Retaining Wall Block Calculator
A retaining wall block calculator divides wall face area by 1/3 sq ft per block: a 20 × 3 ft wall needs 180 blocks, or 198 with 10% extra.
Retaining wall block calculator formula
A retaining wall block calculator counts how many blocks cover the face of your wall. Blocks = wall length × wall height ÷ face area of one block, rounded up. The tool uses 1/3 sq ft, which is a block with a 12 inch by 4 inch face. A checkbox adds 10 percent for cuts and breakage.
The formula in plain words
Find the area of the face you will see, in square feet. Divide by the square feet one block covers. Round up. If you tick the extra box, multiply by 1.1 before you round.
Worked examples
These match the calculator exactly.
- Low garden wall: 20 ft long, 3 ft high, $4 per block
- Face area: 20 × 3 = 60 sq ft
- Blocks: 60 ÷ (1/3) = 180
- Cost: 180 × $4 = $720
- With 10% extra: 198 blocks, $792
- Longer, taller wall: 40 ft long, 4 ft high, $4 per block
- Face area: 160 sq ft
- Blocks: 160 ÷ (1/3) = 480
- Cost: 480 × $4 = $1,920
- With 10% extra: 528 blocks, $2,112
You can check example 1 by hand. A 3 ft wall is 9 courses of 4 inch blocks, and each course is 20 blocks 12 inches long. That is 180, and the tool shows the same.
Block counts by wall size
| Wall (length × height) | Face area | Blocks at 1/3 sq ft |
|---|---|---|
| 10 × 2 ft | 20 sq ft | 60 |
| 20 × 3 ft | 60 sq ft | 180 |
| 30 × 3 ft | 90 sq ft | 270 |
| 40 × 4 ft | 160 sq ft | 480 |
Add the 10 percent if you want spares. The counts do not include cap blocks.
What else the wall needs
Blocks are not the whole job. Count these as well:
- A compacted gravel base under the first course, with a depth set by the block maker.
- Drainage stone behind the wall, often about a foot wide.
- A perforated drain pipe at the base of the wall.
- Cap blocks or coping on top.
- Geogrid or other reinforcement for taller walls, as the maker specifies.
For the base, see the paver base calculator, which works the same way for compacted stone.
When to call a professional
A retaining wall holds soil, and soil is heavy. Walls above a few feet, walls that support a driveway or a building, and walls on a slope with water behind them can fail. Many places require a permit and an engineer’s plans for taller walls. Check with your local building department, and ask a licensed engineer to design a wall that carries heavy loads.
How to measure
Measure the length of the wall along its front face, including curves. Measure the height from the top of the base to the top of the cap. If the maker tells you to bury the first course, add its 4 inches to the height, since those blocks still have to be bought.
Stepped walls are easiest to break into sections. Measure each section as a rectangle and add up the blocks.
Ordering tips
Blocks are sold by the pallet, and a pallet holds a set number of units. Divide the count from the tool by the pallet count on the product page, then round up. Order the corner and cap pieces at the same time so that the colors match. Blocks from different batches can vary slightly in shade, so one order for the whole wall is better than two.
Setback and batter
Many block systems step each course back slightly, so the wall leans into the hill. That lean helps it hold. It also means the finished face is a bit shorter in plan than the base. The face area in the tool is still the wall’s length times its height, so the count holds. Follow the maker’s rules on setback and reinforcement for your block.
Common mistakes
- Forgetting the buried first course.
- Leaving out cap blocks.
- Ignoring drainage behind the wall.
- Mixing block sizes. If your blocks are not 12 × 4 inches, divide the area by your own face area.
- Building tall without a permit or plan.
Related pages
Compare with the concrete block calculator for structural block walls. For the cost of a finished wall, use the retaining wall cost calculator.
Frequently asked questions
How many blocks do I need for a retaining wall?
Multiply the wall length by its height to get face area, then divide by the area of one block face. The calculator uses exactly 1/3 sq ft, a 12 × 4 inch face, so a 20 × 3 ft wall is 60 ÷ (1/3) = 180 blocks.
Does the tool match counting courses by hand?
Yes. A 12 × 4 inch block is exactly one third of a square foot, so 60 sq ft is 180 blocks. Counting courses gives the same 180: 9 courses of 20 blocks.
Should I add extra for cuts and breakage?
Yes. Tick the 10 percent box to add spares for corners, cuts and cracked units. With it on, the 20 × 3 ft wall needs 198 blocks.
How tall can a retaining wall be?
Rules vary. Many places require a permit or an engineer's design for walls above a certain height, often somewhere around 3 to 4 feet, or for walls that hold up a slope near a building. Check with your local building department.
What goes behind and below the wall?
A compacted gravel base goes under the first course. Drainage stone and a perforated pipe go behind the wall so water does not build up pressure. Follow the block maker's instructions.
Estimates only. Verify quantities with your supplier before purchasing.