How many blocks for a retaining wall?
Standard 12 × 4 in blocks including 10% waste, no cap row.
| Wall length | 2 ft high | 3 ft high | 4 ft high |
|---|---|---|---|
| 10 ft | 66 blocks | 99 blocks | 132 blocks |
| 20 ft | 132 blocks | 198 blocks | 264 blocks |
| 30 ft | 198 blocks | 297 blocks | 396 blocks |
| 50 ft | 330 blocks | 495 blocks | 660 blocks |
Retaining wall block sizes compared
Bigger blocks mean fewer to lay but much heavier lifting.
| Block size | Typical weight | Blocks per 10 sq ft of wall face |
|---|---|---|
| 12 × 4 in (7.5 in deep) | 20–35 lb | 30 |
| 16 × 6 in (11 in deep) | 42–64 lb | 15 |
| 18 × 8 in (12 in deep) | 75–105 lb | 10 |
How many retaining wall blocks do I need?
Divide the wall length by the block length and you’ve got blocks per course. Divide the height by the block height for courses, then multiply the two. Add about 10% for cuts and breakage.
Take a 20 ft wall, 2 ft high, in 12 × 4 in block. That’s 20 per course × 6 courses. Cap it and you get 5 block courses plus caps, which is the calculator’s default.
The gravel nobody counts
A block wall lasts on two separate gravels, and most people only order one. The first is a compacted base trench, 6 in deep and twice the block depth wide. The second is 12 inches of clean crushed stone directly behind the full wall face, there for drainage. Skip the backfill and the wall usually starts leaning after two winters.
How much gravel goes behind a retaining wall?
Behind the face goes the drainage column, and it isn’t interchangeable with the base gravel. Every manufacturer, and the CMHA guidance behind them (CMHA is the body NCMA and ICPI merged into), asks for at least 12 inches of clean angular stone. It runs the full height and full length of the wall, sits directly against the back of the block, and fills the open cores of hollow units.
The volume is just the wall face area times one foot. A 20 ft wall standing 4 ft above grade is 80 sq ft of face, so 80 cubic feet, which is 2.96 cubic yards. Add 10% for settling, handling and the buried course and you’re ordering about 3.3 cubic yards. The retaining wall gravel calculator above works this out from length and height alone.
Tonnage is where estimates start to disagree, and it’s worth knowing why before you argue with a supplier. #57 stone is quoted at roughly 2,700 lb per cubic yard (about 1.35 tons) for crushed limestone, around 1.45 tons as a general estimating value, and closer to 1.5 tons for granite. Real densities drift about 10% either way depending on source, gradation and how wet the pile is.
So those 3.3 cubic yards are about 4.4 tons of limestone, and closer to 5 for granite. The calculator uses 1.35 tons per cubic yard. Ask your yard what their own conversion is rather than trusting any calculator to three decimal places. In bags it looks absurd. A cubic yard is 54 half-cubic-foot bags, so you’d be carrying roughly 180 of them. Order bulk.
Specify the stone by grade so the yard knows what you mean. That’s #57 in the US, or 19 mm or 20 mm clear stone at a Canadian counter. #57 under ASTM D448 passes a 1½ in sieve entirely, 95 to 100% passes 1 in, and no more than 10% passes the No. 4. Open enough to drain, angular enough to lock together. Allan Block writes its wall rock spec as 0.25 to 1.5 in with no more than 10% passing the #200 sieve, and #57 sits comfortably inside that.
Don’t substitute pea gravel or sand. Both hold moisture, and neither has the shear resistance to stay put once the wall loads up.
Separate the stone from the native soil with a non-woven geotextile so silt can’t migrate in and blind the column. Run a 4 in perforated drain pipe along the bottom at the back of the trench.
Retaining wall base gravel: leveling pad depth, trench width and tonnage
The leveling pad is the footing. It’s also the one part of the wall you can’t go back and fix. Common practice is a minimum of 6 inches of compacted crushed stone, extending 6 inches in front of and 6 inches behind the block. So a 12 in deep unit wants a 24 in trench, which is what Allan Block specifies. For walls under 4 ft the same manual drops to an 18 in wide trench at 4 in deep, plus whatever depth the buried course needs.
This calculator uses twice the block depth. That’s 24 in for a 12 in unit but only 15 in for a 7.5 in garden block. Want the full block-depth-plus-12-inches pad? Dig wider than the default.
Trench depth is the pad plus the buried course, and the published rules genuinely differ. Allan Block’s installation page tells you to dig 6 inches for the wall rock plus 1 inch for every foot of wall height to bury block. Its own embedment article sets a flat minimum of 6 inches of buried block, adding an inch per foot only above 6 ft. Other practice quotes roughly 10% of exposed height, or half a unit.
On a 4 ft wall those all land within an inch or two of each other. Bury the whole first course of a 6 or 8 inch unit and dig 12 to 14 inches overall. With a 4 inch garden block you’d need two buried courses to reach 6 inches, which is one more reason those units suit low walls only.
Use crushed angular stone for the pad and compact it hard. Around 95% Standard Proctor is the usual specification, which in practice means several passes with a plate compactor and a screed rail to get it dead level. Sand and pea gravel are out for the same reason as behind the wall. No shear resistance, so they creep under a concentrated load.
For quantities, a 20 ft wall in 12 × 4 block needs a 15 in wide, 6 in deep trench. That’s 12.5 cubic feet or 0.46 cubic yards. Add 20% for over-dig and you’re at 0.56 cubic yards, roughly three-quarters of a ton. The same wall in 18 × 8 × 12 block takes a 24 in trench, so 0.89 cubic yards, or about 1.2 tons.
How many blocks for a 20 ft wall at 2 ft, 3 ft and 4 ft high?
A retaining wall block calculator is really doing two divisions and a lot of rounding up. Blocks per course is wall length over block length. Courses is wall height over block height. Multiply the two.
Take 20 ft, which is 240 inches. In 12 × 4 in block that’s 20 blocks per course, and 6, 9 or 12 courses at 2 ft, 3 ft and 4 ft high. So 120, 180 and 240 blocks before any allowance. Cross-check it against face area. A 4 ft by 20 ft wall is 80 sq ft, and a 12 × 4 face covers a third of a square foot, so 240 again. Then add about 10% for cuts, corners and the ones that arrive broken.
If you cap the wall, the top course stops being block. The calculator swaps it out, so a 4 ft wall becomes 11 block courses plus a cap row, or 220 units, 242 with waste, and 22 caps. The 2 ft version is 110 blocks and 22 caps, the 3 ft version 176 and 22.
One thing to watch. Caps are counted off the wall block length, on the assumption your cap matches it. Plenty don’t. If you’re running 18 in wall units under a 12 in capstone, divide the wall length by the cap length yourself and treat that row as a rough guide only.
Bigger units change the count more than people expect. A 16 × 6 in block gives 15 per course, and 4, 6 or 8 courses at those same three heights. With a cap row you’re at 50, 83 and 116 blocks plus 17 caps. An 18 × 8 block gives 240 divided by 18, which is 13.3 and rounds up to 14 per course, and 3, 5 or 6 courses. Call it 31, 62 and 77 blocks plus 16 caps.
That fractional block per course is real work. On a running bond every other course starts with a cut half anyway, which is why any segmental retaining wall material calculator has to round up and then add waste on top of that.
How tall can a retaining wall be without engineering or a permit?
The number to know is 4 feet, and the trap is where it’s measured from. The IBC exempts retaining walls not over 4 feet in height measured from the bottom of the footing to the top of the wall, unless the wall supports a surcharge or impounds Class I, II or IIIA liquids.
Bottom of the footing means the underside of your leveling pad. Put 6 inches of pad and a 6 inch buried course under the wall and you’ve already spent a foot of that allowance. You’re left with roughly 36 inches of exposed face before you cross the line, which is a good deal less wall than most people picture when they hear four feet.
Local rules move the line in both directions, so it’s worth a call to the building department before you order anything.
Philadelphia wants a permit on any wall over two feet, Montgomery County in Maryland triggers at 3 ft of retained height, and California follows the 4 ft from bottom of footing wording.
Canada measures it lower and differently. Under the Ontario Building Code a wall over 1.0 m (3 ft 3 in) next to public property or a building access is a designated structure requiring engineered design. The City of Toronto requires a permit above 1.0 m of retained height, and CMHA recommends an engineer above 1.2 m wherever local code is silent.
Separately from permits, manufacturers publish their own unreinforced limits — and those aren’t negotiable. Allan Block puts AB Classic at a 4.0 ft gravity maximum, and Versa-Lok rates its 12 in deep Standard unit at 4 ft in optimal conditions. The 4 in Pavestone garden block is rated for total wall heights up to 2 ft. Poor soil, a slope at the toe or any load on top pulls all of those down.
Past the gravity limit you’re into reinforced wall territory, which means geogrid. These are sheets of polymer grid laid between courses and stretched back into the fill, so the wall and the soil behind it behave as one mass. CMHA practice puts the grid length at 0.6 times the wall height with a 4 ft minimum, laid every other course. On an 8 in unit that’s a layer every 16 inches.
Compact the fill in lifts of 8 inches or less, at least two passes, and never run the plate compactor directly over the grid. A surcharge above the wall pushes you here regardless of height. None of that comes out of a calculator. It comes off an engineered plan.
Retaining wall block sizes compared: 12 × 4, 16 × 6 and 18 × 8
Compare block sizes on face coverage before anything else, since that’s the number that actually matters. A 12 × 4 in unit covers a third of a square foot, so you need three per square foot of wall. A 16 × 6 covers 0.667 sq ft, or one and a half per square foot. An 18 × 8 covers a clean square foot each. That same 80 sq ft wall is 240 units, 120 units or 80 units depending which you pick.
Weight tells the other half of the story. The Pavestone 12 in block measures 11.75 by 6.75 by 4 in and weighs 22 lb. A Versa-Lok Standard at 16 by 12 by 6 in weighs 82 lb, and an AB Classic at 18 by 12 by 8 in weighs 75 lb.
Canadian yards carry a different lineup (Techo-Bloc, Permacon, Barkman, Expocrete and Rinox at RONA, Home Depot Canada and Canadian Tire). The face-coverage math is identical. Each maker’s own published gravity height limit still governs.
Depth resists the soil, not face size, so the shallow garden block gets capped low. At 6.75 inches deep it’s rated to 2 ft of total wall height, while the 12 inch deep units are good for around 4 ft ungridded.
Setback is the other spec to check, because every system leans the wall back into the bank by a fixed amount per course. Versa-Lok’s pin holes give a 3/4 in setback per unit. AB Classic’s raised lip gives a 6 degree batter, and AB Stone offers 12 degrees. Six degrees over 4 ft of wall pulls the top back about 5 inches, which changes both your excavation width and where the finished face lands against a string line.
Capstones are a separate product and rarely the same size as the wall units. Allan Block’s Universal Capstone is 18 by 12 by 4 in, and the company notes that capping adds 3.5 inches to the finished wall height. So count your courses to the top of the block and add the cap afterwards. Caps are bonded down with concrete adhesive rather than stacked dry, and one 29 oz tube covers roughly 60 ft of wall.
When you’re choosing between sizes on price, compare cost per square foot of face rather than per unit. Three 12 in blocks at the calculator’s default $3.50 is $10.50 a square foot against $3.50 for a single 18 × 8. But that gap is an artifact of one default price applied to both sizes, and a 75 lb AB Classic typically costs several times what a 22 lb garden block does. Put your own quoted prices in before you draw a conclusion.
How deep should footings be for a retaining wall?
For a segmental block wall, deep enough for the leveling pad plus the buried course. That means a minimum of 6 inches of compacted crushed stone for the pad, plus a minimum of 6 inches of buried block. It doesn’t need to reach the frost line. A mortared or poured wall does.
The frost exemption catches people out. CMHA treats a segmental wall as a flexible structure. The blocks are stacked dry and the pad is free-draining crushed stone, so the assembly rides small heave movements instead of cracking. Frost-depth footing rules were written for rigid foundations and simply don’t transfer.
Rigid walls are the opposite case. Mortared CMU or poured concrete must bear below your local frost line, and that varies a lot. It’s nothing in Florida, around 6 inches in Dallas, 36 to 48 inches across much of the Midwest and Northeast, and deeper again in northern Minnesota. In Canada it’s 1.2 m (48 in) in southern Ontario, 1.5 m (60 in) in Ottawa and Montreal and 1.8 to 2.4 m (72 to 96 in) across the Prairies.
If an inspector does insist on frost depth for a block wall, thicken the leveling pad rather than burying more block. The extra stone gets you down there without spending exposed height.
Do I need a footing for a 2 foot retaining wall?
Not a concrete one. A 2 foot block wall sits on a compacted crushed-stone leveling pad, and that pad is the footing.
It’s 6 inches deep and runs the block depth plus 6 inches front and back. Bury the first course and you’re excavating roughly 12 inches below finished grade at the toe.
Pouring concrete under a dry-stacked wall is actively worse than gravel, because it traps water beneath the base and leaves the wall nothing to drain into. The exceptions are walls that are rigid by nature (mortared block, mortared stone, poured concrete), all of which want a real footing below frost.
Don’t read ‘no concrete’ as ‘no preparation’, though. Dig down to firm undisturbed soil, strip out topsoil and roots, compact the pad hard, and get that first course level to within about an eighth of an inch, front to back as well as side to side. Every error left in the base course is still there, magnified, by the time you reach the cap.
Common questions
How many blocks for a 4-foot high, 20-foot wall?
In 12 × 4 in block you’re looking at 20 per course × 11 block courses under a cap row, so 220. Add roughly 10% waste and it’s about 242 blocks and 22 caps, or 264 units in total.
How much gravel goes behind a retaining wall?
A 12-inch-wide column of clean crushed stone, the full height and length of the wall. That works out at about 0.75 cu yd per 20 sq ft of wall face, plus 10% for settling.
How deep should the base be?
A 6-inch compacted gravel base in a trench twice the block depth, with the first course buried about half its height. Don’t skimp on the compaction.
Do I need an engineer?
The IBC exemption runs to 4 ft from the bottom of the footing to the top of the wall, not exposed height. A 6 in pad plus a 6 in buried course spends a foot of that, leaving roughly 36 in of exposed face. A surcharge (slope, driveway, structure) means engineering at any height. Local rules often run lower: Philadelphia at 2 ft, Ontario and Toronto at 1.0 m (3 ft 3 in) of retained height.
How much does a retaining wall cost?
In the US as of 2026, block walls typically run $20–55 per sq ft of wall face installed. DIY materials come in around $8–15 per sq ft, and that moves a fair bit with block choice and region.
How wide should the base trench be for a retaining wall?
Dig it to the block depth plus 6 inches front and back. That’s 24 inches for a 12 in deep unit, which is what Allan Block specifies, or 18 inches wide for walls under 4 ft. This calculator uses twice the block depth, identical at 12 in but narrower on a 7.5 in garden block, so widen the trench if you’re following the leveling pad rule to the letter.
What size gravel goes behind a retaining wall?
Clean angular crushed stone, and #57 is the usual call. Under ASTM D448 that’s everything through a 1½ in sieve, 95 to 100% through 1 in and no more than 10% through the No. 4. Allan Block writes its wall rock as 0.25 to 1.5 in with under 10% fines. Never pea gravel or sand. They hold water and deform under load.
How many tons of gravel for a 20 ft wall 4 ft high?
About 3.3 cubic yards behind the wall plus roughly 0.6 in the base trench, so call it 3.9 cubic yards. Tonnage depends who you ask, since #57 gets quoted anywhere from 1.35 tons per cubic yard for limestone up to 1.5 for granite. Somewhere between 5.3 and 5.9 tons, then. Your supplier’s own yard weight beats any of those figures.
Do I need a drain pipe behind a retaining wall?
Allan Block calls for one on any reinforced wall, any gravity wall over 4 ft, and any site with poor drainage. Use 4 in perforated PVC or corrugated pipe laid at the lowest point toward the back of the trench. Vent it to daylight at least every 50 ft (Allan Block’s published minimum), and more often than that in clay, where water moves slowly.
When does a retaining wall need geogrid?
Once you pass the manufacturer’s gravity limit (about 4 ft for most 12 in deep units, 2 ft for a 4 in garden block), or whenever anything loads the ground above the wall. CMHA (formerly NCMA) practice sets grid length at 0.6 times wall height with a 4 ft minimum, laid every second course. That layout belongs on an engineered plan, not in a calculator.
How many capstones do I need, and how are they fixed down?
One per block length along the wall plus about 10%, so 22 caps for a 20 ft wall in 12 in units. They’re bonded with concrete adhesive rather than stacked dry, and Allan Block reckons one 29 oz tube per 60 ft of wall. Caps also add height. Allan Block’s put another 3.5 inches on top of your course count.
What size concrete footing does a mortared retaining wall need?
Rule of thumb, twice the wall thickness wide and at least as thick as the wall is wide. So an 8 inch mortared block wall wants a 16 inch wide footing 8 inches deep, with its underside below the local frost line. Anything tall or cantilevered gets designed properly. Engineers usually start between half and two-thirds of wall height for base width.
Will a block retaining wall heave in winter?
Not normally. The mortarless joints let a segmental wall flex, and the open crushed stone in the pad and behind the face drains rather than holding the water that heave needs. Failures blamed on frost are almost always blocked drainage instead: fines in the wall rock, no geotextile separation, or a drain pipe with nowhere to daylight.
What if the soil under the leveling pad is soft?
Dig it out. Excavate the soft or organic material down to firm undisturbed subgrade and make the depth back up with the same crushed stone, compacted in lifts of no more than 6 to 8 inches. Never make it up with soil, and never bridge a soft pocket. A wall tends to settle at the one spot you decided not to bother with.