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MaterialCalc — Retaining Wall Block Calculator

Estimate prepared 09/01/2026. Estimates only; verify against your plans before ordering.

Retaining Wall Block Calculator

Above finished grade. A course below grade is added automatically.

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Cuts at ends and curves. Curved walls run higher.

Result

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Block count is wall face area divided by block face area — but the face area is taller than the wall you see, because at least one course is buried below grade. That buried course is the most commonly forgotten line on a retaining wall order, and it is roughly 10% of the block on a 3 ft wall.

Assumptions used

These are the numbers behind the result. Where one of them is soft, the cost range widens rather than pretending to a precision the data does not have.

Retaining wall block face area
96 in²

A common 12x8 in face. Check your block — face size varies a lot by product line.

Allan Block and Pavestone product data — Industry standard, varies by supplier, checked 09/01/2026

Compacted base under retaining wall
6 in

Segmental wall installation guides — Industry standard, varies by supplier, checked 09/01/2026

Drainage stone behind wall
12 in

Water behind a wall is what pushes it over. This is not the place to economize.

NCMA segmental retaining wall guidance — Industry standard, varies by supplier, checked 09/01/2026

Wall height buried below grade
0.1

Roughly one course in ten, minimum one full course, sits below finished grade.

NCMA segmental retaining wall guidance — Rough figure — widens the estimate range, checked 09/01/2026

Crushed gravel density
105 lb/ft³

Compacted 3/4 in crushed stone. Varies by quarry and moisture; loose stone runs lighter.

USGS aggregate bulk density range — Industry standard, varies by supplier, checked 09/01/2026

Compaction allowance
0.2

Loose stone loses roughly a fifth of its volume once compacted, so order above the finished volume.

Standard earthwork practice — Rough figure — widens the estimate range, checked 09/01/2026

Prices verified 09/01/2026 (US national average, big-box retail shelf price and regional ready-mix quotes). Regional adjustment: Blended from RSMeans city cost index bands and BLS construction wage differentials.

The formula

Built height = exposed height + buried depth, where the buried depth is about a tenth of the wall height and at least one full course. Blocks = (wall length × built height) ÷ block face area. Caps = wall length ÷ block face width. Drainage stone = wall length × 12 in wide × built height, converted to tons at 105 lb per cubic foot.

Worked examples

40 ft wall, 36 in exposed

Wall length: 40 Exposed wall height: 36 Waste: 10

About 240 blocks over 6 courses including the buried one, plus 40 caps. Around 4 tons of drainage stone behind it — that is a full truckload nobody budgets for.

25 ft garden wall, 18 in exposed

Wall length: 25 Exposed wall height: 18 Waste: 10

Three courses, roughly 80 blocks. Low enough to be a straightforward weekend job with no engineering and usually no permit.

60 ft wall, 48 in exposed

Wall length: 60 Exposed wall height: 48 Waste: 10

Above the 4 ft threshold where most jurisdictions want an engineered design, and tall enough to need geogrid layers tied back into the slope.

Choosing materials

  • Bury at least one full course. The first course below grade is what keeps the base of the wall from kicking out, and it is not optional on any wall over about 18 in.
  • Block face size varies enormously between product lines. This calculates on a common 12x8 in face — check your actual block and adjust, because a 6 in high block nearly doubles the count.
  • The base trench is wider than the wall: about twice the block depth, so the wall sits on compacted stone rather than on its front edge.
  • Cap blocks are glued, not stacked. One tube of concrete adhesive bonds roughly 15 caps, and caps left dry-laid walk off the wall within a couple of seasons.
  • Curved walls use more block than the straight-line length suggests, because the joints open on the outside of the curve. Add 5% on top of the normal waste for anything with a radius in it.

Common questions

How many blocks do I need for a retaining wall?

Divide the wall face area — length times the full built height, including the buried course — by the block face area. With a common 12x8 in block that is 1.5 blocks per square foot of wall face. A 40 ft wall 3 ft high needs about 240.

How deep should the first course be buried?

About a tenth of the wall height, and never less than one full course. On a 4 ft wall that is roughly 5 in, so a full 8 in course goes below grade. It is what stops the base from sliding forward.

How much gravel goes behind a retaining wall?

A 12 in wide zone of clean drainage stone the full height of the wall, with a perforated pipe at the bottom. On a 40 ft wall 3 ft high that is around 4 tons — plan the delivery, because it will not fit in a pickup.

Do I need geogrid?

Above about 4 ft, or on any wall retaining a slope or a driveway, yes. Geogrid layers extend back into the retained soil every few courses and tie the wall to the mass behind it. Below 3 ft on level ground it is usually unnecessary.

Do I need a permit for a retaining wall?

Most jurisdictions require one above 4 ft measured from the bottom of the footing, and many require an engineered design at that point too. Walls holding back a driveway or structure often need a permit at any height.

How do I handle a curved wall?

Segmental blocks are tapered so they form curves naturally, but the joints open on the outside of the arc and you use more block per foot of run. Add about 5% over the straight-line count, and set out the curve before the first course goes down.

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