Retaining Wall Calculator
A retaining wall looks simple — it is just stacked blocks — but ordering the materials is where most DIY walls go wrong. Skip the buried base course and the first row can slide forward under soil pressure. Forget the drainage gravel and water builds up behind the wall until it pushes the whole thing outward; poor drainage, not the blocks, is behind most residential wall failures. This calculator counts what you need to buy: wall blocks per course times the number of courses (including the buried one), cap blocks for the finished top, compacted gravel for the 6-inch base, and clean drainage gravel for the 12-inch zone behind the wall. Enter your wall below and you get the full materials list in cubic yards or cubic metres, plus tonnage for the gravel order.
How to use this calculator
- Measure the wall length along the face where the wall will run, and the exposed height — the part you will see above finished grade when the job is done. Do not include the buried course in the height; the calculator adds it.
- Pick your block from the product tag, not the catalog nickname. You need three numbers: face length, course height, and depth front-to-back. A common block is 16 × 8 × 8 in (400 × 200 × 200 mm).
- Choose the cap block length. Caps are thinner finishing units for the top row — they are counted and priced separately from the wall body.
- Keep one buried course selected unless your wall sits on an existing concrete footing. That buried row is what stops the base course sliding outward under soil pressure.
- Set the waste allowance. Ten percent covers cut blocks at corners and ends, plus breakage. Go to 15% if the wall has curves or many corners.
- Read the results: wall blocks to order, cap blocks, base gravel and drainage gravel. Gravel is shown in cubic yards (or cubic metres) and in tons, so you can order whichever way your supplier sells.
- Take the list to the supplier and confirm the block face size on the pallet tag before you pay — the same wall needs 30% more blocks if the units are 6 in tall instead of 8 in.
Formula
Worked example
Take a garden wall: 20 ft long, 3 ft of exposed height, built with standard 16 × 8 × 8 in blocks and 12 in caps, one buried course, 10% waste. Step one — blocks per course: (20 × 12) ÷ 16 = 15 exactly. Step two — courses: (3 × 12) ÷ 8 = 4.5, which rounds up to 5 visible courses, plus 1 buried = 6 courses. Step three — base blocks: 15 × 6 = 90 blocks. Step four — with waste: 90 × 1.10 = 99, so order 99 blocks. Step five — caps: (20 × 12) ÷ 12 = 20 caps. Step six — base gravel: 20 ft × (8 + 6) in × 6 in = 20 × 1.167 × 0.5 = 11.67 ft³ ÷ 27 = 0.43 yd³, plus 20% compaction = 0.52 yd³ (≈ 0.73 tons). Step seven — drainage gravel: 20 × 3 × 1 ft = 60 ft³ ÷ 27 = 2.22 yd³ (≈ 3.11 tons).
Metric version: a 6 m long wall, 0.9 m exposed, 400 × 200 × 200 mm blocks, 300 mm caps, one buried course, 10% waste. Blocks per course: 6 ÷ 0.4 = 15. Courses: 0.9 ÷ 0.2 = 4.5 → 5 visible + 1 buried = 6. Base blocks: 15 × 6 = 90; to order: 99. Caps: 6 ÷ 0.3 = 20. Base gravel: 6 × 0.3524 × 0.1524 × 1.2 = 0.39 m³ (≈ 0.71 tonnes). Drainage gravel: 6 × 0.9 × 0.3048 = 1.65 m³ (≈ 3.01 tonnes).
Measurement tips
- Read the block tag, not the nickname. "Standard" means different face sizes by brand — a 16 × 6 in unit needs a third more blocks than a 16 × 8 in unit for the same wall.
- Level the base trench obsessively. Every course inherits the first course's mistakes — a base that is out by half an inch at one end is out by inches at the top.
- Compact the base gravel in lifts. Dump 6 in at once and only the top compacts; the bottom stays soft and the wall settles.
- Put landscape fabric between the drainage gravel and the soil. Without it, fines wash into the gravel within a season or two and the drain clogs.
- Run a perforated drain pipe at the base of the gravel zone, sloped to daylight. Gravel alone stores water; the pipe removes it.
- Backfill and compact behind the wall as you build, course by course — not all at the end. Dumping a full height of soil against a finished wall is how walls get pushed out of plumb.
Common mistakes
- Skipping the buried course to save blocks. The base row then sits on grade with nothing holding it, and soil pressure slides it forward — the classic DIY wall failure.
- Counting area instead of courses. A 3 ft wall of 8 in blocks needs 5 courses (36 ÷ 8 = 4.5, rounded up), not 4.5 courses worth of blocks. Partial courses still need a full row.
- Forgetting drainage gravel. Water trapped behind the wall creates hydrostatic pressure that no stack of blocks can resist long-term. The gravel zone is structural, not decorative.
- Using the exposed height for the whole count. The buried course adds a full row of blocks — on a 3 ft wall that is a sixth of the order.
- Ordering exact count with no waste. Corners, ends and curves all eat cut blocks, and pallets always hide a cracked unit or two.
- Building over 4 ft without checking the rules. Above about 4 ft of exposed height most building codes want an engineered design and a permit — and often geogrid reinforcement.
Factors that affect accuracy
- Exposed height. Taller walls need more courses, more gravel, and past about 4 ft they cross into engineered-design territory in most places.
- Block face size. Smaller units mean more blocks per course and more courses — the same wall can need 30–50% more blocks with a 6 in tall unit versus an 8 in one.
- Buried courses. One buried course is standard; soft soil, slopes above the wall, or frost-prone ground can call for more.
- Drainage. The 12-inch gravel zone plus a drain pipe is what keeps water pressure off the wall — skimp here and the block count stops mattering.
- Soil type. Clay holds water and pushes harder than sandy soil; poor soils argue for deeper base gravel and often geogrid.
- Surcharge loads. A driveway, patio, or slope above the wall adds load the blocks alone were not sized for — another reason tall or loaded walls need an engineer.
Sources & assumptions
- Blocks per course = wall length ÷ block face length, rounded up; courses = exposed height ÷ block height (rounded up) + buried courses.
- Base gravel = length × (block depth + 6 in) × 6 in deep, plus 20% for compaction; drainage gravel = length × height × 12 in.
- Block-count method follows EverydayTools' retaining wall block formula; backfill guidance per CalculatorzHub.
- Gravel weight assumes 1.4 tons per cubic yard — confirm density with your supplier.
What this calculator doesn't do
- This sizes materials, not structure — it doesn't design geogrid layers or soil reinforcement.
- Walls over 4 ft (1.2 m) of exposed height usually need an engineered design and a permit; check your local building code.
- Gravel tonnage assumes 1.4 tons per cubic yard — confirm the density with your supplier, since it varies by stone type.
- Curves, corners and stepped bases need extra blocks this calculator doesn't add.
Frequently asked questions
How many retaining wall blocks do I need?
Divide the wall length by the block face length and round up — that is blocks per course. Divide the exposed height by the block height, round up, and add the buried courses — that is total courses. Multiply the two, then add 10% waste and round up again. A 20 ft × 3 ft wall in 16 × 8 in blocks works out to 90 base blocks, 99 to order.
Do I really need to bury the first course?
Yes, for almost every wall. The buried course locks the base row against sliding forward under soil pressure. The rule of thumb is about 1 inch of burial per 8 inches of exposed height — for most garden walls that means one full course below grade. Only a wall on an existing concrete footing can skip it.
How deep should the gravel base be?
Six inches of compacted crushed stone is the standard for walls up to 4 ft tall, in a trench about 6 inches wider than the block (3 inches each side). Compact it in thin lifts — dumping all 6 inches at once leaves the bottom soft. Taller walls may need a deeper base.
How much gravel goes behind a retaining wall?
Plan a 12-inch-wide zone of clean drainage gravel running the full length and height of the wall: volume = length × height × 1 ft, divided by 27 for cubic yards. A 20 ft × 3 ft wall needs about 2.2 cubic yards (roughly 3.1 tons). This gravel is the wall’s drainage system — water passes through it instead of pushing on the blocks.
How tall can a retaining wall be without an engineer?
About 4 ft (1.2 m) of exposed height is the common limit — below that, most building codes treat a segmental wall as landscaping. At or above it, expect to need a permit, an engineered design, and usually geogrid reinforcement. A slope, driveway, or structure above the wall can force engineering at any height, so check your local code first.
Does a retaining wall need drainage?
Yes — always. A perforated drain pipe at the base of a 12-inch clean-gravel zone, sloped to daylight, is what keeps hydrostatic pressure off the wall. Trapped water is the most common cause of retaining wall failure, far ahead of anything wrong with the blocks themselves.
What is geogrid and do I need it?
Geogrid is a synthetic mesh laid between block courses and extending back into the soil — it ties the wall to the earth behind it. It is typically required above 4 ft and worth considering on shorter walls that hold back heavy loads like driveways. This calculator does not size geogrid; that is engineer territory.
Are cap blocks included in the block count?
No — caps are counted separately, and this calculator lists them on their own line. Caps are thinner finishing units for the top row: divide the wall length by the cap length and round up. A 20 ft wall with 12 in caps needs 20 caps. Not every wall needs them, but they protect the top course and give a finished edge.
Next step
Take the block, cap and gravel totals to the landscape supplier, and confirm the block face size on the pallet tag — then check your local building code before you dig, since walls over about 4 ft usually need a permit.
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