Retaining Wall Calculator

How to use this calculator

Enter the length of the wall and how tall you want it to stand above grade. Pick your block size from the list or enter your own, and the calculator works out the four things you actually have to order: wall blocks, cap blocks, gravel for the levelling pad, and drainage stone for behind the wall.

The defaults follow standard practice — one buried course, a 6-inch pad in a 24-inch trench, and a 12-inch column of drainage stone. Change any of them and the numbers follow.

A block wall is four material orders, not one

This is where most wall projects go wrong at the counter. People price the blocks, buy the blocks, and discover on the day that the wall also needs several tons of two different kinds of stone.

Material	What it does	Typical spec
Wall blocks	The wall itself, including the buried course	Face size varies by maker
Cap blocks	Finish the top, glued down	One course along the length
Levelling pad gravel	Flat, compacted, load-spreading base	6 in deep, 24 in wide
Drainage stone	Carries water down instead of into the soil behind	12 in wide, clean ¾ in stone

On the 20-foot example above, the stone alone comes to about 3.5 cubic yards, or roughly 4.8 tons. That is a delivery, not a few bags.

The buried course

A retaining wall does not start at ground level. The first course is set below grade so the soil in front of it holds the base of the wall in place; without it the bottom course can simply slide forward under load.

Total height = exposed height + buried depth   |   Courses = total height ÷ block height

The usual rule is to bury one full course. A common alternative is 10% of the finished wall height, which works out about the same on a low wall and deeper on a tall one.

Here is why it matters to your order:

Wall	From exposed height	With buried course	Short by
20 ft long, 2 ft exposed	60 blocks	80 blocks	20
20 ft long, 3 ft exposed	100 blocks	120 blocks	20
40 ft long, 3 ft exposed	200 blocks	240 blocks	40

It is always exactly one course — which on a long wall is a lot of blocks, and a second delivery charge.

Working out the blocks

Both halves of the block count round up, separately, because you cannot buy part of a block:

Courses = ⌈total height ÷ block height⌉
Blocks per course = ⌈wall length ÷ block width⌉
Wall blocks = courses × blocks per course

Two things this does not capture. Curves and corners eat blocks, because units get cut to close the angle, and the offcuts are rarely reusable. Setback — the slight backward lean each course takes — shortens the top courses very slightly on a curved wall. Neither is worth modelling precisely; a waste allowance covers both.

Use 5% on a straight run with square ends, and 10% or more if the wall curves, steps up a slope, or turns a corner.

The levelling pad

Everything above ground rests on this, and it is the one layer you cannot fix later.

The standard is 6 inches of compacted gravel minimum, in a trench roughly twice the depth of the block, with the pad extending about 6 inches beyond each edge of the bottom block. State DOT specifications for segmental walls use the same 6-inch minimum, and call for the native soil beneath it to be compacted as well before the pad goes in.

Go deeper where the ground is poor. A contractor's rule of thumb is 3 to 6 inches under a wall below 2 feet, more as walls get taller, and considerably more over clay. Build the pad up in 3-inch lifts, compacting each one — a single 6-inch dump does not compact properly through its full depth, and the settlement shows up as a dip in the wall two years later.

Drainage stone, and why walls fail

Retaining walls very rarely fail because the blocks were not strong enough. They fail because water got into the soil behind them.

Saturated soil is heavier than dry soil and it pushes sideways with far more force — and in a freezing climate that trapped water expands as well. The fix is to give water an easier path than into the fill: a column of clean ¾-inch stone at least 12 inches wide directly behind the blocks, a 4-inch perforated drain pipe at the bottom of it running out to daylight, and non-woven filter fabric between the stone and the native soil so silt does not wash in and clog it.

Use clean, angular, open-graded stone. Anything with fines in it packs tight and stops draining, which defeats the point.

When the wall needs an engineer

There are two thresholds worth knowing, and both are measured on the exposed height.

Exposed height	What changes
Under 3 ft	Normally a straightforward gravity wall — the mass of the blocks does the work
3 to 4 ft	Soil reinforcement (geogrid laid back into the fill every few courses) is normally required
Over 4 ft	Engineered design and a permit in most jurisdictions

Three things push you over those thresholds regardless of height: a surcharge on top of the wall (a driveway, a parking area, a pool, another wall above it), saturated or clay soil, and a slope rising behind the wall. Any of those and the wall is carrying far more than its height suggests.

Tiering is not a loophole. Two 3-foot walls stacked close together behave like one 6-foot wall unless they are separated by a generous bench, and that separation distance is itself an engineering question.

Worked example: a 20 ft wall, 3 ft above grade

Using 12 × 8 inch blocks, one buried course, a 6 in × 24 in pad and 12 in of drainage stone:

1. Total height: 36 in exposed + 8 in buried = 44 in
2. Courses: ⌈44 ÷ 8⌉ = 6
3. Blocks per course: ⌈240 ÷ 12⌉ = 20
4. Wall blocks: 6 × 20 = 120
5. Cap blocks: 20
6. Levelling pad: 240 × 24 × 6 ÷ 1,728 = 20 cu ft = 0.74 cu yd
7. Drainage stone: 240 × 12 × 44 ÷ 1,728 = 73.3 cu ft = 2.72 cu yd
8. Stone weight: 3.46 cu yd × 1.4 = 4.8 tons

Add 5% waste and the block order becomes 126 plus 21 caps.

Common mistakes

• Counting courses from the exposed height. One course short, every time — and on a long wall that is a second delivery.
• Skipping the drainage stone. It is the cheapest material on the list and the one that decides whether the wall is still straight in ten years.
• Backfilling with the soil you dug out. If it was clay going in, it is clay going back, and clay holds water against the wall.
• A thin or uncompacted pad. Everything above it inherits the settlement. Build it in 3-inch lifts.
• Forgetting the pipe outlet. A drain pipe that does not run out to daylight or a drain is just a buried tube full of water.
• Assuming tiering avoids engineering. Two close-set walls act as one taller wall.
• Using nominal block dimensions. Face sizes vary between manufacturers far more than you would expect. Check the spec sheet.

A note on accuracy

This calculator gives clean quantities from the wall you describe. Real walls curve, step up slopes, turn corners and sit on ground that is not level, and every one of those adds blocks. Treat the result as a planning figure, use the waste allowance honestly, and confirm the block dimensions against the manufacturer's spec sheet before ordering. For anything over 4 feet of exposed height, or with a surcharge above it, the quantities here are a budget estimate and the real numbers come from the engineered design.

Where these figures come from

The base, drainage and reinforcement guidance built into this page comes from the sources below. The arithmetic — courses, blocks per course, and volume to weight — is straightforward geometry you can check in the full working the calculator shows. Our worked example's levelling pad figure agrees with an independently published quantity for the same 20-foot, 24-inch trench.

• Section 9071 — Segmental Block Retaining Walls Source: Iowa Department of Transportation. The 6-inch minimum levelling pad and the requirement to compact the native soil beneath it.
• Segmental Retaining Wall Specification (PDF) Source: Western Interlock. A manufacturer specification for the drainage fill: clean ¾-inch minus stone, minimum 12 inches behind the wall.
• Segmental Block Retaining Walls Source: Journal of Light Construction. Base depth by wall height, compacting in 3-inch lifts, and how clay subgrade changes the excavation.
• Retaining Wall Calculator Source: Inch Calculator. An independent implementation of the course and block-per-course method, used here to cross-check our worked example.

Frequently asked questions

How many blocks do I need for a retaining wall?

Multiply the number of courses by the blocks per course, rounding each up. Courses is the total wall height divided by the block height, and total height includes the buried first course. A 20 ft wall standing 3 ft above grade in 12 by 8 inch blocks needs 6 courses of 20, which is 120 blocks, plus 20 caps.

How deep should the first course of a retaining wall be buried?

One full course is the usual rule, so 8 inches for a standard 8-inch block. A common alternative is 10% of the finished wall height, which comes out about the same on a low wall and deeper on a tall one. The buried course stops the base of the wall sliding forward under load.

Do I need gravel behind a retaining wall?

Yes, and it is the single most important material on the list. A column of clean three-quarter inch stone at least 12 inches wide behind the blocks, with a 4-inch perforated pipe at the bottom running out to daylight and filter fabric between the stone and the soil, carries water away. Saturated soil is heavier and pushes much harder than dry soil, and that is what makes walls fail.

How tall can a retaining wall be without an engineer?

Four feet of exposed height is the usual threshold for an engineered design and a permit, and above about 3 feet soil reinforcement such as geogrid is normally required. A surcharge above the wall, a slope rising behind it, or clay soil can push you over those limits at a lower height.

How thick should the base of a retaining wall be?

Six inches of compacted gravel is the usual minimum, in a trench roughly twice the block depth, with the pad extending about 6 inches past each edge of the bottom block. State DOT specifications use the same 6-inch minimum. Build it up in 3-inch lifts and compact each lift; soft or clay subgrade wants more.

Do I need a permit for a retaining wall?

Usually once the exposed height passes 4 feet, though some jurisdictions set it lower and many require one for any wall carrying a surcharge such as a driveway. Check with your building department before you order, because a permit normally means an engineered design too.

What is geogrid and when do I need it?

Geogrid is a polymer mesh laid horizontally back into the fill behind the wall every few courses, tying the soil mass to the blocks so the whole thing acts as one unit. It is normally needed above about 3 feet of exposed height, and sooner if there is a slope or a load above the wall. The manufacturer's installation guide specifies the spacing and the length.

Can I build two shorter walls instead of one tall one?

Tiering is not an automatic way around the engineering threshold. Two walls set close together behave like one taller wall, because the upper wall loads the lower one. They only act independently when separated by a generous bench, and that separation distance is itself an engineering question.