Retaining Wall Calculator

How to Calculate Retaining Wall Blocks

A segmental retaining wall is built in rows (courses) of identical blocks. Work out how many blocks fit in one row, then how many rows the wall needs.

• Blocks per row = wall length ÷ block length, rounded up
• Rows = height above ground ÷ block height, rounded up, + buried rows
• Wall blocks = blocks per row × rows × (1 + extra %), rounded up
• Cap blocks = blocks per row × (1 + extra %), rounded up

Work in inches: multiply the wall length and height in feet by 12.

Gravel base and drainage stone

• Base gravel (ft³) = length × trench width × base depth, where the trench width is the block depth plus 6 in at the front and 6 in at the back
• Drainage gravel (ft³) = length × drainage width × built height
• Cubic yards = cubic feet ÷ 27

Worked Example

A wall 20 ft long and 2 ft high, built from 4 in high × 11.75 in long × 6.75 in deep blocks, with one buried row:

1. Blocks per row: 240 in ÷ 11.75 in = 20.4 → 21
2. Rows: 24 in ÷ 4 in = 6, plus 1 buried = 7 rows
3. Wall blocks: 21 × 7 = 147, × 1.05 = 154.35 → 155 blocks
4. Cap blocks: 21 × 1.05 = 22.05 → 23 caps
5. Base gravel: 20 × (18.75 ÷ 12) × (6 ÷ 12) = 15.6 ft³ = 0.58 yd³
6. Drainage gravel: built height is 7 × 4 = 28 in, so 20 × 1 × (28 ÷ 12) = 46.7 ft³ = 1.73 yd³

Tips and Common Mistakes

Don’t forget the buried course

The first row sits below the ground on the gravel base, so it adds blocks without adding visible height. Leaving it out is the most common reason people run short.

Use the real block length

Block sizes on the label are often nominal. Some retailers list the 11.75 in RockWall block at 11.63 in, for example. Over a long wall the difference adds up, so measure a block if you’re close to a pallet boundary.

Corners, curves and steps

Outside corners and curves use cut or special corner blocks, and walls that step up a slope need extra buried blocks at each step. Raise the extra-blocks percentage, or add each section separately.

Drainage matters more than the blocks

Water pressure behind a wall is a common reason block walls bulge or tip. Fill the drainage zone with clean crushed stone, not pea gravel or soil, and follow the manufacturer’s guidance on filter fabric and a drain pipe at the base.

Height limits

Small landscape blocks are usually rated for short walls only; check the maximum height on the product. Walls over 4 ft, or walls holding up a driveway, a slope or a building, need an engineered design. Taller walls also often need geogrid reinforcement.

For the leveling base, the gravel calculator converts cubic yards to tons. If you’re pouring a concrete footing instead, use the concrete calculator.