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Retaining Wall

What it's for

Takes off a segmental block retaining wall: the dry-stacked landscape block that sits on a gravel pad. You get blocks, courses, caps, base gravel, drainage stone and drain pipe, with a section drawing of the wall. It makes one code check: it tells you when the wall is tall enough that the IRC line on the screen calls for an engineer. It does not design the wall.

Step by step

The example is the screen as it opens: a wall 40' 0" long and 3' 0" high, in block 4" high and 1' 0" long, with 10% waste.

Retaining Wall inputs at their defaults: WALL LENGTH 40' 0", WALL HEIGHT 3' 0", BLOCK HEIGHT 4", BLOCK LENGTH 1' 0", LOAD ABOVE THE WALL None (Slope, Driveway and Wall above beside it), SLOPE BELOW THE WALL Level (3H:1V, 2H:1V, Steeper), HOLLOW-CORE BLOCK off and WASTE FACTOR 10%. The answer bar reads 484 blocks, a green 48" or less unbalanced fill — IRC R404.4, and 11 courses × 40/course · 44 caps. RESULTS repeats 11 courses × 40/course · 44 caps, ≈ 1.48 yd³ base gravel · ≈ 5.43 yd³ drainage stone, and the green check. 1 2 3 4 5 6
Four dimensions, the load above the wall, the slope below it, the hollow-core switch and the waste factor.
  1. Enter WALL LENGTH along the face of the wall. Follow the curve if it has one.
  2. Enter WALL HEIGHT: the exposed face, from finished grade at the toe to the top of the top block, not counting the cap. Do not add the buried part. The calculator buries whole courses for you: at least 6", and 1" for each foot of exposed height (Allan Block's rule), on level ground in front of the wall. The 48" check judges the unbalanced fill, so if the ground behind will come up to the top of the cap, the fill is this height plus the cap (3" to 4"). A face of 44" or more can then retain 48" or more: treat it as over 48" and have it engineered.
  3. Enter BLOCK HEIGHT and BLOCK LENGTH of the unit you are buying, measured on its face. Use the real size, not the catalog name.
  4. Set WASTE FACTOR. It is applied to the blocks and the caps. The gravel, stone and pipe carry no waste.
  5. Set LOAD ABOVE THE WALL: None, Slope (ground rising away behind the wall), Driveway (a driveway, parking or other traffic) or Wall above (another wall, a footing or a building above it). A terrace closer than 2× the lower wall's height counts as a wall above. With any load above, the IRC calls for an engineer once the wall is over 24" (R404.4), and the calculator reads that 24" on the whole block stack, buried courses included. That is the wall's height, not the fill it holds back: a 1' 8" face in 4" block stands 28" with its two buried courses.
  6. Set SLOPE BELOW THE WALL: Level, 3H:1V, 2H:1V or Steeper — the ground falling away in front of the wall. A slope buries more, from the NCMA Segmental Retaining Wall Design Manual's Table 5-1: the exposed height ÷ 10 at 3H:1V and ÷ 7 at 2H:1V, 6" at least. Steeper than 2H:1V is past that table and needs a geotechnical design. HOLLOW-CORE BLOCK — turn it on for units with open cores that you fill with the drainage stone (Allan Block is one). The stone then covers the cores as well as the 12" behind the wall.
  7. Read RESULTS. On the defaults: 484 blocks, 11 courses × 40/course · 44 caps, ≈ 1.48 yd³ base gravel · ≈ 5.43 yd³ drainage stone, and a green 48" or less unbalanced fill — IRC R404.4. Then tap BASIS & WHAT WAS ASSUMED.

Reading the results

Retaining Wall scrolled down. The answer bar at the top keeps 484 blocks and the green 48" or less unbalanced fill — IRC R404.4. RESULTS reads 11 courses × 40/course · 44 caps, ≈ 1.48 yd³ base gravel · ≈ 5.43 yd³ drainage stone and the green check, then the BASIS & WHAT WAS ASSUMED row and the link Check before you cut · Measure twice. The section drawing shows a 3' 0" exposed face, 8" and 6" dimensions below grade, a 2' 0" gravel base, drainage stone behind the wall and a drain pipe, labelled CAP UNITS — 44 PCS, DRAIN STONE ≈ 5.43 CY, SRW BLOCK — 40/COURSE, BASE GRAVEL ≈ 1.48 CY and PERFORATED DRAIN PIPE (4"), over the title strip RETAINING WALL, 11 CRS — 484 BLK, NOT TO SCALE. MATERIALS starts with SRW block (4"×12") — incl. 2 buried courses (embed ≥ 6": 6" min. or 1" per ft of height, Allan Block — verify…), 484 pcs. 12 13 14 15
The results card, the basis row, the section drawing and the top of the material list. The answer bar at the top keeps the answer in view wherever you scroll.
  1. The headline is blocks to buy, waste included. The line under it shows where it came from: courses times blocks per course. The course count includes the buried courses, so the default 3' 0" wall in 4" block is 11 courses: 9 exposed and 2 buried to reach the 6" minimum.
  2. Caps are one course of cap units along the length, waste included.
  3. Base gravel and drainage stone are in cubic yards. The material list gives the section each was figured on. On the shot: base gravel 6" deep × 24" wide, drainage stone 12" behind wall + drain pipe (with hollow-core block, the cores too: Allan Block's 1.4 ft). The stone runs the full built height, buried courses included. The material list orders each up to the next 1/4 yd³, never less than the need, and gives the weight at 120 lb per cubic foot: on the defaults ≈ 1.48 yd³ of base gravel orders 1.50 yd³ and ≈ 5.43 yd³ of stone orders 5.50 yd³. The results card and the drawing keep the figured amounts, with ≈ in front when the two decimals shown are rounded.
  4. The check. With nothing above the wall, green reads 48" or less unbalanced fill — IRC R404.4; with a load above, 24" or less with a load above — IRC R404.4. ENGINEER REQUIRED — LOAD ABOVE A WALL OVER 24" (IRC R404.4) shows when there is a load above and the block stack is over 24". ENGINEER REQUIRED — OVER 48" UNBALANCED FILL (IRC R404.4) shows when the exposed stack is over that: whole courses times the block height, the wall the app draws and orders, not the height you typed. A block that does not divide 48" can round a 4' 0" wall past the line: 4' 0" in 7-5/8" block is 7 courses = 4' 5-3/8". When rounding is what crosses 48", the results show both stacks, the one drawn (over 48") and the one a course lower (under it), and a BURY THE EXTRA COURSE switch appears. Turn it on and the same stack is set one course lower: the extra course goes below the toe grade, so the exposed face is the shorter stack and the check passes, with the same blocks, caps and stone. The pad sits one course deeper; size that dig in Excavation. The results read Extra course buried: 6 courses = 3' 9-3/4" exposed, 2 buried, and the drawing dimensions both buried courses. The quantities still figure, but a wall over the line needs an engineered design. Lower the wall or take it to an engineer. Terraces closer than 2× the lower wall's height, or with the upper taller than the lower, are one wall: engineer required. Where your local code sets a stricter rule for tiered walls, it governs. Spacing terraces farther apart does not make them independent walls either: the slope as a whole still has to be stable.
  5. BASIS & WHAT WAS ASSUMED opens to the height rule — the height you enter is the exposed face, and the drawing dimensions the stack as built, rounded to full courses plus the burial — then the stack the check was judged on, which test ran (the 48" fill test alone, or the 24" test on the whole stack when there is a load above), the extra-course note when you buried one, then IRC 2021 — verify local code and the note on local amendments.
  6. The drawing is a section through the wall: cap, block, the gravel pad, the stone column and the drain pipe. Pinch to zoom, drag to pan, double-tap to reset, and use the expand button for full screen. See Drawings.
  7. The buried course is dimensioned on the drawing below the grade line, under the exposed height. If your height is not a whole number of courses, the height on the drawing is the rounded-up stack, not the number you typed, and the check reads that stack. The cut sheet and History print it as Exposed Stack next to the height you typed, with Load Above Wall and Buried Courses. History's Open in Retaining Wall puts both choices back.
  8. MATERIALS. The block row says how many buried courses it includes, the embed they reach and the rule that set it, and tells you to check it against the manufacturer's spec. The list also has Cap adhesive (one 29 oz tube per 60 ft of cap), Drain outlets to daylight (one at least every 50 ft of drain) and a Filter fabric strip over the stone, all from Allan Block's manual. A slope below the wall adds a line: Slope below: buried for NCMA Table 5-1; keep a 4' level bench in front and have the slope's stability checked, red when it is steeper than 2H:1V. Over 3' 2" exposed, an amber line says a gravity wall in clay needs geogrid by Allan Block's chart. The app does not ask the soil, so geogrid is not counted: check the maker's chart.
Retaining Wall is a Pro calculator. SHARE PDF and SAVE are Pro as well, a one-time purchase. SHARE PDF and the list icon at its right end stay dim until you have entered your own measurements. SAVE stays lit, but until then a tap only answers Enter your job's numbers first and saves nothing.

Common mistakes