investigate that weekend, not next year. Walls talk before they fail; the trick is listening.