Retaining Wall Calculator
Blocks (real sizes or custom), poured concrete, timber, or stone — with caps and base.
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A retaining wall holds back soil and turns a slope into flat, usable yard. It can be built several ways — stacked segmental block (the most popular DIY type), poured concrete, timber, or dry-stacked stone — and the build steps are quite different for each, so the guide below changes with the material you pick above. Under about 4 feet most of these are a strong weekend project; over that, get an engineered design.
DifficultyIntermediate
TimeA weekend for a short wall; longer for tall, poured, or curved runs
SkillsHeavy lifting and careful leveling; poured walls add formwork and rebar
PermitsWalls over about 4 ft (or any wall holding up a structure, driveway, or surcharge) usually need an engineered design and a permit. Every type needs gravel backfill and drainage behind it.
Tools you'll need
- Plate compactor (rent it)
- Shovel and wheelbarrow
- 4-ft level
- Rubber mallet
- Masonry saw or block splitter
- Landscape fabric, drain pipe & drainage gravel
Step by step — segmental block wall
- Dig the base trench Excavate a trench wide enough for the block plus backfill, deep enough to bury the bottom course (and below frost for tall walls).
- Compact a gravel base Add and compact ~6" of gravel, then screed it dead level — every block above follows this course.
- Set the first course perfectly level Lay the base blocks, checking level front-to-back and side-to-side on every one. Slow here saves the whole wall.
- Stack and backfill with gravel Stack courses with the built-in setback, sweep block tops clean, and backfill behind each course with drainage gravel (wrap a perforated drain pipe in fabric at the base).
- Cap it Glue cap blocks on with masonry adhesive for a finished top edge.
Common mistakes
An unlevel first course
Errors multiply up the wall. Get the base course perfect in both directions before stacking.
No drainage gravel or pipe
Soil behind a wall holds water, and water pressure pushes walls over. Backfill with gravel and add a drain pipe.
Building too tall without engineering
Walls over ~4 ft need engineering and often geogrid reinforcement and a permit.
Tools you'll need
- Form plywood/lumber + form ties and bracing
- Rebar (#4) and tie wire
- Circular saw and drill
- Concrete vibrator or a mallet to tap forms
- 4-ft level and string line
- Wheelbarrow or concrete pump
Step by step — poured concrete wall
- Pour a reinforced footing Pour a concrete footing below the frost line, wider than the wall, with vertical rebar dowels left sticking up to tie the wall into.
- Tie the rebar grid Build a rebar mat — commonly #4 (1/2") bars at 12–18" on center both ways — tying verticals to the footing dowels. Keep steel about 2–3" inside the form faces (the cover).
- Build and heavily brace the forms Set plywood/lumber forms to the wall thickness, plumb and tied across with form ties. Wet concrete pushes hard — over-brace and stake the forms so they can't bow or blow out.
- Add weep holes and drainage Place weep holes or a drain pipe at the base and gravel backfill behind the wall so water pressure can escape.
- Pour in lifts and consolidate Pour in 1–2 ft lifts, vibrating or tapping the forms to release air pockets and avoid honeycombing on the face.
- Strip and cure Leave forms on a day or two, strip them, and keep the wall damp while it cures.
Common mistakes
No rebar or undersized/over-spaced bars
A poured wall needs a proper steel grid tied to footing dowels — roughly #4 bar at 12–18" both ways. Without it the wall can crack and fail.
Forms not braced enough
Wet concrete is heavy and pushes outward; under-braced forms bow or blow out mid-pour. Over-brace, tie, and stake them.
No weep holes or drainage
Water behind a solid wall has nowhere to go. Add weep holes/drain pipe and gravel backfill, or hydrostatic pressure builds.
Pouring all at once with no consolidation
Pour in lifts and vibrate; dumping it in traps air and leaves honeycombed, weak spots.
Tools you'll need
- Circular saw
- Drill with a long bit
- Sledgehammer
- Rebar or timber spikes
- 4-ft level
- Landscape fabric & drainage gravel
Step by step — timber wall
- Set a level gravel base Dig a trench and compact a gravel base, screeded level for the first course.
- Pin the first course Lay the bottom timbers level and pin them to the ground with rebar driven through drilled holes.
- Stack with a setback and stagger joints Stack courses with a slight backward batter, staggering the joints, and pin each course to the one below with spikes or rebar.
- Add deadmen (tiebacks) For walls over ~3 ft, run "deadmen" timbers back into the slope every ~4 ft on alternating courses and anchor them — they keep the wall from pulling away.
- Backfill with gravel Backfill behind the wall with gravel and a drain pipe for drainage as you go up.
Common mistakes
No deadmen on a tall wall
Above ~3 ft, timber walls need tieback "deadmen" into the slope or they bulge and lean out.
Not pinning the courses together
Pin each course with rebar or timber spikes so courses can't shift.
Using non-ground-contact lumber
Use timbers rated for ground contact, or they rot out in a few years.
Tools you'll need
- Mason's hammer and chisel
- Heavy gloves
- Pry bar
- 4-ft level
- Landscape fabric & gravel
Step by step — dry-stacked stone wall
- Dig a base trench Excavate and set a compacted gravel footing for the base course.
- Set the biggest stones first Place the largest, flattest stones as the base, tilted slightly back into the slope.
- Batter the wall back Stack so the wall leans into the hill — about 1" of setback per foot of height — and bridge each joint with the stone above ("one over two").
- Chink and level as you go Use smaller stones to shim, level, and lock the face; keep each course stable before adding the next.
- Backfill with gravel Backfill behind the wall with gravel for drainage as you build.
Common mistakes
Building the face vertical
Dry-stack walls must batter back into the slope (~1"/ft) or they topple forward over time.
Aligning the joints
Stagger joints — set each stone to bridge the seam below ("one over two") — for a wall that locks together.
No gravel backfill
Even a stone wall needs gravel drainage behind it to relieve water pressure.
Safety
- Blocks, stone, and timbers are heavy — lift with your legs and use a hand truck.
- Wear eye protection when cutting block, stone, or rebar; cut masonry wet to limit silica dust.
- Wet concrete and mortar are caustic — wear gloves and rinse skin promptly.
- Call 811 before digging, and never undermine an existing structure with your excavation.
Frequently asked questions
How tall can a DIY retaining wall be?
Generally up to about 3–4 ft. Taller walls — or any wall holding up a driveway or structure — need an engineered design, soil reinforcement (geogrid or tiebacks), and usually a permit.
How much material do I need?
Pick your wall material above — concrete block (small, standard, or large), poured concrete, timber, or natural stone — then enter length and height. The calculator switches its math to match: block and cap counts, cubic yards of concrete, 6×6 timbers, or tons of stone, plus base gravel for all of them.
How much rebar does a poured concrete wall need?
A typical short wall uses about #4 (1/2") rebar on a 12–18" grid both directions, tied to vertical dowels in the footing and kept 2–3" inside the form faces. Always follow an engineered design for taller walls or those holding a surcharge.
What size are retaining wall blocks?
They vary by product. Small garden-wall blocks are roughly 10" long × 3" high, a common standard block is about 16" long × 6" high × 10" deep, and large units run 18" long × 8" high or more. The calculator has presets for these plus a Custom option where you enter your block's length, height, and depth.
Do I need drainage behind the wall?
Yes — every type. Gravel backfill plus a perforated drain pipe (and weep holes in a poured wall) relieves the water pressure that otherwise topples retaining walls.
Next steps for this project
Gravel
How much gravel you need for a driveway, path, or base — in tons and cubic yards.
Paver
Pavers (any size), gravel base and sand for a patio or walkway.
Excavation
Cubic yards of dirt to dig and haul, plus truckloads.
Concrete Slab
How much concrete you need for a slab, driveway, or patio — in cubic yards and bags.