Retaining Wall Calculator
Enter your wall length, height, and block size to get the number of wall blocks and caps you need — plus base gravel, drainage backfill, and cap adhesive.
Free · no sign-up ·
You need · incl. +5% waste
- Blocks / course
- 20
- Wall face area
- 40 ft²
- Base gravel
- 0.56 yd³
- Backfill gravel
- 1.48 yd³
- Cap adhesive
- 1 tube
- Exact wall blocks
- 80
Includes the leveling-pad gravel in the base trench and a 12-inch drainage zone of backfill gravel behind the wall. Order the gravel with the Gravel or Crushed Stone calculator if you want it by the ton.
Walls over about 3–4 ft usually need an engineer and a permit, and proper drainage behind the wall is what stops it bulging. Check local rules before building tall.
The formula
Courses = Wall height ÷ block height. Blocks per course = Wall length ÷ block length. Total blocks = Courses × blocks per course (+ waste). Caps run the wall length; base + backfill gravel come from the trench and drainage zone.
How to estimate a retaining wall
A retaining-wall estimate is really four numbers, and most calculators only give you the first. You need the wall blocks (courses × blocks per course), the cap blocks that finish the top, the leveling-pad gravel that goes in the trench under the first course, and the drainage gravel that backfills behind the wall so water pressure does not push it over. This calculator gives you all four plus the adhesive for the caps, so you can order the whole job at once.
Block count comes from two divisions. The number of courses is your wall height divided by the block height (a 2-foot wall in 6-inch block is 4 courses). The blocks per course is the wall length divided by the block length. Multiply them for the wall total, then add about 5% because you will cut blocks at ends and corners and the odd one arrives broken. Caps run a single row along the top, so they equal one course of blocks by length.
The gravel is what separates a wall that lasts from one that bulges. A 6-inch compacted gravel leveling pad in the base trench keeps the first course dead level and drains the footing. Behind the wall, a 12-inch zone of clean drainage gravel (wrapped in fabric, ideally with a perforated pipe) relieves the water pressure that fails most DIY walls. Anything over about 4 feet tall typically needs an engineer and a permit — check locally before you build high.
Frequently asked questions
How many blocks do I need for a retaining wall?+
Divide the wall height by the block height for the number of courses, and the wall length by the block length for blocks per course, then multiply. A 20 ft long, 2 ft high wall in 12×6-inch block is 4 courses × 20 = 80 blocks, about 84 with waste. Enter your numbers above for an exact count.
How do I calculate courses for a retaining wall?+
Courses = wall height ÷ block height, rounded up. A 24-inch-high wall built from 6-inch-high blocks is 4 courses; from 8-inch blocks it is 3. The calculator works it out when you set the wall height and block height.
How many cap blocks do I need?+
Caps run as a single row along the top of the wall, so the count is the wall length divided by the cap length — the same as one course of blocks if the caps match the block length. The calculator includes caps by default; turn them off if your system has none.
How much gravel do I need behind a retaining wall?+
Plan a roughly 12-inch-wide zone of clean drainage gravel behind the wall, the full height of the wall. The calculator estimates that backfill volume plus the leveling-pad gravel in the base trench, and adds them for a total in cubic yards.
How deep should the base of a retaining wall be?+
Dig a trench and set a compacted gravel leveling pad about 6 inches deep (more for tall walls), then bury the first course so a portion sits below grade. The calculator uses a 6-inch base by default — adjust it for your design.
Do I need adhesive for retaining wall caps?+
Yes — cap blocks are glued down with exterior construction adhesive so they do not shift. One tube covers roughly 25 linear feet of cap, so a 20-foot wall needs about one tube. The calculator estimates tubes from your cap run.
How tall can a retaining wall be without an engineer?+
As a rule of thumb, walls up to about 3–4 feet can often be built without engineering, but the exact limit and whether you need a permit vary by location and soil. Above that, or for tiered or surcharged walls, get an engineer — water and soil pressure rise fast with height.
How much extra block should I order?+
Add about 5% to the wall block count for cuts at ends and corners, curves, and the occasional broken block. The calculator includes an adjustable waste allowance.
What size are retaining wall blocks?+
Common segmental blocks are about 12 inches long and 4, 6, or 8 inches high, with larger units running 18 inches long. The calculator lets you pick the block length and height so the count matches the product you are buying.
Is this retaining wall calculator free?+
Yes — free, no sign-up, and it runs entirely in your browser, so nothing you enter is stored or sent anywhere.