Retaining Wall Calculator
Count the blocks and caps for a segmental retaining wall, plus the base stone and drainage gravel that most block calculators leave out entirely.
What this calculator counts
Block counts are the easy part, and most calculators stop there. The material that actually derails a retaining wall project is everything underneath and behind it: the compacted base you set the first course on, and the drainage stone that keeps water pressure from pushing the wall over. This calculator gives you all three.
The block math itself is simple. Divide the wall length by the block face length to get blocks per course, divide the wall height by the block height to get the number of courses, then multiply. A 20 foot wall at 3 feet high using a standard 12 inch by 8 inch block is 20 blocks across and 5 courses up, so 100 blocks plus waste.
Bury the first course
The single most common mistake on a DIY block wall is setting the first course on the surface. It needs to be buried, and the rule of thumb is one inch of buried block for every foot of exposed wall height, with a minimum of one full course.
So a 3 foot wall wants the bottom course fully below finished grade. A 4 foot wall wants the bottom course buried plus a bit more. This is not optional detailing, it is what stops the base course from kicking out under load.
Enter your exposed height in the calculator and add a course for the buried one when you order. Or enter the total height including the buried course if you prefer to think that way, just be consistent.
The base is the whole job
Everything about whether a wall stays straight in ten years happens in the first afternoon, before a single visible block is placed.
Dig a trench twice the depth of your block. For a standard 8 inch deep block that is a 16 inch wide trench. Depth needs to hold 6 inches of compacted crusher run plus the full height of the buried course. Fill with crusher run in two lifts, compacting each one, and screed the top dead level. Level matters more than almost anything else, because every error in the base course multiplies as you go up.
Use crusher run or a similar base stone with fines in it, not clean gravel. Clean stone does not lock together and will not hold compaction. The calculator adds 20 percent to your base order to cover what the material loses when compacted, which is a real loss people routinely forget and then run short on.
Drainage, or why walls fail
Retaining walls almost never fail because the blocks were too weak. They fail because water built up behind them. Saturated soil is dramatically heavier than dry soil and it pushes with hydrostatic pressure the wall was never designed to resist.
The fix is a drainage chimney: a vertical column of clean crushed stone directly behind the block, running the full height of the wall, at least 12 inches wide. Water hits the stone, falls straight down instead of soaking into the soil behind the wall, and exits through a perforated drain pipe at the bottom that daylights out the end or through the face.
Use clean #57 stone for this, not crusher run. You want water moving through it freely, which is the opposite of what you want in the base. Put geotextile fabric between the drainage stone and the backfill soil so silt does not migrate in and clog it.
| Layer | Material | Why |
|---|---|---|
| Base, under the wall | Crusher run, compacted | Fines lock together and hold a level bearing surface |
| Drainage, behind the wall | Clean #57 stone | No fines, so water passes straight through |
| At the base of the drain | Perforated pipe | Carries water out rather than letting it pool |
| Between stone and soil | Geotextile fabric | Stops silt washing into the drainage stone |
When you need an engineer
In most of the United States, a segmental block wall taller than 4 feet of exposed height requires an engineered design and a building permit. Some jurisdictions set it at 3 feet. Some trigger it at any height if there is a surcharge above the wall, meaning a driveway, a pool, a structure or a slope that loads the soil behind it.
Above that threshold you are generally into geogrid territory. Geogrid is a structural mesh that runs back into the retained soil at set intervals and ties the wall to the mass of earth behind it, turning the whole assembly into a gravity structure. Spacing and length are specific to your soil and wall height, which is exactly what the engineer is calculating.
Two shorter walls terraced up a slope are often a better answer than one tall one, both for cost and for permitting. Keep the horizontal gap between terraces at least twice the height of the lower wall, otherwise the upper wall is surcharging the lower one and you are back to needing an engineer anyway.
A worked example
Say you are building a 24 foot wall, 3 feet of exposed height, using a standard 12 inch face by 8 inch high block.
- Blocks per course: 24 feet is 288 inches, divided by 12 inch blocks, so 24 per course.
- Courses: 3 feet exposed is 36 inches, divided by 8 inch blocks, so 4.5 which rounds to 5 courses. Add one buried course, call it 6.
- Blocks: 24 times 6 is 144, plus 5 percent waste, so order about 152.
- Caps: 24.
- Base: a 16 inch wide trench, 24 feet long, 6 inches deep is about 0.6 cubic yards loose, so order around 0.7 yards or one ton of crusher run.
- Drainage stone: 12 inches wide by 24 feet by 3 feet tall is about 2.7 cubic yards, roughly 3.7 tons of clean #57.
The drainage stone alone outweighs the base by a factor of five, and it is the line item people forget entirely when they budget. On a wall that size you are ordering more stone by weight than block.
Common questions
How many retaining wall blocks do I need?
Divide the wall length by the block face length for blocks per course, divide the wall height by the block height for the number of courses, then multiply the two and add about 5 percent for waste. A 20 foot wall at 3 feet high with standard 12 by 8 inch blocks works out to 100 blocks plus waste, plus one buried course.
How deep should the base be for a retaining wall?
Six inches of compacted crusher run under the first course, in a trench twice the depth of your block. On top of that the first course sits fully below finished grade. The general rule is one inch of buried block for every foot of exposed wall height, with one full course as the minimum.
How tall can a retaining wall be without an engineer?
Four feet of exposed height is the common threshold across most US jurisdictions, though some set it at three. Any surcharge above the wall, such as a driveway, pool or slope, can trigger the requirement at lower heights. Check with your local building department before ordering material.
What gravel goes behind a retaining wall?
Clean crushed stone, usually #57, in a column at least 12 inches wide running the full height of the wall. It has no fines so water drains straight through it. Do not use crusher run behind the wall, its fines hold water, which is the opposite of what you need there.
Do I need drainage pipe behind a retaining wall?
For anything over about 2 feet, yes. A perforated pipe at the base of the drainage stone carries water out to daylight instead of letting it pool at the bottom of the wall. Without it the drainage stone just becomes a reservoir and you get the hydrostatic pressure you were trying to avoid.
How much does a retaining wall cost?
Material alone for a segmental block wall usually runs $15 to $30 per square face foot once you add the base stone, drainage gravel and caps, though block style and local pricing move that a lot. That is materials only, this calculator does not include labor. A contractor-built wall typically prices closer to $40 to $60 per square face foot once labor, equipment and any engineering are folded in.
These figures are planning estimates. Bulk material weight shifts with moisture and screen size, and suppliers round differently, so confirm the final quantity with your yard before you order.