Retaining Wall Calculator

Blocks = ceil( ceil( wall length ÷ block length ) × ( ceil( wall height ÷ block height ) + 1 ) × ( 1 + wastage ÷ 100 ) )

L

Length of the wall face, in feet

H

Height of the wall above the ground, in feet

b

Length of one block along the wall, in inches

h

Height of one course of block, in inches

d

Depth of the base trench, in inches

w

Wastage allowance as a percentage of the block count

Use it once you have chosen the block system, because the block length and the course height are the two inputs that decide the answer and both come from the manufacturer rather than from the wall. Measure the length along the face of the wall rather than the length of the excavation: a wall that curves, or one that steps down a slope, is measured in straight segments, and each segment rounds up on its own. Measure the height as the height above the finished ground on the low side — the side the wall is holding up — because that is the height the wall and the drainage behind it have to resist. The wastage allowance is small and it is small for a reason: a dry-stacked wall is cut only at the two ends and where it steps, not around every opening the way siding is. Five percent covers those ends and a couple of blocks broken in handling. If you are building on a slope, run each step as its own wall and add the answers, because a stepped wall is several short walls rather than one tall one.

1. A 30 foot wall, 3 feet high, in 12 by 6 inch blocks

   1. Blocks per course: 30 feet ÷ 1 foot per block = 30 blocks (the block is 12 inches long)
   2. Courses: 3 feet ÷ 0.5 feet per course = 6 visible courses, plus the buried course = 7
   3. Blocks: 30 × 7 = 210, plus 5% wastage = 220.5, rounded up to 221 blocks
   4. Cap blocks: one per block along the top course = 30
   5. Base gravel: 30 × 1 foot of trench width × 0.5 feet deep = 15 cubic feet = 0.56 cubic yards (0.42 m³)

   These are the page defaults and the arithmetic divides exactly at every step, which is why the answer is the one to check the method against: 30 blocks a course, 7 courses, 221 blocks. The seventh course is the buried one — six courses of a 6 inch block is exactly 3 feet, which is the wall you can see, and the block in the trench is what stops the bottom course from being pushed out by the soil behind it.

2. A 50 foot wall, 4 feet high, in 16 by 4 inch blocks

   1. Blocks per course: 50 feet ÷ 1.3333 feet per block = 37.5, rounded up to 38 blocks
   2. Courses: 4 feet ÷ 0.3333 feet per course = 12 visible courses, plus the buried course = 13
   3. Blocks: 38 × 13 = 494, plus 5% wastage = 518.7, rounded up to 519 blocks
   4. Cap blocks: one per block along the top course = 38
   5. Base gravel: 50 × 1 × 1 = 50 cubic feet = 1.85 cubic yards (1.42 m³)

   This is the example that shows why the counting is done per course rather than by area. The wall face is 200 square feet, and 200 square feet of 16 by 4 block is 37.5 blocks a course on average — but you cannot buy half a block at the end of a course, so every one of the 13 courses needs 38. Counting by area would have given 488 blocks; the per-course count gives 494 before wastage. A shorter block makes the error bigger: at 12 inches, a wall length that is not a whole number of feet costs you a block a course.

3. A taller 24 foot wall, 6 feet high, in 12 by 6 inch blocks

   1. Blocks per course: 24 feet ÷ 1 foot per block = 24 blocks
   2. Courses: 6 feet ÷ 0.5 feet per course = 12 visible courses, plus the buried course = 13
   3. Blocks: 24 × 13 = 312, plus 5% wastage = 327.6, rounded up to 328 blocks
   4. Cap blocks: 24
   5. Base gravel: 24 × 1 × 0.5 = 12 cubic feet = 0.44 cubic yards (0.34 m³)

   Twice the height of the first example and only half again as many blocks, because the length halved at the same time — but this is the wall where the counting stops being the interesting question. A dry-stacked wall six feet high is a structure: at that height the manufacturer's own tables call for geogrid layers reaching back into the soil, and the wall needs an engineer rather than a calculator. This page will still tell you how many blocks and how much gravel, and it will be right about both.

The block count is exact for the block and the wall you describe; everything behind the wall is an assumption. The buried course is counted as one extra course in every case, which is normal practice and is the reason the course count is one higher than what you can see, but a wall built on rock or on an existing concrete footing may not need it. The cap blocks are counted as one per block along the top course, which assumes the cap is the same length as the block — many systems sell a 16 or 18 inch cap for a 12 inch block, so if yours differs, divide the wall length by the cap length instead and expect the block count to move slightly as well. The base gravel is a volume for a trench 12 inches wide by the depth you entered, which is the block depth plus working room on both sides; a wider trench is more gravel and no more blocks. Three things that decide whether the wall stands up are not in this arithmetic at all. Drainage is the first: gravel backfill behind the wall, a weep hole in the bottom of every course, and filter fabric between the gravel and the soil, because a dry-stacked wall that holds water is a wall that fails. Geogrid is the second: above roughly two feet the manufacturer's tables start calling for grid layers, and a wall over four feet should be designed rather than estimated. Compaction of the base is the third, and a base that was not tamped in lifts is the most common reason a low wall settles and opens at the joints. Nothing here is a structural design, and none of it replaces the block manufacturer's installation guide.

How many blocks do I need for a 30 foot retaining wall?

221 blocks of 12 by 6 inches for a wall three feet high: 30 blocks along each course, seven courses once the buried one is counted, so 210 blocks, plus 5% wastage, rounded up. You will also need 30 cap blocks and about 15 cubic feet of gravel for the base — 0.56 cubic yards. The height is what moves the answer: the same wall at six feet takes 410 blocks and 13 courses.

Why does the calculator count one more course than I can see?

Because the bottom course is buried. A dry-stacked wall starts in a trench, and the first course sits below the excavated line so that the soil in front of it can be backfilled over it. That block does not show in the finished wall, but it is bought, carried and laid like every other one, and it is what stops the bottom of the wall being pushed out. Six inches of course height and a three foot wall therefore means seven courses, not six.

Can I build a retaining wall without mortar?

Yes, and for a garden wall under about four feet that is the normal way. Segmental blocks are shaped so that each course sets back slightly and the ones above lock into the ones below, and the friction between courses plus the weight of the wall is what holds the soil back. Mortar would add almost nothing to that and would crack as the wall moves. What a dry-stacked wall does need is a compacted gravel base, gravel backfill, filter fabric and a weep hole in the bottom course — water is what destroys these walls, not the absence of mortar.

Do I need to deduct anything for drainage or the cap?

No, and both are counted separately rather than deducted. The cap blocks are counted one per block along the top course, which is why they appear as their own number: a 30 foot wall takes 30 caps, and a 50 foot wall in 16 inch blocks takes 38. Drainage stone behind the wall is a separate order from the base gravel and cannot be worked out from the wall dimensions alone, because it depends on the depth and width of the backfill zone — a common rule is a foot of clean gravel directly behind the blocks, with filter fabric on the soil side.

How deep should the base trench be?

Six inches of compacted gravel plus the buried course is the usual minimum for a garden wall, which is why the default here is six inches of gravel. The rule the block manufacturers use is that the buried depth should be about one tenth of the wall height plus the gravel bed, and it goes up on a slope or where the ground freezes. Trench depth matters more than it looks: the base is the only part of a dry-stacked wall that is fully hidden, and a wall that settles is a wall whose base was not compacted in thin lifts.

How tall can a dry-stacked wall be?

About four feet is the practical limit for a wall with no reinforcement, and above roughly two feet the manufacturer's tables start calling for geogrid layers that tie the wall back into the soil. A wall over four feet, a wall carrying a driveway or a building load, or a wall on a slope that has been cut into should be designed rather than estimated, because the failure mode is not the blocks — it is the soil behind them sliding. This page counts blocks and gravel; it does not check the slope, the soil or the drainage.

• Approximate Conversions from U.S. Customary Measures to Metric — the chart behind feet, inches, cubic yards and cubic metres on this page — National Institute of Standards and Technology
• The International System of Units (SI) brochure — the metre and the cubic metre the outputs are converted to — Bureau International des Poids et Mesures (BIPM)