Read This Before You Buy AnythingHEIGHT & PERMIT
CHECK FIRST Confirm the height rule where you live
That is called a surcharge, and it is the single thing most likely to turn a weekend wall into an engineered one no matter how short it is. A wall holding up a slope is carrying load from soil you cannot see.
What is genuinely true and what is not. It is commonly said that walls above roughly 3 to 4 ft need a permit and an engineered design, and the range is honest, because the trigger height is set locally and it is not the same everywhere. Some jurisdictions measure from the bottom of the footing rather than from finished grade, which makes a wall taller on paper than it looks. Some drop the trigger far lower when there is a surcharge. Some apply setback rules from a property line or an easement regardless of height. There is no universal number, and anyone who gives you one confidently is guessing. Your local building department is the authority. Call them before you order a pallet, not after.
Wall & Block0 SQ FT
= 20 ft 0 in
Type it the way your tape reads it. This box understands 12ft 6in, 12' 6", 12 6 and 6 1/2, and it echoes back what it understood. Measure along the face of the wall. A curved wall needs a little more, so measure the curve, not the chord.
= 3 ft 0 in
This is what you will see when the job is done, measured from the finished grade in front of the wall to the top of the cap. The buried base course sits below this and is added by the calculator. Note that some building departments measure total wall height from the bottom of the base course instead, which can push you over their trigger height. Ask them which they use.
Block size
Segmental retaining wall units are not standardized the way brick and block are. 12 in long by 8 in high by 9 in deep is a common yard unit and it is the default here, but 16 x 6 x 12 and 18 x 8 x 12 are just as common and a tapered unit measures differently front and back. Take the three numbers off your product's spec sheet, because the block count and the pad width both depend on them.
The cap is part of the exposed height, which is why it is subtracted before the courses are worked out. Caps are usually glued down with a masonry adhesive rather than set in mortar, and they are often a different length from the wall unit, so set both numbers from your own product.
Buried base course
The common rule of thumb is about 1 in of buried depth for every 1 ft of wall height, with a minimum of one full course. It is a rule of thumb and not a code figure, manufacturers state it differently, and a wall at the bottom of a slope or in front of a swale usually wants more. Both numbers are yours to change. What is not optional is that the first course goes below grade: it is what stops the wall being pushed out at the bottom.
Base, drainage and setback
Pad width here is the block depth plus this figure, so 12 in gives you 6 in in front and 6 in behind, which is a common minimum. Setback is the built in lean of the unit: most segmental blocks step back somewhere between about 1/8 in and 1 1/2 in per course depending on the lip or pin position, and yours is on the spec sheet.
Gravel Density, Fabric & Geogrid
Crushed base and clean 3/4 in drainage stone both run roughly 1.3 to 1.5 tons per cubic yard depending on the stone and how wet it is. Ask your quarry what density they invoice against and use theirs. The overage is shown as its own line, never folded into the volume.
Only fill these in if you are reading them off an engineered drawing. Geogrid is the sheet reinforcement that ties the wall back into the soil behind it, and it turns a stack of blocks into a reinforced soil mass. How many layers, at which courses, and how far back each one runs are outputs of a design that accounts for your soil, your wall height and whatever is sitting on top of it. A calculator cannot produce them, and this one will not pretend to.
Cross Section1:SCALE
Drawn to your real course heights, your real setback and your real buried depth. The dashed line is finished grade in front of the wall.
Blocks To BuyLIVE
Wall blocks to buy
0 blocks
Waste allowance, shown as its own line
5% for a straight wall with square ends. 10% once there are curves, because an outside curve opens the joints and an inside curve makes you cut the back off units. 15% for tight radius work, steps, corners or a wall that has to die into a slope at both ends. Split face units also chip in handling, and a chipped face is scrap.
Blocks, Caps & Courses#WALL
Base & Drainage#BASE
Materials Ticket#SRW
Enter the wall size to build the ticket.
Order the whole wall in one delivery. Split face concrete units shift color between production runs, and the drainage stone is cheap compared to rebuilding the wall, so never trim that line.
How This Is CalculatedSHOW WORK
Courses, and why the cap comes off first
The exposed height you enter is measured to the top of the finished wall, so it includes the cap. The cap height comes off before the courses are counted, then what is left is divided by the block height and rounded up. The buried base course is added on top of that count.
Work the anchor case through: a 20 ft wall, 3 ft exposed, using 12 in long by 8 in high units with a 4 in cap. 36 in of exposed height minus a 4 in cap is 32 in of block, which is 4 courses. The burial rule of 1 in per foot gives 3 in, and the minimum of one full course rounds that up to 1 buried course. So 5 courses in total. 20 ft is 240 in, divided by a 12 in face is 20 blocks per course. 5 x 20 = 100 blocks, plus 20 caps. The finished wall stands 5 courses of 8 in plus a 4 in cap, which is 44 in, with 8 in below grade, leaving exactly the 36 in of exposed height you asked for.
The buried base course
The first course goes below finished grade. This is not optional and it is not about looks. Soil pushes on a retaining wall horizontally, and the bottom course is the pivot: bury it and the soil in front acts against that push, leave it on the surface and the base course slides out and the wall follows it.
The common rule of thumb is about 1 in of burial per foot of wall height, with a minimum of one full course. Treat that as a starting point, not a law. It is a manufacturer style guideline rather than a code figure, different makers state it differently, and a wall at the bottom of a slope, in front of a drainage swale or beside a road usually wants noticeably more. Both the rate and the minimum are inputs on this page for that reason.
Drainage is what keeps the wall up
The most common way a segmental retaining wall fails is not the blocks and it is not the height. It is water. Saturated soil behind a wall builds hydrostatic pressure, and water is heavy: a wall that was comfortable holding back damp soil can be pushed over by the same soil after a wet week, because the water adds load the wall was never carrying when you built it.
The answer is a drainage zone of clean, open graded stone directly behind the wall, from the base up, with a perforated drain pipe at the bottom that daylights somewhere lower than the wall. Clean stone means washed and angular with no fines in it, because stone with fines in it clogs and becomes soil. The stone gives the water a path down, the pipe takes it away, and the pressure never builds. Wrapping the stone in a filter fabric keeps the soil behind it from washing into the stone and blinding it over the years.
Two practical points the calculator cannot do for you. The pipe has to actually go somewhere: a pipe that ends in the middle of a lawn or runs uphill is decoration. And the stone zone wants to be capped near the top with a few inches of compacted soil or clay so surface water runs off the wall rather than straight down into the drainage zone.
The leveling pad and the trench
The wall sits on a compacted gravel leveling pad, not on soil and not on concrete. The pad is dug wider than the block, commonly the block depth plus about 6 in in front and behind, so the base course has support past its own edges. Pad depth is typically around 6 in of compacted crushed base for a modest wall, more as walls get taller.
The trench you dig is deeper than the pad, because it has to hold the pad and the buried course. Trench volume on this page is that full depth times the pad width times the wall length, so you know what is coming out of the ground as well as what is going into it. Loose spoil is bulkier than the hole it came out of, so allow more room for the pile than the number suggests.
Compaction matters more than depth. A pad that is 6 in deep and properly compacted in lifts beats a 10 in pad that was dumped and raked, because settlement under the base course shows up as a wave along the top of the wall within a season, and there is no fixing it from the top.
Setback and where the top of the wall ends up
Segmental units are made to lean back into the slope, and each course steps back by a fixed amount set by the lip or the pin holes. The default here is 3/4 in per course, but real units range from about 1/8 in for a near vertical wall to 1 1/2 in or more for a heavily battered one, and yours is on the spec sheet.
The consequence people forget is horizontal. Over the courses of your wall the top face sits back from the base face by the setback times one less than the course count, so on the anchor wall that is 3/4 in x 4 = 3 in. On a six foot wall with a 1 in setback it is closer to 8 in. If the top of the wall has to land on a property line, a path edge or the face of an existing step, set the base course out to allow for it, because you cannot recover the difference at the top.
Geogrid: when, and why not from here
Geogrid is a stiff sheet laid between courses that runs back into the fill behind the wall, tying the blocks and the soil into a single reinforced mass. It is what makes a tall segmental wall work at all. It is typically needed once a wall gets tall, and often needed at any height when there is a surcharge such as a slope, a driveway or a structure above it, and in poor or clay soils.
Where it goes is an engineering answer. The number of layers, which courses they sit on, how far back each one runs and what strength grade to use all come out of a design that accounts for the soil type, the wall height, the slope above and below, the water and the surcharge. This page will multiply layers by length by wall length for you once you have those numbers from a drawing, and it will not invent them. If nobody has given you the numbers, that is the signal to go and get an engineered design, not to guess.
For the gravel volumes on their own, see the gravel calculator. For any depth of fill in cubic yards, see the cubic yard calculator. For a mortared block wall instead of a segmental one, see the concrete block calculator, and for the footing under it, the concrete calculator.
Retaining Wall Quick ReferenceSCREENSHOT ME
Courses and blocks for common walls, 12 in by 8 in units with a 4 in cap
| Wall | Exposed | Courses | Per course | Blocks | Caps |
|---|---|---|---|---|---|
| 10 ft | 2 ft | 4 | 10 | 40 | 10 |
| 20 ft | 3 ft | 5 | 20 | 100 | 20 |
| 20 ft | 4 ft | 7 | 20 | 140 | 20 |
| 30 ft | 2 ft | 4 | 30 | 120 | 30 |
| 40 ft | 3 ft | 5 | 40 | 200 | 40 |
| 50 ft | 4 ft | 7 | 50 | 350 | 50 |
The four things that fail a retaining wall
| Cause | What it looks like | The fix at build time |
|---|---|---|
| No drainage | Wall bulges or tips out after heavy rain | Clean stone behind, perforated pipe that daylights |
| Base course not buried | Bottom course slides out, wall leans forward | Bury the first course, roughly 1 in per ft of height |
| Pad not compacted | Wave along the top of the wall in year one | Compact the pad in lifts, check level as you go |
| Too tall with no reinforcement | Wall rotates outward from mid height | Engineered design, geogrid, and a permit |
Drainage stone and base gravel per 10 ft of wall
| Item | Size used | Cu ft | Cu yd | Tons at 1.4 |
|---|---|---|---|---|
| Leveling pad | 21 in wide x 6 in deep | 8.8 | 0.32 | 0.45 |
| Drainage stone, 3 ft wall | 12 in wide x 44 in high | 36.7 | 1.36 | 1.90 |
| Drainage stone, 4 ft wall | 12 in wide x 60 in high | 50.0 | 1.85 | 2.59 |
| Trench spoil, 3 ft wall | 21 in wide x 14 in deep | 20.4 | 0.76 | varies |
Setback: where the top of the wall lands
| Setback per course | 5 courses | 8 courses | 12 courses |
|---|---|---|---|
| 1/8 in, near vertical | 0.5 in | 0.9 in | 1.4 in |
| 1/2 in | 2.0 in | 3.5 in | 5.5 in |
| 3/4 in, common | 3.0 in | 5.3 in | 8.3 in |
| 1 in | 4.0 in | 7.0 in | 11.0 in |
| 1 1/2 in, heavy batter | 6.0 in | 10.5 in | 16.5 in |
Now Price The Job
- GravelTons and yards of stone
- Concrete BlockCMU, courses, core fill
- BrickBricks, cubes, mortar
- ConcreteFootings and slabs
- Cubic YardAny depth of fill
- Square FootageOdd shapes and curves
- PaversPatio above or below
- DeckBoards and joists
- FencePosts, rails, pickets
- DrywallSheets, mud, tape
- InsulationBatts and blown R value
- PaintGallons per coat
Retaining Wall Calculator FAQ
How tall can I build a retaining wall without a permit?
There is no universal answer, and that is the honest one. It is commonly said that walls above roughly 3 to 4 ft need a permit and an engineered design, and that range is a fair description of what many jurisdictions do, but the trigger height is set locally and it is not the same everywhere.
Several things move it. Some departments measure total height from the bottom of the base course rather than from finished grade, which makes a wall taller on paper. Some drop the trigger sharply when there is a surcharge such as a slope, a driveway or a structure above the wall. Some apply property line or easement setbacks regardless of height, and some treat a series of stacked short walls as one tall wall if they are close together. Call your local building department before you order material. They are the authority and the call is free.
How many blocks do I need for a 20 ft retaining wall 3 ft high?
With 12 in long by 8 in high units and a 4 in cap, 100 wall blocks and 20 caps before waste. The 36 in of exposed height less the 4 in cap is 32 in of block, which is 4 courses, and the buried base course makes 5. 240 in of length divided by a 12 in face is 20 blocks per course, and 5 x 20 = 100.
Add your waste allowance on top as its own line: 5% for a straight wall, 10% or more with curves. Also budget about 1.4 cubic yards of leveling pad and drainage stone per 10 ft of wall at this height, which the calculator breaks out for you.
How deep should the first course of a retaining wall be buried?
The rule of thumb is about 1 in of buried depth for every 1 ft of wall height, with a minimum of one full course. On a 3 ft wall the rule gives 3 in, so the minimum takes over and you bury one whole course, typically 8 in for a common unit.
Treat it as a starting point rather than a rule. It is a manufacturer style guideline, not a code figure, different makers state it differently, and a wall at the bottom of a slope, in front of a swale or beside a road usually wants more. Both the rate and the minimum are adjustable on this page. What is fixed is that the first course has to go below grade at all: the buried course and the soil in front of it are what stop the bottom of the wall sliding out.
Why does a retaining wall need drainage stone behind it?
Because water is the most common reason retaining walls fail. Soil behind a wall soaks up rain and holds it, and saturated soil pushes far harder than damp soil does. That extra push is called hydrostatic pressure, and it is load the wall was never carrying on the day you built it.
A zone of clean, open graded stone directly behind the wall gives the water a path straight down instead of a reason to lean on the blocks, and a perforated pipe at the bottom carries it away to a point lower than the wall. Clean means washed and angular with no fines, because stone with fines in it silts up and turns into soil. Wrap the stone in filter fabric so the backfill cannot wash into it, and cap the top few inches with compacted soil so surface water runs off rather than in.
How much gravel do I need under a retaining wall?
A leveling pad of about 6 in of compacted crushed base, dug wider than the block, is typical for a modest wall. The common width is the block depth plus roughly 6 in in front and 6 in behind, so a 9 in block sits on a 21 in wide pad. That works out near 0.32 cubic yards per 10 ft of wall.
The trench is deeper than the pad because it holds the pad and the buried course together, so on the anchor wall it is 14 in deep. Compaction matters more than depth: a properly compacted 6 in pad beats a dumped 10 in one, because any settlement under the base course shows up as a wave along the top of the wall and cannot be fixed from above.
What is setback, and does it change where the top of the wall lands?
Setback is the built in lean. Segmental units have a lip or a pin position that steps each course back from the one below, so the wall leans into the slope it is holding. Real units range from about 1/8 in to 1 1/2 in per course, and yours is on the spec sheet. The default here is 3/4 in.
It absolutely changes where the top lands. The top face sits back from the base face by the setback times one less than the course count, so five courses at 3/4 in puts the top 3 in behind the base, and a twelve course wall at 1 in puts it 11 in behind. If the top of the wall has to meet a property line, a path or an existing step, set the base course out to allow for it, because there is no recovering that distance at the top.
When does a retaining wall need geogrid?
Typically once the wall gets tall, and often at any height when there is a surcharge, meaning a slope, a driveway, a parking area, a pool or a structure sitting above the wall. Poor or clay soils bring it in sooner. Geogrid is a stiff sheet laid between courses that runs back into the fill and ties the blocks and the soil into one reinforced mass.
The spacing comes from an engineered design, not from a calculator. How many layers, which courses they sit on, how far back each one runs and what grade of grid to use all fall out of a design that accounts for your soil, your wall height, the slope above and below and the surcharge. This page will multiply the numbers for you once a drawing gives them to you. If no drawing exists, that is the signal to get one.