Retaining Wall Calculator
Plan a segmental retaining wall end to end. Enter the wall length and height with your block size to get a course-by-course block count, optional cap stones, the base leveling gravel and the drainage backfill behind the wall (with compaction and waste allowances), gravel tonnage, and an optional material cost breakdown, in metric or imperial.
Formula
Worked example
Metric: a 10 m wall, 1 m high, with 45 cm × 20 cm blocks and a cap row. Per course = ⌈1000 ÷ 45⌉ = 23, courses = ⌈100 ÷ 20⌉ = 5, so 4 field courses give 23 × 4 = 92, and with 5% waste that rounds to 97 blocks plus 24 caps. Base pad 0.40 m × 0.15 m over 10 m, +20% ≈ 0.72 m³; backfill 0.30 m × 1 m over 10 m, +10% ≈ 3.30 m³.
How the block count is worked out
A segmental retaining wall is built from identical blocks stacked in horizontal rows called courses. The calculator divides the wall length by the length of one block to find how many blocks sit in a single course, then divides the wall height by the height of one block to find how many courses are stacked up. Both divisions are rounded up, because a partial block still has to be bought and cut. If you add a cap row, the top course is finished with cap stones instead of field blocks, so the field block count uses one fewer course and the caps are counted separately. A waste allowance, typically five to ten percent, is then multiplied in to cover cuts at ends, returns at corners and units that arrive chipped.
Base gravel versus drainage backfill
A durable wall needs two separate volumes of stone, and many calculators only count one. The first is the base leveling pad: a shallow trench of compacted gravel under the first course that spreads the load and gives a flat, frost-stable surface. It is wall length times trench width times pad depth, and because gravel can lose up to twenty percent of its volume when tamped, the calculator adds twenty percent so you order enough. The second is the drainage backfill: a column of clean, angular stone placed directly behind the wall face for its full height, usually at least thirty centimetres (twelve inches) wide. This is what lets water drain down and out rather than building pressure behind the wall. It is wall length times backfill width times wall height, with a ten percent allowance for voids and uneven excavation.
Tonnage, cost and good practice
Aggregate is sold by weight as well as by volume, so the calculator multiplies the total gravel volume by a density of about 1.5 tonnes per cubic metre and also reports cubic yards for US suppliers. Turn on the cost estimate to price the three main materials: field blocks and cap stones each, plus gravel by the tonne. The total is a planning figure since local prices, delivery and pallet quantities vary. Always confirm the exact nominal block dimensions with your supplier, line the back of the wall and the gravel with geotextile fabric so silt cannot clog it, and slope any drain pipe at the base toward a daylight outlet. These are estimates: confirm the design against your site and local building rules before you order.
Common retaining wall block sizes
| Block type | Length | Height | Typical use |
|---|---|---|---|
| Standard SRW | 45 cm / 18 in | 20 cm / 8 in | Garden and yard walls |
| Large SRW | 30 cm / 12 in | 20 cm / 8 in | Taller engineered walls |
| Small/edging | 30 cm / 12 in | 10 cm / 4 in | Low borders and beds |
| Cap stone | 45 cm / 18 in | 7-10 cm / 3-4 in | Finished top course |
Nominal as-laid dimensions for popular segmental units, confirm with your supplier.
Frequently asked questions
How many retaining wall blocks do I need?
Divide the wall length by one block’s length to get the blocks per row, divide the wall height by one block’s height to get the number of rows, then multiply the two. For a 10 m wall, 1 m high, with 45 cm × 20 cm blocks that is 23 × 5 = 115 blocks before a cap row, plus a few percent for cuts and breakage. With a cap row the top course becomes cap stones, so the field count drops by one course.
How much gravel do I need for a retaining wall?
You need two amounts. The base leveling pad is wall length times trench width times pad depth (commonly 15 cm or 6 in deep), and you add about 20% for compaction. The drainage backfill is clean stone behind the wall, at least 30 cm (12 in) wide times the wall height times the length, with about 10% for voids. This calculator reports both, the combined cubic yards, and the total tonnage.
Do I need cap stones?
Cap stones are not structurally required, but they give a clean, finished top and protect the cores of the top course from water. Turn on the cap row option to count them; they usually equal one course of units and are often glued down with construction adhesive rather than pinned. If your field blocks already have a finished face on top, you can leave the cap row off.
How tall can I build without an engineer?
Many jurisdictions allow homeowner-built segmental walls up to about 1.2 m (4 ft) before an engineered design and permit are required. Taller walls, walls on slopes, or walls carrying a load such as a driveway or pool need professional design for drainage, geogrid reinforcement and a buried base course. Always check your local building code first.