Retaining Wall Block Calculator
Enter your wall's length and height in feet to get the wall face square footage that every block count is figured from. The chart below turns that number into blocks, because retaining wall blocks are not a standard size.
Total area: 100.0 sq ft
How it works
Enter the wall as length by height in feet. It is the vertical face you are measuring, not the ground the wall sits on. Use the Add area button for a wall that steps down a slope or turns a corner, and enter each straight run at its own height.
Include the buried base course in the height. The wall you order is taller than the wall you end up looking at, and leaving that course out is the most common way a retaining wall order comes up short.
Take the total to the chart below and divide by the face area of the block you are buying. Face area is the visible front of the block — width times height — not the depth that runs back into the hill.
Reference chart
Blocks needed by block face size (nominal sizes — confirm on your product page)
| Block face (W × H) | Face area | Per 100 sq ft | 20 × 3 ft wall |
|---|---|---|---|
| 12 × 4 in | 0.33 sq ft | 300 | 180 |
| 12 × 6 in | 0.50 sq ft | 200 | 120 |
| 16 × 6 in | 0.67 sq ft | 150 | 90 |
| 16 × 8 in | 0.89 sq ft | 113 | 68 |
| 18 × 8 in | 1.00 sq ft | 100 | 60 |
Wall face, not ground area — and the course you never see
A retaining wall is ordered by the area of its vertical face. That sounds obvious until you are standing in front of one with a tape measure, at which point it is easy to measure the line the wall runs along and multiply it by the wrong second number. Length times height is the figure you want. The depth of the block, the width of the trench, and the area of ground behind the wall all matter elsewhere in the job, but none of them belongs in this calculation.
The height to use is not the height you will see when the job is done. The base course is set below finished grade so the soil in front of it holds the bottom of the wall in place, which means the wall you build is at least one course taller than the wall you look at. Block manufacturers publish a burial depth in their installation instructions and it grows with wall height, so read the guide for the block you actually bought rather than working from a rule of thumb. The table below assumes a single buried course, which is the minimum to plan for.
Face area of a block is the visible front: width times height, divided by 144 to get square feet. It is not the depth running back into the slope, and on a block with a tapered back those two numbers are very different. Measure a block off your own pallet if you can, because the sizes below are nominal and manufacturers round them.
Courses needed by exposed height, including one buried base course
| Exposed height | 4 in blocks | 6 in blocks | 8 in blocks |
|---|---|---|---|
| 2 ft | 7 | 5 | 4 |
| 3 ft | 10 | 7 | 6 |
| 4 ft | 13 | 9 | 7 |
Base, drainage, and what goes in behind the wall
Retaining walls fail from the bottom. The base is a trench of compacted gravel, levelled carefully, and the time spent getting it flat is repaid on every course above it — a base that is out by half an inch is still out by half an inch at the top, except by then the wall is leaning. Compact in layers rather than dumping the full depth and running a plate over it once.
Behind the wall goes drainage stone, not the soil that came out of the hole. Water collecting behind a wall pushes on it, and that pressure is what takes walls down long after they were built straight. Clean angular stone lets the water reach a perforated pipe at the bottom and leave. Estimate it the same way as the base: the width of the backfill zone times the wall height times the length, divided by 27 for cubic yards. The gravel calculator will convert either figure to tons if your supplier sells by weight.
Trench width and depth come from the block manufacturer's installation guide and from what your local code requires. Both scale with the height of the wall, and a tall wall wants a wider base than anything in the table below. Treat these figures as a way to size a delivery for a short garden wall, not as a specification.
Compacted base gravel for a 6 in deep trench (cubic yards)
| Wall length | 12 in wide | 18 in wide | 24 in wide |
|---|---|---|---|
| 20 ft | 0.37 | 0.56 | 0.74 |
| 40 ft | 0.74 | 1.11 | 1.48 |
| 60 ft | 1.11 | 1.67 | 2.22 |
How tall you can build, and when the wall needs an engineer
There is no national height limit for a retaining wall, but there is almost always a local one. Many US jurisdictions put the trigger for a permit or an engineer's stamp somewhere near 4 ft, and it is measured from the bottom of the base course rather than from the ground you are standing on — so a wall that looks comfortably under the limit can be over it once the buried course counts. Your building department has the number that applies to you, and asking costs nothing next to taking a finished wall back down.
Whatever the local limit is, it drops when something loads the soil behind the wall. A driveway or parking pad at the top, a slope that keeps rising behind it, or a second wall close above are all surcharges, and they change the engineering rather than just the block count. So does ground that holds water. None of these show up in a square footage figure, which is why the number this page gives you is a materials estimate and not a design.
Blocks with a moulded lip step back a fixed amount on every course, which is what leans the wall into the hill. The setback is small per course and adds up: at three quarters of an inch a course, ten courses put the top of the wall about 7 1/2 inches behind the bottom. It does not change how many blocks you need, but it does move where the top of the wall lands, so check it against the line you were aiming for before the first course goes down.
Frequently asked questions
How many blocks do I need for a retaining wall?
Multiply wall length by wall height for the wall face area, then divide by the face area of one block. A 20 ft wall standing 3 ft tall is 60 sq ft: with a 12 by 6 in block face (0.50 sq ft) that is 120 blocks, and with a 12 by 4 in face (0.33 sq ft) it is 180. Block size moves the answer more than anything else, which is why the chart above lists five common faces instead of assuming one.
How do I calculate retaining wall square footage?
Length times height, both in feet, and it is the vertical face you measure rather than the ground the wall sits on. A wall 30 ft long and 2 ft 6 in tall is 30 × 2.5 = 75 sq ft. Add the buried base course to the height before you multiply, because those blocks get ordered too even though nobody ever sees them.
Do I need to bury the first course of a retaining wall?
Setting the base course below finished grade is standard practice, and it is what stops the bottom of the wall being pushed out. Count it when you order: a wall you want standing 3 ft tall is ordered as 3 ft plus at least one course. Your block's installation guide gives the burial depth for the height you are building, and it increases as the wall gets taller.
How deep should a retaining wall base be?
Follow the trench depth in your block manufacturer's installation guide, because it is written for that specific block and it scales with wall height. The usual shape of it is a trench deep enough to hold a compacted gravel base plus the full height of the base course, so the top of that course finishes at or just below grade. Confirm against your local code before you dig, especially if the wall carries anything above it.
How much gravel goes behind a retaining wall?
There are two separate quantities: the compacted base under the wall and the drainage stone behind it. The base is a trench, so length times width times depth divided by 27 — a 20 ft wall on a 12 in wide, 6 in deep base takes about 0.37 cu yd. Drainage stone is figured the same way from the width of the backfill zone and the height of the wall. Use the gravel calculator to convert either one to tons.
How tall can a retaining wall be without an engineer?
There is no single national figure. Many US jurisdictions set the threshold for a permit or an engineer's stamp near 4 ft, measured from the bottom of the base course rather than from ground level, so the buried course counts against the limit. A driveway above the wall, a slope rising behind it, or a second wall close by all lower the threshold further. Ask your local building department before ordering.
Link to this calculator
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<a href="https://diymaterialcalculators.com/retaining-wall/">Retaining Wall Block Calculator</a> — free online calculator