🧱 Retaining Wall Block Calculator

How the Block Math Works

Segmental retaining wall (SRW) blocks stack in courses, so the estimate is simple multiplication:

• Blocks per course = Wall length (in inches) ÷ block face width.
• Number of courses = Wall height (in inches) ÷ block face height. Remember the first course sits partially buried — see below.
• Cap blocks = one per block in the top course (roughly length ÷ cap width).

This calculator also estimates your base trench gravel (6 in deep × 12 in wide under the full wall) and drainage backfill (12 in of clean crushed stone behind the full height of the wall) — the two materials DIYers most often forget to order.

The Buried First Course (Don't Skip This)

Every properly built retaining wall buries its first course about 10% of the wall height, minimum one inch per foot of wall. A 3-ft wall should have its first course roughly 4 inches below grade, sitting on 6 inches of compacted crushed stone.

Drainage Is What Keeps Walls Standing

Retaining walls rarely fail from weak blocks — they fail from water pressure (hydrostatic pressure) building up behind them. Every wall over 2 feet needs:

• 12 inches of clean crushed stone (¾" clean, no fines) directly behind the blocks, full height.
• A perforated drain pipe at the base of the backfill, sloped to daylight at the wall ends.
• Filter fabric between the gravel and the retained soil so the drainage zone doesn't clog with silt.

Our calculator includes the backfill gravel tonnage automatically. Size the drain pipe run with the Linear Footage Calculator.

How Tall Can a DIY Retaining Wall Be?

• Up to 3 feet: DIY-friendly with standard SRW blocks and good base prep.
• 3 to 4 feet: Doable, but check local codes — many jurisdictions require a permit above 3 or 4 feet.
• Over 4 feet: Requires geogrid soil reinforcement and usually an engineer's stamp. Failure at this height can be dangerous and expensive — hire it out.

Also note: if the slope above the wall carries a driveway or structure (a "surcharge"), the engineering threshold drops.

Frequently Asked Questions

How many retaining wall blocks do I need per square foot?

A standard 12" × 4" face block covers 1/3 sq ft, so you need 3 blocks per square foot of wall face. A 16" × 6" block covers 2/3 sq ft (1.5 blocks per sq ft), and an 18" × 8" jumbo block covers 1 sq ft.

How much gravel goes behind a retaining wall?

Plan a 12-inch wide column of clean crushed stone the full height of the wall, plus a 6-inch deep base trench. For a 20-ft long, 3-ft tall wall that's roughly 2.6 cubic yards (about 3.5 tons) including the base.

Do I need concrete footings for a block retaining wall?

No — segmental (dry-stack) retaining wall blocks sit on compacted crushed stone, not concrete. A rigid concrete footing can actually crack with frost movement. Mortared or poured walls are a different system and do need footings.

How much does a retaining wall cost in 2026?

DIY with standard SRW blocks: roughly $15–$25 per square foot of wall face in materials. Professionally installed: $25–$50+ per square foot depending on height, access, and block style.

Can I build a retaining wall on a slope?

Yes — step the base trench so each section of the first course stays level and buried. Never follow the slope with the courses; blocks must always stack dead level.