Most failed retaining walls did not fall because the blocks were too weak. They tipped over for two boring reasons that show up on every tear-out. The trench was too shallow, so frost heave lifted the first row out of the ground. Or nobody drained the backfill, so water froze behind the wall and pushed it forward like a wedge. Counting blocks is easy. The base and the drain are what keep the count from being a landfill later.
The two failures to avoid
Watch any wall that has bowed outward and you will see the same fingerprint. The bottom row sits on top of the soil instead of in a trench, and the gap behind it is packed with clay that never dries out. When winter freezes that clay, it expands sideways and shoves the wall out at the base. A wall built right fights this twice. It buries the first course below the frost so it cannot heave, and it leaves a gravel path behind it so water runs out instead of freezing.
How a block is sized
A standard landscape block measures 18 inches long by 6 inches tall on the face. Laid with the next row tight against it, each block covers about 0.75 sq ft of wall. That single number is the whole counting trick. Take the wall length times the wall height in feet to get square feet, divide by 0.75, and you have the block count. No rows and columns to miscount.
Walk a 20 ft by 2 ft wall
Say the wall runs 20 ft along a driveway edge and stands 2 ft above grade. That is 40 sq ft of face. Divide by 0.75 sq ft per block and you get about 53 blocks. Add 5 percent for chipped corners and the occasional split block, and the order lands near 56 blocks. The first row gets buried halfway. That means you actually dig the trench deeper than 2 ft of visible wall, because the bottom half-course hides below grade.
Set the base
Dig the trench 12 to 18 inches deep and wider than the block by a few inches. Put in 4 to 6 inches of crushed gravel and compact it with a plate tamper, not a hand tamper. The buried block sits on this gravel, its top half flush with where the grade will be. A gravel base 4 to 6 inches thick spreads the wall weight and lets water run under it, which is the cheapest insurance you will buy on the whole job.
Plan the drainage
Behind the block, do not shovel the clay back in. Use clean drain stone for the first 12 inches up from the base, and run a perforated pipe along the footing if the wall is over 2 ft tall. The pipe exits through the wall ends or out to daylight. Without this layer, every rainstorm turns into pressure against the face of the block, and even a perfect base will lose the argument over a few winters.
Read the result
For that 20 by 2 ft wall, the material list is short. About 56 face blocks, 4 to 6 inches of compacted gravel in the trench, a cubic yard or two of drain stone behind the wall, and a perforated pipe if the wall sits higher than a garden border. Geogrid is optional under 3 ft, but for anything taller, fold a layer between courses every 16 inches.
A wall stands for a decade when the base is buried below frost, the backfill drains, and the block count covers the waste. Skip the drain and you are not saving money, you are just scheduling the rebuild.
Retaining Wall Calculator
Learn why walls tip over, then count 18x6 inch blocks by the face area, set a 4 to 6 inch gravel base and plan the drainage that keeps frost heave out.
How to use
- Measure the wall length and finished height in feet
- Pick the block face size, usually 18 by 6 inches
- Dig a trench and set 4 to 6 inches of compacted gravel
- Bury the first block row halfway below grade
- Add 5 percent waste for broken corners and cuts
- Read the block count and the gravel base
Supported formats and limits
- Walls over 4 ft tall need an engineer
- Default 5 percent waste already added
- Does not include geogrid or drain pipe
- Block face assumed at 0.75 sq ft each
Examples
20 ft long, 2 ft high wall with 18x6 blocks lands near 56 blocks after 5 percent waste.Common errors
- Wall length and height must be positive
- Block width and height must sit between 6 and 24 in
- Waste factor must stay under 20 percent
Privacy and browser support
All calculations run locally in your browser. No measurements are uploaded.
FAQ
Why do so many retaining walls lean or fall over
Two causes cover most failures. The trench was too shallow, so frost heave lifted the base, or water pooled behind the wall because there was no drain.
How big is the face of one 18x6 block
Each block covers 0.75 sq ft of wall once laid, so a 40 sq ft wall needs about 53 blocks before waste.
How deep should the gravel base be
Spread 4 to 6 inches of compacted crushed gravel in the trench and set the first block row halfway into it. Buried blocks cannot ride up on frost.
Do I really need drainage stone behind the wall
Yes. Gravel backfill and a perforated pipe behind the base carry water away. A wall holds back dirt; it cannot hold back water pressure.
When do I need an engineer
Above 4 ft tall, or on a slope, next to a driveway, or where a failure would hurt someone. Low garden walls under 3 ft are a DIY job.
How is the block count worked out
Wall area in square feet divided by 0.75 sq ft per block, then rounded up with 5 percent waste for breakage.