🧱 Retaining Wall Calculator

How to Calculate a Segmental Retaining Wall

A segmental retaining wall (SRW) is dry-stacked concrete units, no mortar, held in place by their own weight, the setback between courses, and - above a certain height - layers of geogrid tying the wall back into the soil behind it. The estimate has four parts: blocks, caps, base stone and drainage stone.

The number people get wrong is total wall height. It is not the height you see. It is the exposed height plus the buried base course.

The buried course rule

Bury the first course by the greater of one full block height or 10% of the total wall height. For a typical 8 in unit that means at least 8 in below finished grade, and more as the wall gets taller. On a 6 ft wall, 10% is 7-1/4 in of extra burial beyond one course.

Two situations demand more burial than the rule:

• Slope at the toe. If the ground falls away in front of the wall, bury deeper - the soil in front is what resists the wall sliding forward, and a slope removes it.
• Traffic or a hard surface at the toe. Frost heave and vehicle loads both argue for extra embedment.

The buried course is not wasted material. It is the single cheapest thing you can do to stop the wall walking forward, and it is the first thing skipped on failed walls.

Base stone and drainage stone are different materials

• Base: 3/4 in minus crushed stone - angular, with fines, so it compacts and locks. Never sand, never pea gravel, never rounded river rock. 6 in compacted for walls to about 4 ft; 8 to 12 in for taller walls or soft subgrade. Extend it 6 in beyond the block on each side.
• Drainage: 3/4 in clean washed angular stone - no fines at all, so water runs straight through. A minimum 12 in wide zone the full height of the wall, plus the hollow cores of the units filled with the same stone.

Use our gravel calculator if you want to price either aggregate by the ton on its own.

Wall Height Drives Everything

Retaining wall design is not linear. The soil pressure on a wall grows with the square of its height, so a 6 ft wall carries roughly four times the total force of a 3 ft wall - not twice. That is why the requirements escalate the way they do.

A "surcharge" is any load above the wall - a slope rising behind it, a driveway or parking area, a pool, a building footing, a fence taking wind load, or a second wall terraced above. Surcharge changes the problem completely, and it is the reason two identical-looking 3 ft walls can have entirely different requirements.

Terraced walls are not two small walls

Stacking two 3 ft walls to get 6 ft of grade change does not avoid the engineering. The upper wall is a surcharge on the lower one unless the horizontal separation between them is at least twice the height of the lower wall. Below that separation, most codes treat the pair as a single tall wall. Designers who ignore this produce the classic failure where the lower wall bulges outward under the upper terrace.

Drainage: Why Retaining Walls Actually Fail

Retaining walls very rarely fail because the blocks were not strong enough. They fail because water got behind the wall and stayed there. Saturated soil weighs roughly 120 lb per cubic foot and pushes far harder than dry soil, and if the water freezes, the expansion does the rest.

Wrap the pipe or wrap the stone, but do not wrap nothing. A sock on the pipe stops fines entering it; fabric between the stone and the retained soil stops the whole drainage zone silting up. Doing both is the belt-and-braces version and costs very little.

Compaction discipline matters as much as the drainage. Backfill in 8 in lifts, compact each one, and keep the plate compactor within 3 ft of the wall face on hand-guided passes only. Running a ride-on compactor or a loaded machine close to the back of a fresh wall pushes the top courses out - the damage is done before you see it.

When You Need an Engineer and a Permit

This is the section that matters more than the block count. A retaining wall that fails does not just crack - it releases tons of soil, and it can take a driveway, a fence, a neighbour's yard or a person with it. Treat the height thresholds as the minimum bar, not the target.

• The common trigger is 4 ft, measured from the bottom of the footing or base to the top of the wall - not from finished grade. Above that, most jurisdictions require a design stamped by a licensed engineer plus a building permit and inspections.
• The trigger varies and you must check locally. Plenty of jurisdictions set it at 3 ft, some at 30 in, some at 2 ft, and some regulate by grade change rather than wall height. Homeowners association rules and easements can be stricter than the code.
• Any surcharge usually forces engineering at any height - a slope above the wall, a driveway, a pool, a structure footing within the influence zone, or a terraced wall above.
• Segmental wall manufacturers publish maximum unreinforced heights for each unit, typically 3 to 4 ft with a level backslope. Above that height their own literature requires geogrid, and geogrid length and spacing come from a design, not from a rule of thumb.
• Geogrid embedment is conventionally at least 60 to 70% of the wall height and never less than about 4 ft, in layers every 2 to 3 courses. Those are planning figures. The actual grid type, spacing and length depend on the soil properties, the slope and the surcharge, and are the engineer's call.
• Soil matters more than the wall. Clay, fill, high water table and organic soils all change the design. An engineer may require a soils report before designing anything tall.

This calculator estimates quantities. It is not a structural design, it does not size geogrid, and it does not replace an engineer or a permit. If your wall is near or above your local threshold, has anything loading it from above, or is holding up something you cannot afford to lose, get it designed.

When to pick a different wall entirely

If the grade change is under about 3 ft and you are comfortable with a plate compactor, a segmental wall is genuinely a DIY job. Above that, the cost of engineering is small compared with rebuilding a failed wall - and far smaller than the liability if it fails onto something.

How to Build a Segmental Retaining Wall

1. Call before you dig. Utility locates are free and mandatory in most places. Then check your setbacks, easements and permit threshold with the building department before you buy anything.
2. Excavate the trench. Width equals the block depth plus 6 in on each side; depth equals the gravel base thickness plus the full buried course. Dig into undisturbed soil - if you hit soft, organic or filled ground, over-excavate and replace it with compacted stone.
3. Compact the subgrade before any stone goes in. This step is invisible in the finished wall and it is the one that decides whether it settles.
4. Place and compact the base stone in lifts. 3/4 in minus crushed stone, no more than 4 in per lift, compacted to a firm, unyielding surface. Screed it dead level - level base, level wall.
5. Set the first course perfectly. Level front to back and side to side, block to block, using a torpedo level on every unit. Spend an hour here to save a day later; every error multiplies up the wall.
6. Backfill the first course as you go - drainage stone behind, cores filled, and the front of the buried course covered. Set the perforated pipe at the base of the drainage stone, holes down, on a slope of at least 1%, and plan where it daylights before you bury it.
7. Stack the remaining courses with the joints staggered and the built-in setback engaged - most systems give 1/8 to 1-1/2 in of batter per course, and the setback is what leans the wall into the hill. Sweep every course clean before setting the next; a single stone chip under a block tips it out of plane.
8. Fill cores and backfill in 8 in lifts, compacting each lift with a plate compactor. Hand-guided passes only within 3 ft of the wall face, and never run heavy equipment behind a fresh wall.
9. Install geogrid where the design calls for it, at the specified courses, pulled taut, with the roll direction perpendicular to the wall face and the correct length embedded. Grid laid loose or too short does nothing.
10. Cap the wall. Cut caps as needed on a masonry saw, then bed them in flexible masonry adhesive - one 10 oz tube runs roughly 15 to 20 lineal feet. Slope the finished grade behind the cap away from the wall.

Mistakes that end in a rebuilt wall

• No buried course. The wall has nothing holding its toe and it creeps forward season by season.
• Sand or pea gravel for a base. Neither compacts into a stable platform. Use angular crushed stone with fines.
• Backfilling with the excavated clay right against the wall. Clay holds water and swells. That is what the clean stone zone is for.
• No drain pipe, or a pipe that goes nowhere. Collecting water and then trapping it is worse than not collecting it.
• Compacting with a machine too close to the face. The top courses bulge outward and you cannot pull them back.
• Assuming a 3 ft wall does not need a permit. Verify with your building department. The fine and the tear-out both cost more than the call.

Materials, Weights and Costs

Face square footage is the unit the trade quotes in - total wall height, including the buried course, times wall length. A 40 ft wall at 3.7 ft total height is 148 face sq ft, which at $35 installed is roughly $5,200. Estimating from the exposed height alone systematically under-quotes every wall.

Delivery and material handling are real costs on this job. A 40 ft, 4 course wall in 12 in units is around 240 blocks at 60 lb each - about 7 tons of block, plus 8 to 10 tons of stone. Get the pallets and the stone dropped as close to the trench as the truck can reach.

Related calculators: gravel and crushed stone for base and drainage tonnage, concrete block for a reinforced CMU wall, mortar coverage if you build the alternative in mortared block or stone, concrete for a poured footing, concrete curb for a low edge that is not retaining anything, and brick for a decorative facing wall.

Frequently Asked Questions

How many blocks do I need for a retaining wall?

Divide the total wall height in inches by the block face height to get courses, then divide the wall length in inches by the block face width to get blocks per course, and multiply. Total height includes the buried base course, not just what you will see. A 40 ft wall 3.7 ft tall in 12 x 8 in face units is 6 courses of 40 blocks, or 240 blocks.

How deep should a retaining wall be buried?

Bury the base course by the greater of one full block height or 10% of the total wall height. For a typical 8 in unit that is at least 8 in below finished grade. Bury deeper if the ground slopes away in front of the wall, since the soil at the toe is what resists the wall sliding forward.

How much gravel base does a retaining wall need?

6 in of compacted 3/4 in minus crushed stone for walls up to about 4 ft, and 8 to 12 in for taller walls or soft subgrade. Make the base 6 in wider than the block on each side. Use angular crushed stone with fines so it compacts and locks - never sand, never pea gravel, never rounded river rock.

What height retaining wall needs a permit and an engineer?

The most common trigger is 4 ft, measured from the bottom of the base to the top of the wall - but it varies. Plenty of jurisdictions set it at 3 ft, some at 30 in, and some regulate by grade change instead. Any surcharge above the wall, such as a slope, a driveway, a pool or a structure, usually requires engineering regardless of height. Check with your local building department before you build.

Do I need gravel behind a retaining wall?

Yes, and it is the single most important thing you will do. A minimum 12 in wide zone of clean washed 3/4 in stone for the full wall height, plus the hollow block cores filled with the same stone. Saturated soil pushes roughly twice as hard as drained soil. Almost every retaining wall failure is a drainage failure.

Do I need a drain pipe behind a retaining wall?

Yes on anything but the lowest garden edging. A 4 in perforated pipe at the base of the drainage stone, holes down, sloped at least 1%, daylighting every 25 to 50 ft and at both ends. Wrap the pipe in a sock or put filter fabric between the stone and the retained soil. Collecting water and then trapping it is worse than not collecting it at all.

What is geogrid and when do I need it?

Geogrid is a stiff polymer mesh laid between courses and extending back into the compacted backfill, which turns the soil behind the wall into part of the structure. It is generally required above 3 to 4 ft, and at any height with a slope or load above the wall. Conventional practice is layers every 2 to 3 courses with an embedment length of 60 to 70% of the wall height and never less than about 4 ft. The actual type, spacing and length must come from an engineered design.

Can I build two short retaining walls instead of one tall one?

Only if you separate them properly. The upper wall is a surcharge on the lower one unless the horizontal setback between them is at least twice the height of the lower wall. Below that, most codes treat the pair as a single tall wall requiring the same engineering. Terraces built too close together produce the classic bulge in the lower wall.

How much does a retaining wall cost?

Segmental block walls run about $25 to $50 per face square foot installed, where face area is total height including the buried course times length. Materials alone are roughly $12 to $25 per face sq ft. Engineering, geogrid, poor access and tall walls all push it up. A 40 ft wall 3.7 ft tall is about 148 face sq ft, so roughly $3,700 to $7,400 installed.

What is the setback or batter on a retaining wall?

Setback is how far each course steps back from the one below, built into the unit by a lip or a pin. Most systems give 1/8 in to 1-1/2 in per course, which leans the wall into the hill. More setback means more stability and more lost space at the top. Do not defeat it by shimming the wall vertical - the batter is part of the design.

Does a segmental retaining wall need a concrete footing?

No. A dry-stacked segmental wall sits on a compacted crushed stone base, which lets the wall flex slightly with ground movement instead of cracking. A concrete footing would make it rigid and is not how the system is designed to work. Reinforced CMU and poured concrete retaining walls are different - those do need a designed, frost-depth footing.

How much drainage stone and base stone will I need in tons?

Both are usually sold by the ton. Crushed base stone runs about 1.4 tons per cubic yard compacted and clean washed drainage stone about 1.35 tons per cubic yard. A 40 ft wall 3.7 ft tall with a 6 in base and a 12 in drainage zone needs roughly 2 tons of base stone and 7 tons of drain rock. The calculator above gives both in cubic yards and tons.

How do I attach the cap blocks?

With flexible masonry or landscape block adhesive, not mortar. One 10 oz tube covers roughly 15 to 20 lineal feet. Sweep the top course clean and dry first, run two beads, and set the caps with a slight overhang at the front. Mortar goes rigid, cracks with seasonal movement, and pops the caps off within a couple of winters.

Can I compact right behind a new retaining wall?

Only by hand-guided plate compactor, and only in lifts of 8 in or less. Keep ride-on compactors and loaded equipment at least 3 ft back from the wall face. Over-compacting close to a dry-stacked wall pushes the upper courses outward, and once the wall is out of plane there is no pulling it back without taking it apart.