Retaining Wall Calculator
Calculate retaining wall blocks course by course, plus the base gravel, drainage stone and cap blocks — with the build-height rules and a full material cost estimate.
Above grade — we add the buried course
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Enter a price per block to estimate cost.
You might also need:Concrete Block Calculator·Gravel Calculator
Retaining wall math
blocks per course = wall length ÷ block width · total = courses × per course
A 20 ft wall standing 3 ft above grade using 12 × 8 inch blocks: one buried course plus four visible ones = 5 courses of 20 blocks = 100 blocks, plus 20 caps. Underneath goes about 0.9 yards of compacted base gravel, and behind it roughly 2.2 yards of clean drainage stone.
The wall build-up, bottom to top
| Layer | Spec | Why |
|---|---|---|
| Compacted subgrade | Level, undisturbed soil | Everything rests on it |
| Base gravel | 6″ of compacted crusher run | Load spreading and leveling |
| Buried first course | 6″ or 10% of height | Resists sliding |
| Drainage stone | 12″ of #57 behind blocks | Relieves water pressure |
| Drain pipe | 4″ perforated, daylighted | Carries water away |
| Geotextile fabric | Between stone and soil | Stops silt clogging the stone |
| Cap blocks | Adhesive-set | Finish and weather protection |
Before you start
- Check the height limit: over 3–4 ft usually means permits and engineering. Terracing two short walls is often easier and cheaper than one tall one.
- Set back each course about 1 inch (most blocks have a built-in lip) so the wall leans into the slope.
- Call 811 first — retaining wall excavation is deep enough to find utilities.
- Base and drainage quantities in detail: gravel calculator and crushed stone calculator. Pouring a footing instead? See the concrete calculator.
Frequently asked questions
How many blocks do I need for a retaining wall?+
Divide the wall face area by the face area of one block. A standard 12 × 8 inch retaining block covers 0.67 sq ft, so a 20 ft long × 3 ft high wall (60 sq ft) needs about 90 blocks plus caps. This calculator works it out by course so you can see the layout.
How deep should the base of a retaining wall be?+
Bury the first course so its top is at or just below grade — typically 6 inches or one-tenth of the wall height, whichever is greater. Below that, put 6 inches of compacted gravel base. The buried course and its base are what stop the wall from sliding forward.
How high can I build a retaining wall without an engineer?+
Most jurisdictions allow DIY segmental walls up to 3–4 feet. Above that you generally need engineering, permits, and often geogrid reinforcement into the slope behind. Walls holding up a driveway or structure need engineering at any height — check your local code before you buy blocks.
Does a retaining wall need drainage?+
Yes, always. Water pressure behind a wall is the number one cause of failure. Put 12 inches of clean drainage stone (#57) behind the blocks, a perforated drain pipe at the base daylighting to the side, and geotextile fabric between the stone and the backfill soil.
How much does a retaining wall cost?+
Materials for a DIY segmental wall run $15–$30 per face square foot (blocks, base, drainage stone, fabric, caps). Professionally installed, expect $35–$75 per face square foot depending on height, access and whether engineering is required.