Retaining Wall Calculator

This retaining wall calculator estimates the blocks, cap blocks and gravel you need for a small segmental block retaining wall. Enter the wall length and the exposed height above grade, choose a block size, and it works out blocks per course, the number of courses including one buried course, and the total with an allowance for cuts.

It also estimates the crushed gravel for the leveling pad under the first course and the drainage zone behind the wall, in cubic yards and tons or in metric. It is meant for low garden and landscape walls. Taller walls, or walls holding back a slope, driveway or building, need an engineered design.

How to Use the Retaining Wall Calculator

1
Choose units and enter the wall size
Pick US or metric, then enter the wall length and the exposed height above the finished ground, not counting caps.
2
Pick the block
Choose the 12 x 4 in garden wall block, the 16 x 6 in segmental block, or enter a custom face length, face height and depth from the spec sheet.
3
Set caps and extra
Keep Include cap blocks on and enter the cap length, and set the extra percentage for cuts and breakage.
4
Adjust the gravel
Change the leveling pad depth (6 in default) and the width of the drainage zone behind the wall (12 in default) if your block maker specifies different values.
5
Read the results
The results show total blocks, blocks per course, courses, caps, built height and the gravel for the base and backfill.

How the retaining wall calculator works

Blocks per course is the wall length divided by the block face length, rounded up. The number of exposed courses is the exposed height divided by the block face height, rounded up, and the calculator adds one buried course below grade to key the wall into the ground. Total blocks is blocks per course times courses, plus your extra percentage. Caps are the wall length divided by the cap length, plus the same extra.

Gravel is split into two parts. The leveling pad is the wall length times the pad depth times a trench width of the block depth plus 12 inches (about 6 inches in front and behind). The drainage backfill is the wall length times the full built height (including the buried course) times the drainage zone width. Weight uses an average of 1.4 tons per cubic yard for crushed stone.

Blocks per course = wall length ÷ block length (round up)
Courses = (exposed height ÷ block height, round up) + 1 buried
Blocks = blocks per course × courses × (1 + extra %)
Caps = wall length ÷ cap length × (1 + extra %)
Base (cu ft) = length × pad depth × (block depth + 12 in)
Backfill (cu ft) = length × built height × drainage width

Worked example: a 30 ft wall, 2 ft high, with 12 x 4 in blocks

With 16 x 6 in blocks (12 in deep) on a 40 ft wall 3 ft high, you get 30 blocks per course and 7 courses, so 221 blocks and 32 caps with 5% extra, plus about 6.67 cu yd of gravel (9.3 tons). In metric, a 10 m wall 0.6 m high with 30 x 10 cm blocks needs 34 per course, 7 courses, 250 blocks and 36 caps with 5% extra, and about 2.82 m³ of gravel.

Blocks per course: 360 in ÷ 12 in = 30.
Exposed courses: 24 in ÷ 4 in = 6, plus 1 buried course = 7 courses.
Blocks: 30 x 7 = 210, plus 5% = 220.5, rounded up to 221 blocks.
Caps (12 in long): 30, plus 5% = 31.5, rounded up to 32 caps.
Built height: 7 x 4 in = 28 in.
Leveling pad: 30 ft x 0.5 ft x (7 + 12 in = 1.58 ft) = 23.75 cu ft, or 0.88 cu yd.
Drainage backfill: 30 ft x 2.33 ft x 1 ft = 70 cu ft, or 2.59 cu yd.
Total gravel: 3.47 cu yd, about 4.9 tons.

Blocks per 10 ft of wall

Blocks for every 10 ft of wall length, including one buried course and before any extra for cuts. Gravel uses the default 6 in pad and 12 in drainage zone.

Block face	Exposed height	Courses	Blocks	Backfill (cu yd)	Base (cu yd)
12 x 4 in	1 ft	4	40	0.49	0.29
12 x 4 in	2 ft	7	70	0.86	0.29
12 x 4 in	3 ft	10	100	1.23	0.29
16 x 6 in	1 ft	3	24	0.56	0.37
16 x 6 in	2 ft	5	40	0.93	0.37
16 x 6 in	3 ft	7	56	1.30	0.37

Tips and common mistakes

Check the real block dimensions. Many blocks sold as 12 in are slightly shorter, which adds a block per course on long walls.
Curves and corners use extra blocks and cuts. Raise the extra percentage for walls with many turns.
Use clean, angular crushed stone for the drainage zone, not round pea gravel or soil, and place a perforated drain pipe at the base behind taller walls.
Compact the leveling pad well and get the first course perfectly level. Every mistake there shows up in the courses above.
Block makers usually call for at least one buried course and more for taller walls. Follow the burial depth in their installation guide.
Many block systems step back slightly with each course (setback). Leave room for it at the top of the wall.

When you need an engineer or a permit

Retaining walls fail when the soil pressure behind them is more than the wall can resist, and a collapsing wall can injure people and damage property. In many US jurisdictions, walls taller than about 4 ft, often measured from the bottom of the footing to the top of the wall, need a building permit and a design by a licensed engineer. Some places set the limit at 3 ft or lower.

Even a shorter wall may need an engineer if it holds back a slope, a driveway, a pool, a building or other load above it, sits near a property line, or is built on poor soil. Taller walls also usually need geogrid reinforcement. Check with your local building department before you start, and treat this calculator as a material estimate only.

Features

Blocks per course and number of courses, including one buried course
12 x 4 in and 16 x 6 in block presets or a custom block size
Optional cap blocks with their own length
Extra percentage for cuts and breakage
Gravel leveling base and drainage backfill in volume and weight
Warning when the wall is tall enough to need an engineer or permit

Frequently Asked Questions

How many retaining wall blocks do I need?

Divide the wall length by the block length to get blocks per course, then multiply by the number of courses. Include one buried course below grade. A 30 ft wall 2 ft high with 12 x 4 in blocks needs 30 blocks per course and 7 courses, which is 210 blocks, or 221 with 5% extra for cuts and breakage.

How many courses of block should be buried?

Most block makers call for at least one course below grade, and a deeper burial for taller walls. That buried course keeps the base of the wall from sliding. This calculator always adds one buried course to your exposed height. If your installation guide asks for more, add that depth to the height you enter.

How much gravel do I need behind a retaining wall?

A common approach is a 12 in wide zone of crushed stone behind the blocks for the full wall height, plus a 6 in deep leveling pad under the first course. For a 30 ft wall 2 ft high with 12 x 4 in blocks, that is about 2.59 cu yd of backfill and 0.88 cu yd of base, or 3.47 cu yd in total, roughly 4.9 tons.

How tall can a retaining wall be without an engineer?

In many US jurisdictions, walls up to about 4 ft, often measured from the bottom of the footing, can be built without an engineered design, but some places set the limit at 3 ft or lower. Walls that hold back a slope, driveway or structure may need an engineer at any height. Always check with your local building department before building.

How many cap blocks do I need?

Divide the wall length by the cap length and round up, then add a little for cuts. A 30 ft wall with 12 in caps needs 30 caps, or 32 with 5% extra. Caps on curved walls are often cut or laid in alternating directions, so buy a few more for curves. Glue caps down with a masonry adhesive made for concrete block.

Do I count the cap block in the wall height?

In this calculator, no. Enter the exposed height to the top of the last full course of wall blocks, and the caps sit on top of that. If you have a finished height in mind that includes the cap, subtract the cap thickness first so you do not end up with one course too many.

Last reviewed September 21, 2026