Retaining wall block calculator

The course you never see

Ask for a 3 ft wall and you need seven courses of 6 in block, not six. Nicolock's installation instructions are explicit: "Bury the first course of block below grade to a depth equal to 10% of the height of the wall or a minimum of 6", whichever is greater." Allan Block sets the same trench from the other side — 6 in of wall rock plus an inch of buried block per foot of height.

That buried course is the anchor. Everything above it is being pushed outwards by the soil, and what stops the bottom sliding is the weight of earth in front of it. Leave it out and you are short a full course — on a 20 ft wall of 16 in block, 15 blocks, plus you have built the wall that will lean.

How the counts work

• Courses = exposed height ÷ block height, then add the buried course(s).
• Blocks per course = wall length ÷ block face length, rounded up. Most of these units are trapezoidal so they can turn curves, so the face is the number that matters, not the back.
• Caps are a separate item number, one row along the top, and they are glued down with concrete adhesive rather than laid dry.

Every one of those dimensions is per-product, which is why this calculator asks instead of assuming. Big-box basic block is 6 x 16 x 12 in, as is VERSA-LOK Standard; Pavestone ProMuro and Allan Block Classic have 18 in faces; Anchor's Windsor Stone is 4 in high and rated for a 2 ft wall. Read your pallet.

The gravel is bigger than the blocks, and it is a safety item

For a 20 ft wall 3 ft above grade: 12 in of clean stone behind the block over the full 3.5 ft of block height is 70 cu ft, and the 6 in leveling pad — 24 in wide, because the trench runs 6 in past a 12 in block front and back — is another 20 cu ft before compaction. That is 90 cu ft, or 95 with the pad's compaction allowance: about 3.5 cubic yards, 4.75 tons. The blocks themselves are about 70 cu ft of concrete. There is more stone in this wall than block.

This page counts the stone alongside the block, and not for tidiness about the shopping list: leaving the drainage out is the failure mode of the wall. Water in the backfill has nowhere to go, and saturated soil pushes far harder than drained soil. The drainage stone, the perforated pipe at the base and the geotextile between stone and soil are not optional extras.

Where this calculator stops

Four feet is the line, and it is measured from the bottom of the footing, not from the lawn. IRC R105.2 exempts from permit only "retaining walls that are not over 4 feet in height measured from the bottom of the footing to the top of the wall, unless supporting a surcharge". A 3 ft exposed wall on a 6 in buried course over a 6 in pad measures exactly 4 ft — you are at the threshold already.

Four feet is a code threshold, not a rating on your block. The cap on the height field comes from IRC R105.2, and it says nothing about what your particular unit can hold up dry. Manufacturers publish a maximum unreinforced height per product and it can be far lower — Anchor's Windsor Stone is listed for 2 ft. This page cannot tell which unit is on your pallet, so read that limit off the manufacturer's literature and let it govern before the permit line does.

Nicolock draws its own line lower: "Walls higher than 36" above grade, terraced walls, weak soils, and other loading conditions require special considerations in design and construction. This should be done by a qualified engineer." A wall taller than that, or one holding up a driveway, slope or tier, needs designing and permitting. This calculator does not size geogrid, and no quantity calculator can.

The wall this counts

Within that limit, the count still describes one particular wall: a straight run of constant height on level ground. Where yours differs:

• Curves and corners are not this arithmetic. Length ÷ face length is a straight-line count. These units are trapezoidal so they can turn, which means an outside curve takes more blocks per foot of face than a straight run and an inside curve fewer. A 90° corner usually needs corner units, bought separately, and the courses interlock differently there.
• Every course is counted at full length. A wall that steps down at its ends, or follows a slope in steps, has short upper courses — this counts them all as long as the bottom one. On a stepped wall the block count is high and the gravel is high with it.
• The setback is not modelled. Segmental block steps back a set amount per course, so the top of the wall sits behind the bottom. That means the excavation behind the wall is wider at the top than "12 in behind the block" suggests, and on a tall wall the difference is real digging. The drainage volume here is a plain rectangular prism against a vertical face.
• Caps are counted as a straight row. No allowance for the mitred cuts a curve or a corner needs, and caps typically overhang, so their length rarely divides the wall the way the wall units do.
• The gravel is stone only. The perforated drain pipe at the base, the geotextile between stone and soil, and the compactable backfill above the drainage zone are all specified parts of the wall and none of them are quantified here. The pipe also displaces a little of the stone.
• Two materials are ordered as one. The drainage zone wants clean open-graded stone; the leveling pad under it is usually dense-graded base. They are different products at different prices, and this page adds their volumes and weighs the total at a single 2,700 lb/cu yd. Crusher run is nearer 2,950, so if you are buying the pad separately, split the two — the cubic feet figure is the sum of those two solids and can be taken apart.
• Nothing here handles the spoil. A 24 in trench along a 20 ft wall produces a pile of soil that has to go somewhere, and on most jobs that is the logistics problem, not the block delivery.

One thing that deliberately cannot bite you here. Nicolock ties the burial depth to "10% of the height of the wall or a minimum of 6", whichever is greater", and it does not say whether that height is the exposed wall or the whole wall from the bottom of the footing. This page reads it as the exposed height — and within the 48 in the field allows, 10% never reaches the 6 in minimum either way, so the buried depth is always the 6 in floor, rounded up to a whole course. Read the rule the other way and the answer on this page does not change. Above about 5 ft it would, and above 4 ft you are off this page anyway.