Retaining Wall Calculator

Three quantities size the wall

A small block retaining wall turns into three shopping-list lines: how many blocks, how much gravel behind them, and how much geogrid, the plastic mesh that ties the wall into the hillside. The calculator above works all three from the same two inputs, the wall's length and height, plus the block you plan to buy. It assumes a level backfill, no surcharge, and a segmental wall of dry-stacked concrete units, which is the kind sold at big-box stores for do-it-yourself builds.

Two limits frame everything that follows. First, height: the 2024 International Residential Code, section R404.4, requires engineered design once a wall retains more than 48 inches of unbalanced fill or resists loads beyond soil. Second, drainage: nearly every failed small wall failed because water had nowhere to go, and the gravel column in section three is what keeps pressure off the blocks. Get those two ideas in place and the arithmetic in the rest of the page is straightforward.

Block count, step by step

Two divisions produce the count. Courses come from height: a 3 ft wall is 36 inches, and 36 divided by a 6 inch block gives 6 courses. Blocks per course come from length: 40 ft is 480 inches, and 480 divided by a 16 inch block gives exactly 30. Multiply and the wall takes 180 blocks.

The table rounds with the ceiling function, not in your favor. A 30 ft wall is 360 inches, and 360 divided by 16 lands on 22.5, so that course takes 23 blocks with a half block of cutting at the end. Buy for the ceiling, not the decimal.

Face area gives a fast cross-check: 120 square feet of face at 1.5 blocks per square foot is 180 blocks, same answer by a different door. If the two routes disagree, one of the block dimensions you typed is wrong. Remember to include the buried first course in the height; the blocks do not care that soil hides them.

The gravel column behind the wall

Block suppliers and masonry associations repeat one rule until it sticks: clean crushed gravel, at least 12 inches deep from the back of the units, for the full height and length of the wall. The NCMA Segmental Retaining Wall Installation Guide calls for clean 1 inch minus crushed stone placed in the cores, between the units, and 12 inches behind them, so water reaching the wall falls instead of pushing. California's Division of the State Architect says the same thing for engineered walls in IR 18-2 (2023 revision): free-draining gravel, minimum 1 ft thick, between facing and soil.

The strip is a rectangle: length times depth in feet times height, then divide by 27 for yards. Two extras ride on top of the table. Hollow block cores filled with gravel add a few percent, and a perforated drain pipe at the bottom of the column needs a bed to sit in. Keep fabric between the gravel and the native soil, or silt will migrate in and clog the column within a few seasons.

Geogrid: length, layers and spacing

Geogrid turns the wall and a slice of hillside into one heavy mass. Two rules size it. Length: the NCMA guide sets the minimum at 60 percent of total wall height, never less than 4 ft. Spacing: DSA IR 18-2 (2023) caps the vertical gap at 32 inches and at twice the block depth, whichever is smaller, and requires a layer inside the top 12 inches.

Run the numbers on the 40 x 3 ft wall. Height 3 ft means grid at least 4 ft deep per layer, since 60 percent of 3 is only 1.8 ft, so the 4 ft minimum governs. With a row every 3 courses, 6 courses give 2 layers, so 2 rows times 40 ft times 4 ft is 320 square feet of grid. Roll widths run 4 ft, and a 4 x 50 ft roll covers 200 square feet, so this wall needs two rolls.

These are planning floors, not engineering. Tall walls, weak soils and slopes above the wall all stretch the lengths and tighten the spacing, and the block maker's published tables for your exact unit take precedence over any rule of thumb.

What the materials cost

Three prices drive the budget, and all three were checked in October 2026. Blocks: big-box listings run from $1.97 for a small Pavestone unit to $5.97 for the 17.44 by 6 in RockWall Large, with premium 6 by 16 in decorative block near $20 apiece. Gravel: the GoodGravel price index, reading 175 yards across 47 states in October 2026, puts crushed stone at a $50 median per cubic yard, with the middle half between $32 and $68. Geogrid: SiteOne listed its Pro-Trade GG2100 biaxial 4 x 50 ft roll at $87.77, which works out to 44 cents per square foot.

What the total leaves out matters as much as what it holds. Base rock under the wall comes to about 0.74 cubic yards for a 40 ft run at 6 inches deep and a foot wide, cap units cover the top course, fabric and a drain pipe add a modest line, and delivery fees can double a small gravel order. Treat the $1,437.62 as the blocks-and-backfill core, then price the edges locally before committing.

Permits, height limits and when to call an engineer

Height rules stack from three sources, and they measure differently. The model 2024 IRC, in section R404.4, requires design by accepted engineering practice, with a 1.5 safety factor against sliding and overturning, for any wall not supported at the top that retains more than 48 inches of unbalanced fill, or one over 24 inches that also resists surcharge loads. Cities then layer their own permit triggers on top: Portland, Oregon requires a building permit above 4 ft measured from the bottom of the footing to the top of the wall, and for any wall carrying a surcharge. Philadelphia skips permits under 2 ft with no surcharge, and offers a plan-free standard for 2 to 4 ft masonry walls that meet its detail sheet.

Three situations call for an engineer whatever the tape measure says: a wall that holds up a driveway, pool or structure; a wall built on a slope that keeps rising behind it; and any wall in clay soil that stays wet. Foundation soil strength, not wall height, is what decides the design, which is why the same 3 ft wall can be routine in sandy ground and a problem in wet clay. A one-hour engineer consult costs less than rebuilding a failed wall.

Building order: base, first course, drainage, caps

The sequence protects the numbers on this page. Dig a trench and lay 6 inches of compacted crusher run or leveling gravel, extending a few inches past the wall on both sides; the NCMA guide treats the leveling pad as the foundation of the whole system. Set the first course level in both directions, checking every unit, because nothing downstream fixes a crooked first row. This is the slowest hour of the job and the one that shows forever.

Stack upward with joints broken, filling cores and the 12 inch zone behind with clean gravel as you go. Backfill in lifts: DSA IR 18-2 (2023), following the NCMA design manual, limits compaction lifts to 8 inches, and mechanical plate compactors stay about 4 ft back, because the standard details allow only hand-operated compaction within 48 inches of the face. Lay geogrid on the set courses, pull it taut, pin it, then place the next lift of soil over it. Fabric separates the gravel column from native soil the whole way up.

Finish with caps adhered to the top course, grade the soil at the top so surface water runs away from the wall rather than into the gravel, and point the drain pipe to daylight. A wall built in that order rarely visits the failure modes in the next section.

Five mistakes that shorten a wall's life

• Backfilling with the wrong rock. Pea gravel rolls, topsoil holds water, and leftover mulch rots. The 12 inch column must be clean, angular crushed stone, or the drainage the wall depends on never happens.
• Skipping the fabric. Without a geotextile between gravel and soil, silt washes into the voids season after season until the column clogs and water ponds behind the blocks.
• Measuring height from finished grade. The permit trigger usually runs from the bottom of the footing, buried course included. A wall that looks 3 ft above the lawn can be a 4 ft wall to the building department.
• Starving the geogrid. Grid shorter than the 60 percent floor, rows spaced past the 32 inch cap, or layers left loose in the backfill all give back the strength the mesh was bought for.
• Dead-end drainage. Gravel without an outlet is a cistern. The NCMA guide is explicit that the fill is not the primary drainage path; the perforated pipe at the heel, sloped to daylight, is what carries water away.

Each failure shows up a year or two after the build, as lean, bulge or settlement, which is exactly why suppliers hand out an installation guide with the first pallet. Follow it and the calculator numbers become the whole project.