Understanding your result
The headline is the total number of wall blocks, with your extra allowance included. The line beneath shows how that breaks down into courses and blocks per course, so you can check it against a sketch.
Courses above ground is rounded up to whole courses, because you cannot lay part of a block. The table shows the height the wall will actually reach. If you asked for 2 feet and the blocks are 6 inches high, four courses hits it exactly. If the blocks are 8 inches high, three courses also gives exactly 2 feet, but a 2 foot 6 inch wall would round up to four courses and stand 2 feet 8 inches.
Base gravel fills a trench under the first course. Drainage gravel fills the space directly behind the wall. They are different materials. The base wants crusher run, which packs hard. The drainage zone wants clean, angular stone with no fines, which lets water through.
The drawing shows a section through the wall, with the buried course, the base, the drainage zone and the pipe.
How we calculate this
The trench width is taken as twice the depth of the block from front to back, with a minimum of 12 inches. The 1.15 on the base allows for compaction.
Caps are assumed to be the same length as the wall blocks. If your caps are a different length, divide the wall length by the cap length instead.
The assumptions behind the numbers
| Assumption | Value used | Why |
|---|---|---|
| Buried course | One, unless you change it | Standard practice for low walls. Follow your block maker if they say more |
| Base depth | 6 in (15 cm) of compacted gravel | The usual minimum under a segmental wall |
| Trench width | Twice the block depth, at least 12 in | Room for the block and for compacted base in front and behind |
| Base compaction | 15% extra | Crusher run loses volume when compacted |
| Drainage zone | 12 in (30 cm) thick unless you change it, full wall height | Clean stone directly behind the blocks |
| Extra blocks | 5% unless you change it | Cuts at ends and occasional breakages |
| Gravel weight | 2,800 lb per cu yd for base, 2,700 for drainage stone | Middle of the usual range. Varies by quarry |
| Setback | Ignored | Blocks step back slightly each course. It does not change the count |
Assumptions last reviewed October 3, 2026.
This calculator is for low landscape walls built with dry-stacked, interlocking blocks. It does not design a wall. It does not account for loads above the wall such as driveways, slopes or buildings, and it does not calculate reinforcement. For anything tall, or anything holding back more than garden soil, the block maker’s guide and a local engineer are the right sources.
Two worked examples
A garden terrace wall
A wall 30 feet long will hold back a 3 foot change in level. The blocks are 16 inches long, 6 inches high and 10 inches deep.
- Blocks per course: 30 × 12 ÷ 16 = 22.5, so 23
- Courses above ground: 36 ÷ 6 = 6
- Total courses with one buried: 7
- Wall blocks: 23 × 7 = 161, plus 5% = 170 blocks
- Caps: 23 plus 5% = 25 caps
- Base trench: 20 in wide, 6 in deep
- Base gravel: 30 × 1.67 × 0.5 × 1.15 = 28.8 cu ft, or 1.5 tons
- Drainage gravel: 30 × 1 × 3 = 90 cu ft, or 4.5 tons
- Drain pipe: 30 ft
The buried course alone is 23 blocks that nobody will ever see. Leaving it out to save money is the most common shortcut on a wall like this, and the most common reason for the bottom course sliding forward a few winters later.
A low edging wall, in metric
A wall 8 meters long and 60 cm high is being built from blocks 300 mm long, 100 mm high and 200 mm deep.
- Blocks per course: 8,000 ÷ 300 = 26.7, so 27
- Courses above ground: 600 ÷ 100 = 6
- Total courses with one buried: 7
- Wall blocks: 27 × 7 = 189, plus 5% = 199 blocks
- Caps: 27 plus 5% = 29 caps
- Base gravel: 8 × 0.4 × 0.15 × 1.15 = 0.56 m³
- Drainage gravel: 8 × 0.3 × 0.6 = 1.44 m³
Small blocks make a low wall look right, but they add up quickly. Nearly 200 blocks for a wall you can step over is a lot of lifting, so have the pallets set down as close to the trench as the truck can reach.
Common mistakes
- Forgetting the buried course. It is a full row of blocks, and it is not optional.
- Building on soil or sand. The first course needs compacted gravel under it. Soil settles and sand washes out.
- Skipping the drainage stone. Backfilling with the soil you dug out traps water against the wall. Use clean stone and a pipe.
- Using round gravel for drainage. Pea gravel rolls and does not lock together. Use angular crushed stone.
- Getting the first course out of level. Every error in the first course is repeated in every course above it. Spend most of your time there.
- Building too tall for the block. Check the height limit in the maker’s guide before you buy.
- Ignoring what is above the wall. A slope, a driveway or a fence at the top adds load that a simple garden wall is not designed for.
Printable shopping list
Print shopping list puts the whole order on one page: wall blocks, caps, base gravel, drainage stone and pipe, with the wall dimensions. Block suppliers sell by the pallet, so ask how many blocks are on one and round up. Order the two kinds of gravel as separate loads, or ask for them to be dumped in separate piles, because once they are mixed they are no use for drainage.
Questions people ask
- How many blocks do I need for a retaining wall?
- Divide the wall length by the block length to get blocks per course, and the wall height by the block height to get courses. Add one course for the buried row, multiply, and add about 5% for cuts and breakages. A wall 20 feet long and 2 feet high in 12 × 4 inch blocks needs 20 per course and 7 courses, which is 147 blocks with the extra.
- How deep should the first course be buried?
- For a low garden wall, one full course is the usual rule. Some block makers specify one inch of burial for every eight inches of wall height. Burying the first course stops the bottom of the wall from kicking out under pressure.
- Do I need gravel behind a retaining wall?
- Yes. Water trapped behind a wall is the main reason walls bulge and fail. A zone of clean crushed stone about 12 inches thick directly behind the blocks, with a perforated pipe at the bottom, lets water drain away instead of building up pressure.
- How tall can I build a block retaining wall myself?
- Most interlocking garden blocks are designed for walls up to 3 or 4 feet without reinforcement, and the limit is printed in the maker's installation guide. In many places a wall over 4 feet, measured from the bottom of the footing, needs a permit and an engineered design.
- What goes under the first course?
- About 6 inches of compacted crusher run or paver base, in a trench roughly twice as wide as the block is deep. Not sand and not bare soil. The base must be level along its length, because every course above copies whatever the first course does.
- Does the calculator handle curved walls and corners?
- It counts a straight wall. Curves use slightly more or fewer blocks depending on the direction of the curve, and corners need extra blocks or special corner units. Count those separately and add them.
Built and reviewed by Bertha
Last updated · How we work out our numbers