How Many Retaining Wall Blocks Do I Need? The Math, Row by Row

You have a slope, a staked line for a block garden wall and a supply yard that wants a number. The count comes from a few measurements and your block’s face size, and the usual way to come up short is forgetting the row that goes underground. Here is how many retaining wall blocks you need, worked out for a 30 ft by 3 ft wall, plus caps, corners, curves and steps.

How Many Retaining Wall Blocks Do I Need?

Multiply the wall’s length by the height of block above grade to get square feet of face, then multiply by the blocks per square foot for your block. Add the buried row and 5%, then round up. For a 30 ft by 3 ft wall in 6 by 16 inch block, that comes to 166 blocks and 24 caps.

Measure the Length and Height

Stake the wall line and measure along it where the block face will sit, following curves with a garden hose and adding any returns where the wall turns back into the slope. For height, stretch a level string from the grade you want at the top out over the low side and measure down to finished grade every few feet. Where the ground falls along the wall, the base steps down a block height at a time, so count each stretch on its own.

Decide, too, whether your height includes the cap. Cambridge’s MaytRx caps are 3 inches tall and the AB Capstone is 4.

Add the Buried Course

The bottom row sits below grade on a compacted stone pad, and it is the row people forget to order. Our retaining wall base depth guide buries about 1 inch of wall for every foot of height, with a minimum of one full course. Allan Block’s estimating worksheet sets a similar floor, at least 6 inches or 1 inch per foot of wall height. On a garden wall in 6-inch block that means one buried row, counted as the wall length in inches divided by the face length of one block, while one row of 4-inch block falls short of 6 inches, so check your maker’s sheet.

Blocks per Square Foot, by Block Face

Many makers list the square feet of face each unit covers, and you can divide your face area by that number. Otherwise, divide 144 by the face height times the face length, in inches, to get blocks per square foot.

• 4 × 12 inches, 0.33 sq ft, 3 per sq ft: garden block such as Rockwood’s Cottage Stone, whose Rochester Concrete Products page says to divide total square feet by 0.33.
• 6 × 16 inches, 0.67 sq ft, 1.5 per sq ft: the Versa-Lok Standard (0.667 sq ft on its cut sheet), Unilock’s Pisa XL Wall (listed at 0.67 face feet) and the stretcher in Cambridge’s MaytRx 6 line.
• 6 × 18 inches, 0.75 sq ft, 1.33 per sq ft: Rockwood’s Classic 6, which covers 3/4 sq ft per block.
• 8 × 18 inches, about 1 sq ft, 1 per sq ft: Allan Block’s AB Classic and Belgard’s Diamond Pro, which Belgard lists at 1 sq ft sizing.

Still choosing a block? Our side-by-side look at the major retaining wall block brands compares Allan Block, Versa-Lok, Keystone, Belgard and Fortus before you settle on a face size.

Worked Example: A 30 ft by 3 ft Wall

Take a straight wall on flat ground, 30 ft long, with 36 inches of 6 by 16 inch block above grade. Its caps are 16 inches long.

• Rows above grade: 36 ÷ 6 = 6 rows.
• Blocks per row: 30 ft × 12 = 360 inches, and 360 ÷ 16 = 22.5 blocks.
• Blocks above grade: 6 × 22.5 = 135, the same as 90 sq ft × 1.5.
• Buried course: one more row of 22.5, for 157.5.
• Extra: 157.5 × 1.05 = 165.4, so order 166 blocks.
• Caps: 360 ÷ 16 = 22.5, and 22.5 × 1.05 = 23.6, so order 24.

That matches the 166 blocks and 24 caps on our 30 ft × 3 ft retaining wall page. With a 4-inch cap on top, this wall stands 40 inches above grade.

In 8 by 18 inch block, each row takes 360 ÷ 18 = 20 blocks, but 36 inches is 4.5 rows. Four rows (32 inches) need (80 + 20) × 1.05 = 105 blocks, and five rows (40 inches) need (100 + 20) × 1.05 = 126. Four rows plus a 4-inch cap land at 36 inches, so settle the row count before you multiply.

Caps by the Linear Foot

Caps run along the top row, so count them by the linear foot. Divide 12 by the cap length in inches for caps per foot, which is 0.75 for a 16-inch cap and 0.67 for an 18-inch one, then multiply by the wall length and add 5%. The 18-inch AB Capstone covers the 30 ft wall in 20 pieces, or 21 with extra.

Cambridge makes MaytRx caps in 12- and 18-inch lengths. If your cap is wedge-shaped for curves, ask the yard how many it takes per foot of straight wall. Caps are glued, and Allan Block’s worksheet allows one 29-ounce tube of adhesive per 60 ft of capped wall.

Once they’re counted, our guide to retaining wall caps covers the overhang, which adhesive to use and how to cut caps around curves. Building a wall people will sit on? See seat wall dimensions for the height, depth and length per person that make one comfortable.

Corners, Curves and Steps

Corners

An outside 90-degree corner needs a corner unit in every row, buried course included. Rockwood’s installation guide staggers it to the opposite side of the corner on each course, and Allan Block’s AB Corner is commonly sold in pairs, for left and right. With 6 rows above grade and 1 below, each outside corner takes 7 corner units.

Some systems split standard units instead. Versa-Lok’s FAQ says “corners, curves and steps can be built using one unit,” and Cambridge splits MaytRx units or sells a pre-made corner stone. Splitting leaves scrap, one more reason for the 5% extra.

Curves

Measure a curve along the bottom row. Each row sets back, so an outside curve, bulging toward the low side, gets shorter as it rises and an inside curve gets longer. Belgard’s Diamond Pro page measures its minimum radius where each curve is tightest, 4 ft at the top course for outside curves and 6 ft at the base course for inside ones. Curves also mean cut block, and our 30 ft × 3 ft wall page tells do-it-yourselfers to order its list, which already has 5% extra, plus 5 to 10% when the yard has curves.

Steps

Rockwood’s guide counts stair risers by dividing the stair height by the riser height, and sizes each side wall by multiplying the tread depth by the number of risers. A 36-inch rise in 6-inch block is 6 risers, and with 12-inch treads each side wall runs 72 inches into the slope.

Each riser row takes the step width divided by the block length, or 3 blocks for a 4 ft step in 16-inch block. Allan Block’s worksheet notes that stairs and step-ups need extra block, so draw them to scale and count block by block.

How Much Extra to Order

Add 5% to blocks and caps for chips, cuts and the odd bad unit, as our calculator does. Allan Block recommends ordering 5% extra block. Go higher for curves, split corners and stairs.

Block ships by the pallet. The Versa-Lok Standard cut sheet lists 40 units, or 26.7 sq ft of face, per pallet, so 166 blocks is 4 pallets plus 6. Ask whether the yard sells loose block before you round up to 5.

The Rest of the Material List

• Base stone: clean 2B, compacted as the leveling pad in a trench about 24 inches wide and 6 inches deep.
• Drainage stone: 12 inches of clean stone behind the block, full height. Our retaining wall drainage guide covers stone size and outlets.
• Perforated pipe: 4-inch, at the base of the drainage stone and run to daylight. Allan Block figures about as much pipe as wall length, and you add the run to the outlet.
• Filter fabric: between the drainage stone and the soil.
• Pins, if your system uses them: two per unit on the Versa-Lok cut sheet.
• Geogrid on taller walls: our calculator estimates it for walls over 3 ft, and the block maker’s chart or an engineer sets the real layers. See when a retaining wall needs geogrid.

Block choice can change the order, too. Belgard rates Diamond Pro for gravity walls up to 4 ft including the buried course, but Diamond 9D only to 3 ft and Easton Stone to 2 ft, and taller Diamond walls need reinforcement designed by an engineer. With its buried row, the 30 ft example has 42 inches of block, too tall for a 9D gravity wall.

Why does a taller wall need reinforcement? Our segmental retaining wall guide compares gravity and geogrid-reinforced block walls and how tall a gravity wall usually goes before it needs grid.

Our retaining wall calculator takes wall length, exposed height and block face, and returns blocks, caps, base and drainage stone, pipe, fabric and geogrid. For the 30 ft example, our sizes page lists 1.9 tons of base stone, 5.4 tons of drainage stone, 40 ft of pipe and 180 sq ft of fabric. To have one built in central Pennsylvania, our retaining wall cost page lists current fixed prices, starting with a 30 ft by 2 ft garden wall.

Walls That Need an Engineer First

A wall over 4 ft, or one holding up a driveway, a slope above it or a building, needs an engineered design, and you count from the engineer’s drawing. Permit rules are set town by town, and our retaining wall permit guide covers where the lines fall and how height is measured.

If you would rather have the wall drawn, our online retaining wall design plans show the layout, sizes and materials on a scaled plan, and Single Space plans start at $349. Build-Ready ($1,795) adds construction details and a quantity takeoff. Since we never stamp drawings, a wall that needs an engineer still needs one.

Before You Call the Supply Yard

Sketch the wall with its length, heights, corners, curves and steps, and get the cut sheet for your block. Count rows in whole courses, add the buried row and 5%, and count caps by the foot. Then call the yard with blocks, caps, corner units and pallets written down.

Frequently Asked Questions

How many retaining wall blocks per square foot?

Divide 144 by the block’s face height times its face length, in inches. A 4 by 12 inch garden block runs 3 per square foot, a 6 by 16 inch block 1.5 and an 8 by 18 inch block 1. Where the maker lists face area per unit, divide by that instead.

Do I need to count the buried course?

Yes. The bottom row sits below grade, and it is the row people forget to order. Allan Block’s estimating worksheet calls for at least 6 inches of buried block or 1 inch per foot of wall height, which on a garden wall in 6-inch block is one full row.

How many caps do I need for a retaining wall?

Divide 12 by the cap length in inches for caps per foot, multiply by the wall length and add 5%. A 30 ft wall with 16-inch caps takes 22.5 caps, or 24 with extra. Allan Block allows one 29-ounce tube of adhesive per 60 ft of capped wall.

How much extra block should I order?

Add 5% to blocks and caps for chips, cuts and bad units, which matches Allan Block’s recommendation. Go higher for curves, split corners and steps, since each means cut block, and Allan Block notes that stairs and step-ups need extra block.

How many retaining wall blocks are on a pallet?

It depends on the block. The Versa-Lok Standard cut sheet lists 40 units, or 26.7 square feet of face, per pallet, so 166 blocks is 4 pallets plus 6. Check your block’s cut sheet, and ask whether the yard sells loose block before rounding up.

How many corner blocks do I need?

One corner unit per course at each outside 90-degree corner, buried course included, so a wall with 6 rows above grade and 1 below takes 7 per corner. Some systems skip special corners: Versa-Lok says corners, curves and steps can be built using one unit.

How tall can a gravity block wall be?

It depends on the block. Belgard rates Diamond Pro for gravity walls up to 4 ft including the buried course, Diamond 9D to 3 ft and Easton Stone to 2 ft. A wall over 4 ft, or one holding up a driveway, a slope or a building, needs an engineered design.

How much drainage stone and pipe do I need?

We place 12 inches of clean drainage stone behind the block for the full height, with 4-inch perforated pipe at its base run to daylight. Allan Block figures about as much pipe as wall length. For the 30 ft by 3 ft example, our sizes page lists 5.4 tons of drainage stone and 40 ft of pipe.

Sources: Allan Block, Material Estimation Worksheet; Allan Block, AB Collection; Rochester Concrete Products, Versa-Lok Standard Cut Sheet; Rochester Concrete Products, Rockwood Classic Installation Manual; Rochester Concrete Products, Cottage Stone; Versa-Lok, FAQ; Unilock, Pisa XL Wall; Cambridge, MaytRx Wallstones; Belgard, Diamond Pro; Belgard, Diamond 9D; Belgard, Easton Stone

Written and reviewed by Thayne Muthler, owner of Muthler Landscaping & Construction. How we check facts: editorial policy.