Retaining wall block calculator

The block count is the part every calculator gets to. The stone behind the wall is the part that shows up as a second delivery. This does both — blocks, caps, the drainage-stone column, and the leveling pad — for a straight segmental wall, in imperial or metric.

01 How the math works

Four take-offs share one wall. Two of them — the drainage stone and the leveling pad — are where the tonnage lives.

Block count is face area times coverage, where coverage is a plain identity: 144 ÷ (face width × face height) in inches. A 12 × 4 in unit is 3.0 per ft²; a 16 × 6 is 1.5; an 18 × 8 is 1.0. Segmental units dry-stack with no mortar joint, so nothing is added to the coursing dimension. The height that drives the count is not the height you see — it is the exposed height plus the buried course, since the bottom course is set below grade.

Caps run the top: 12 ÷ cap width per linear foot. Behind the blocks sits a column of clean crushed stone the full height of the wall, at least 12 in (305 mm) wide, so water drains instead of pushing on the wall. Under the first course sits a compacted leveling pad, 6 in (150 mm) thick, run 6 in past the block on the front and back.

02 Worked example

Take a straight 20 ft (6.10 m) garden wall, 3 ft (0.91 m) of exposed height, standard 12 × 4 in units 8 in deep, 12 in caps, the bottom 4 in course buried, a 12 in drainage column, and a 6 in leveling pad run 6 in past the block front and back.

Blocks: total height is 3 ft exposed plus a 4 in (102 mm) buried course = 40 in, so 10 courses of 20 blocks = 200 blocks. That is the same as face area 66.67 ft² (6.19 m²) × 3.0 per ft². Add 5% for breakage and cuts: 210 blocks.

Caps: 20 ft × 1.0 per ft = 20, plus 5% = 21 caps.

Drainage stone: a 12 in (1 ft) column, 3.33 ft tall, 20 ft long = 66.67 ft³ = 2.47 yd³ (1.89 m³) of clean crushed stone, about 6,667 lb — 3.33 tons (3,024 kg).

Leveling pad: base width is the 8 in block depth plus 12 in of overhang = 20 in (1.67 ft); at 6 in thick over 20 ft that is 16.67 ft³ = 0.62 yd³ (0.47 m³), about 1,667 lb (756 kg) of compacted granular base.

03 When this calculator is wrong

The block count is the easy part. The stone behind and beneath the wall is where the order goes short, and the height is where it goes short before that.

• The buried course is real block. Manufacturers bury the bottom course, adding roughly 1 in of embedment for every 8 in of exposed height, minimum one full course. A 3 ft wall buries a 4 in course — a full extra row over the whole length. Count the wall from exposed height alone and you under-order by a course.
• The drainage stone outweighs the blocks. NCMA's SRW design manual and every manufacturer install guide put at least a 12 in (305 mm) column of clean crushed stone behind the units, full height. On the 20 ft wall that is 3.33 tons (3,024 kg) — more weight than the blocks, and the line most calculators leave off entirely.
• Height, backfill, or a slope above changes who signs the drawing. A wall over 4 ft (1.2 m) bottom-of-footing to top-of-wall, or any wall carrying a surcharge — a driveway, a slope, a structure above — needs a registered design professional and usually a permit (IBC §105.2, §1807.2). This calculator sizes materials for a straight gravity wall; it does not design one.
• Nominal face is not the milled face. The 3.0-per-ft² identity uses a nominal 12 × 4. Real units run a touch under nominal, and a pronounced setback shortens the upper courses. Where the pallet label and the identity disagree, the label wins.

04 What to do with the result

Order blocks and caps to the piece or the pallet your supplier sells, rounded up. Give the yard the drainage stone and leveling pad as volume: 66.67 ft³ is 2.47 yd³, and aggregate sells by the cubic yard or the ton.

Round the aggregate up, never down. Returning half a ton of crushed stone costs almost nothing; stopping a wall halfway to wait on a second delivery costs a day and a delivery fee. On a segmental wall the drainage stone is not optional trim — it is what keeps water from loading the wall — so if the calculator and your gut disagree, order the extra increment. The exception is high-cost items in matched lots: caps and colored block come in dye lots, and once a lot is gone the replacement will not match, so buy those once and buy them right.

05 Common questions

How many retaining wall blocks do I need per square foot?

It is a plain identity: 144 ÷ (face width × face height) in inches. A 12 × 4 in unit is 3.0 per ft² (32.3 per m²), a 16 × 6 is 1.5, and an 18 × 8 is 1.0. Multiply by the wall face area — exposed height plus the buried course — then add waste.

How much gravel goes behind a retaining wall?

A column of clean, free-draining crushed stone (ASTM #57, 3/4 in) at least 12 in (305 mm) wide, run the full height of the wall directly behind the blocks. On a 20 ft × 3 ft wall that is about 2.47 yd³ (1.89 m³), roughly 3.33 tons (3,024 kg) at 100 lb/ft³.

How deep and wide is the base for a retaining wall?

A compacted granular leveling pad 6 in (150 mm) thick, extending at least 6 in (150 mm) beyond the block on the front and back — so base width is the block depth plus 12 in. The bottom course of block sits on this pad, below finished grade.

Do I have to bury the first course of a retaining wall?

Yes. Set the bottom course below grade — a minimum of one full course, and about 1 in more for every 8 in of exposed height. It is what keeps the base of the wall from kicking out, and it means the blocks you order include a course you never see.

How tall can I build a retaining wall without an engineer?

Commonly up to 4 ft (1.2 m) measured from the bottom of the footing to the top of the wall, with no slope or load above it (IBC §105.2). Taller than that, or with a surcharge above, and the wall needs a registered design professional and usually a permit. Local code governs — confirm before you dig.