Retaining Wall Calculator

Estimate a Retaining Wall Without the Guesswork

A retaining wall holds back soil, levels a slope, and creates usable garden space — but ordering the wrong amount of block is an expensive mistake. This retaining wall calculator turns four simple measurements (wall length, wall height, block face length, and block height) into a complete material list: total wall blocks, how they stack into courses, the cap blocks for the top row, and the cubic feet of compacted gravel for the base. Add an optional price per block and you'll also see the material cost in your currency.

The Formulas Behind the Numbers

The math mirrors how a wall is actually built, row by row:

Courses = ⌈ wall height × 12 ÷ block height ⌉ Blocks per course = ⌈ wall length × 12 ÷ block face length ⌉ Total blocks = courses × blocks per course Cap blocks = blocks per course Base gravel (ft³) = wall length × 0.5 × 0.5

Heights and lengths are entered in feet and converted to inches (× 12) so they divide cleanly by the block dimensions. Both block counts are rounded up with the ceiling function, because you can't buy a fraction of a block and the wall must reach full height and length. The base figure assumes a standard 6 in deep by 6 in wide gravel trench (0.5 ft × 0.5 ft) running the length of the wall.

A Worked Example: A 20 ft Garden Wall

Say you're terracing a backyard slope with a wall that is 20 feet long and 3 feet high, using common segmental blocks with a 12 in face length and 4 in height.

• Courses: 3 ft × 12 = 36 in, ÷ 4 in = 9 courses.
• Blocks per course: 20 ft × 12 = 240 in, ÷ 12 in = 20 blocks.
• Total wall blocks: 9 × 20 = 180 blocks.
• Cap blocks: 20 caps for the finished top.
• Base gravel: 20 × 0.5 × 0.5 = 5 ft³, about 0.19 yd³.

At a price of $2.50 per block, the 180 wall blocks come to $450 in material, before caps, gravel, and adhesive. Add a 10% waste buffer and you'd order roughly 198 blocks to cover cuts and breakage.

Why Cap Blocks Are Counted Separately

Cap blocks finish the top of the wall and shed water away from the cores below. They're usually a different product — flatter and often solid — and are bonded with construction adhesive rather than dry-stacked. Because of that, the calculator lists them on their own line so you can place the right order with your supplier instead of assuming the wall blocks include a top.

Practical Tips for a Wall That Lasts

• Level the base obsessively. Every block above sits on the first course, so a level, compacted gravel base is the single most important step.
• Bury the first course. Embed about one block for short walls, deeper for tall ones, to anchor the wall against soil pressure.
• Plan drainage. Add drainage gravel and a perforated pipe behind the wall; this tool sizes the base trench, not the backfill.
• Check local rules. Walls over about 4 feet often require an engineer and a permit — keep tiers shorter or stepped where you can.

Pair It With Your Other Estimates

A retaining wall rarely stands alone. If you're pouring a concrete footing for a taller or engineered wall, size it with the concrete calculator. Building a matching mortared garden wall on top? Cross-check unit counts with the brick calculator. Used together, these tools give you a defensible material order before you ever call the yard.