How many retaining wall blocks do I need?

Divide the wall face area by the face area of one block. A 30-foot wall 2 feet tall using 12×4-inch blocks needs 6 courses of 30 blocks — about 189 blocks after adding 5% extra.

How deep should the base be for a retaining wall?

Dig a trench about 18 inches wide with 6 inches of compacted crusher run, and bury the first course of blocks roughly 1 inch deep for every 8 inches of wall height. A level, compacted base is the single biggest factor in whether a wall stays straight.

Do I need gravel behind a retaining wall?

Yes. Backfill at least 12 inches behind the wall with 3/4-inch clean gravel, and add a perforated drain pipe at the base for walls over 2 feet. Water pressure, not soil weight, is what pushes most residential walls over.

How tall can I build a retaining wall without an engineer?

Most manufacturers rate standard gravity blocks to 3–4 feet unreinforced, and many municipalities require a permit and engineering above 3 or 4 feet. Taller walls need geogrid reinforcement. Always check local code before building.

How much does a retaining wall block weigh?

Standard 12×4-inch blocks weigh 20–30 pounds; larger 16×6 and 18×6 blocks run 50–80 pounds. A 200-block wall can total 3–6 tons, so plan delivery rather than hauling by car.

Retaining Wall Calculator – Blocks, Base & Backfill

Use this retaining wall calculator to find how many blocks, cap blocks, base gravel, and drainage backfill your wall needs. Enter the wall length and height, pick your block size, and you get the full materials list with courses and rows worked out, plus 5% extra blocks for cuts.

Wall length (ft)

Wall height (ft)

Block size (face W × H)

How to calculate wall blocks by hand

courses = wall height ÷ block height\nblocks per course = wall length ÷ block face width\ntotal blocks = courses × blocks per course × 1.05

Worked example: a 30 ft wall, 2 ft tall, with standard 12" × 4" blocks. Courses: 24" ÷ 4" = 6. Per course: 360" ÷ 12" = 30 blocks. Total: 6 × 30 = 180, plus 5% → 189 blocks, and 30–32 cap blocks for the top course.

What goes into a retaining wall

Component	Spec	Rule of thumb
Base trench	18" wide, 6" crusher run	Compact in 2" lifts, dead level
Buried first course	1" per 8" of wall height	A 32" wall buries ~4"
Drainage backfill	12" of 3/4" clean gravel	Behind the full wall height
Drain pipe	4" perforated, at base	Walls over 2 ft
Setback (batter)	Lean into slope	Built into most block lips
Geogrid + engineering	Above 3–4 ft	Check local permit rules

The two failure points of DIY walls are a sloppy base and no drainage. Water pressure behind the wall — not the soil itself — is what tips most residential walls, which is why the gravel backfill and drain pipe matter as much as the blocks.

FAQ

How many blocks do I need?

Wall face area ÷ block face area, +5%. A 30 ft × 2 ft wall with 12×4" blocks ≈ 189 blocks.

How deep should the base be?

6" of compacted crusher run in an 18"-wide trench, plus bury 1" of block per 8" of wall height.

Do I need gravel behind the wall?

Yes — 12" of clean 3/4" gravel, with a perforated drain pipe for walls over 2 ft.

How tall without an engineer?

Usually 3–4 ft max for gravity walls; taller needs geogrid and often a permit.

How heavy are the blocks?

Standard blocks 20–30 lbs; large formats 50–80 lbs. A 200-block wall is 3–6 tons — get it delivered.

Need base or backfill quantities on their own? Use the gravel calculator, or browse all landscape material calculators.