Retaining Wall
Free online retaining wall calculator for landscaping and construction projects. Enter wall height, length, and block size to get the number of blocks, cap blocks, backfill gravel volume, weight, and total cost with charts.
About This Calculator
Our Retaining Wall Calculator helps homeowners, landscapers, and contractors estimate the exact number of retaining wall blocks, cap blocks, and backfill gravel needed for any retaining wall project. Whether you are building a small garden retaining wall, a driveway border, or a large terraced landscape wall, this tool provides accurate material and cost estimates to help you plan your budget and avoid ordering excess materials.
The calculator works by dividing the wall height by the block height to determine the number of rows, and the wall length by the block length to determine the number of columns — both rounded up. The total number of blocks is rows × columns. If you select the cap row option, one row of standard blocks is replaced with cap blocks. Backfill volume is calculated as thickness × length × height, and weight is derived using the standard gravel density of 84.03 lb/ft³ (1,346 kg/m³). Cost estimates use your input prices for blocks, cap blocks, and gravel.
What is a Retaining Wall?
A retaining wall is a structure designed to hold back soil or rock from a building, structure, or area. Retaining walls are used for landscaping, erosion control, terracing sloped yards, creating usable flat areas, and preventing landslides. They are commonly made from interlocking concrete blocks, natural stone, timber, or poured concrete. The wall must be properly designed with adequate drainage (backfill) to withstand lateral earth pressure and water buildup behind the wall.
How to Build a Retaining Wall
Building a retaining wall involves: (1) Excavating a trench for the base, (2) Adding a compacted gravel base (typically 4-6 inches deep), (3) Laying the first course of blocks level and square, (4) Stacking subsequent courses in a staggered pattern, (5) Adding backfill gravel behind each course, (6) Wrapping with geotextile fabric, and (7) Installing cap blocks for a finished look. Always consult local building codes and consider hiring a professional for walls over 3 feet tall.
Regional Notes
US: Common block sizes include 8×16 inches and 6×18 inches. Block prices range from $2 to $8 each. Gravel costs $30-$80 per ton. Building permits are typically required for walls over 3-4 ft.
UK: Standard concrete segmental blocks are 440×215 mm (17.3×8.5 in). Block prices range from £1.50 to £5 each. Gravel costs £40-£100 per tonne. Planning permission may be needed for walls over 1 meter.
India: Common block sizes include 400×200×200 mm and 300×150×150 mm. Block prices range from ₹35 to ₹100 each. Crushed stone costs ₹1,000-₹2,500 per tonne. Local municipal approval may be required for tall walls.
Frequently Asked Questions
How many retaining wall blocks do I need?
To calculate the number of retaining wall blocks needed, divide the wall height by the block height to get the number of rows, and divide the wall length by the block length to get the number of columns, both rounded up. Multiply rows by columns. If you include a cap row, subtract one row from the block count.
What size blocks are typically used for retaining walls?
Common retaining wall block sizes range from 4×8 inches up to 12×24 inches. Popular sizes include 8×16 inches for segmental retaining wall (SRW) blocks, 6×18 inches for medium-height walls, and 4×12 inches for decorative garden walls. The size depends on the wall height and structural requirements.
What is backfill and why do I need it for a retaining wall?
Backfill is the gravel or crushed stone placed behind a retaining wall to provide drainage. It prevents hydrostatic pressure from building up behind the wall, which can cause cracking or collapse. A minimum backfill thickness of 12 inches (1 foot) of gravel or similar granular material is recommended for proper drainage.
How much gravel do I need for retaining wall backfill?
Backfill volume equals the thickness × length × height of the backfill area, typically matching the wall dimensions. For a 10 ft long, 3 ft high wall with 1 ft thick backfill, you need 30 cubic feet (1.11 cubic yards) of gravel, weighing approximately 2,521 lb. The calculator computes this automatically.
Should I include a cap row on my retaining wall?
A cap row, also called a coping row, is the topmost row of blocks that often extends slightly beyond the wall face for a finished appearance. It is optional but recommended for aesthetic purposes. Adding a cap row removes one row of standard blocks and replaces it with a row of cap blocks. The backfill calculation is unaffected.
How do I calculate retaining wall material costs?
To calculate retaining wall costs, multiply the number of blocks by the price per block, the number of cap blocks by the cap block price, and the backfill weight by the gravel price per unit weight. Add all three totals. In the US, retaining wall blocks typically cost $2 to $8 per block, and gravel costs $30 to $80 per ton.
What is the best material for retaining wall backfill?
Crushed gravel or crushed stone (3/4-inch to 1-inch diameter) is the best material for retaining wall backfill. It provides excellent drainage and compaction. Avoid using sand or soil directly behind the wall, as they retain water and can build up pressure. Always wrap the backfill in geotextile fabric to prevent soil migration.
Do I need a building permit for a retaining wall?
Building permit requirements for retaining walls vary by location. In the US, most municipalities require a permit for walls over 3 to 4 feet in height. In the UK, planning permission is typically required for walls over 1 meter (3.3 ft). In India, local municipal rules apply. Always check with your local building department before starting construction.