The Formula Behind This Calculator
Blocks per course = round up (wall length (in) / block length (in))
Courses = round up (total height (in) / block height (in))
Blocks to buy = round up (per course x courses x 1.05), caps = round up (per course x 1.05)
Base gravel (tons) = length (ft) x 2 ft wide x 0.5 ft deep x 100 lb per cu ft / 2,000
A segmental wall is a grid: the face is courses stacked to the height and blocks repeated to the length, so the count is the two round ups multiplied. The 5 percent allowance covers end cuts, the occasional cracked unit, and matching a spare for the future. Drainage stone uses the standard 12 inch column behind the blocks for the full exposed height, and the base trench is a separate, wider, shallower volume. Every number here assumes a straight wall: curves waste more blocks to cuts and corners waste a few specialty units, both reasons to add a second look before ordering for anything but a straight run.
Worked Example: A 20 Foot Garden Wall, 2 Feet Above Ground
A garden terrace wall runs 20 feet, standing 2 feet above grade. With one 4 inch course buried, the total build height is 2 feet 4 inches, entered as 2.33 feet. Blocks are the standard 12 by 4 inch units.
- Per course: 20 feet is 240 inches. 240 / 12 = 20 blocks per course.
- Courses: total build height is 28 inches. 28 / 4 = 7 courses, which is 6 visible courses plus the 1 buried course.
- Blocks: 20 x 7 = 140 blocks before waste, 147 with the 5 percent allowance.
- Caps: 20 across the top, 21 with allowance.
- Base gravel: 20 x 2 x 0.5 = 20 cubic feet = about 1.0 ton.
- Drainage stone behind the 2 foot exposed wall: 20 ft x 1 ft x 2 ft = 40 cubic feet, about 2 tons.
Read the course line twice, because it is where most hand estimates go wrong: the buried course is invisible in the finished wall and unforgettable in the block count. Enter total build height in the tool, not the height you see, and the arithmetic above is what the calculator returns. At $2.50 to $5 per block, the 147 blocks run about $370 to $735, caps and stone extra, a fair picture of why small walls are mostly a materials errand and large walls are mostly a labor contract.
A Complete Guide to Planning a Segmental Retaining Wall
A retaining wall is a structure pretending to be landscaping. It holds back tons of soil, in the rain, forever, and the segmental block systems that make it look easy work because of the unglamorous details beneath and behind them. This guide covers the structure the block count sits on: base, batter, drainage and backfill.
The Base Trench Decides Everything
The trench is dug wide enough for the block plus working room behind it, typically the block depth plus 12 inches, and deep enough for 4 to 6 inches of compacted gravel plus the buried course. The gravel is compacted in lifts until it is hard and level, and level here means checked with a long level in both directions, because every error in course one telegraphs to the top of the wall, amplified. Blocks are laid on a thin leveling bed of coarse sand or stone dust only where the system allows it. Spend the extra hour here. A wall that starts level has a chance; a wall that starts wavy has a schedule.
Batter, Setback and Why Walls Lean Back
Segmental blocks are designed with a slight setback per course, the batter, so the wall leans into the hill it holds. That lean is structural, not a defect, and it is why the top course of a tall small block wall sits noticeably behind the bottom course's face. It also means the wall footprint is deeper than the block: plan the excavation and the property line conversation accordingly. Walls are glued or pinned per the manufacturer's system on cap courses and many block lines; follow the system you bought, not a video about a different one.
Drainage: The Wall Behind the Wall
Soil plus water is the load; the wall only ever holds back the water's ambitions. Behind the blocks goes a column of clean, angular drainage stone at least 12 inches thick, separated from the native soil by landscape fabric so fines do not migrate in and clog it, with a perforated pipe at the base daylighting to an outlet. Backfill is compacted in lifts as the wall rises, never dumped against a finished wall in one load. Most failed garden walls show the same autopsy: no drainage stone, dirt backfilled tight to the blocks, and a winter of freeze and pressure. The drainage line in this calculator is small money with an outsized job.
When to Stop and Call a Professional
This calculator and guide cover garden and terrace walls up to about 4 feet of exposed height in ordinary soils, with nothing but planting above them. Beyond that, or where a driveway, structure or steep slope loads the top, walls become engineered structures with geogrid reinforcement, footing design and permits in most jurisdictions. That line is not a sales tactic; it is where the physics stops being forgiving. Build the garden wall yourself with confidence, and hire the hillside. For the base and bedding materials in bulk, the Gravel Calculator and Sand Calculator size deliveries the same way.
Caps, Adhesive and Finishing the Top
The cap course is the wall's signature and its weather seal. Caps are typically secured with exterior masonry adhesive rather than pins, overhang the face by about an inch for a finished shadow line, and are cut at corners with a diamond blade or a splitter, which is the one tool rental most wall builders end up making. Leave caps off until the backfill behind the top course is placed and compacted, because you will be working and occasionally kneeling along that top edge all day. Behind the caps, finish grading should slope the soil surface away from the wall so roof and yard water do not pour over the face and streak it. Plant the top bed, mulch it lightly rather than deeply, and in the first season walk the wall after heavy rains: any course that has crept forward announces itself early, when the fix is an afternoon, rather than in year five, when it is a rebuild.