Retaining Wall Calculator

A retaining wall calculator works out how many wall blocks and cap units you need, plus the base gravel to set them on, from the wall's length and height and your block size. It's built for stackable segmental blocks — the standard DIY retaining system.

Your measurements

Result

Block walls are estimated by face area: the wall's length × height divided by one block's face. This also counts your courses and the cap row, and sizes the compacted gravel base the wall sits on.

How it is calculated

Blocks = wall face area (length × height) ÷ block face area (length × height). Courses = height ÷ block height. Caps = length ÷ cap length. Base gravel = length × ~1.5 ft trench width × base depth, in cubic yards. Tall or load-bearing walls need engineered design, geogrid reinforcement and drainage.

Steps

Enter the wall length and finished height.
Enter your block's face length and height.
Set the base gravel depth (6 in is typical).
Read the block count, courses and cap units.
Order the base gravel and a drainage layer behind the wall.

Worked example

A 20 ft long, 3 ft high wall in 12 × 6 in blocks needs about 120 blocks, 6 courses, 20 caps and ~0.6 yd³ of base gravel.

Common questions

How many blocks for a retaining wall?

Wall face area ÷ one block's face area. A 20 ft × 3 ft wall in 12×6 in blocks is about 120 blocks.

Do I need a gravel base?

Yes — a compacted gravel base (often 6 in) under the first course is what keeps the wall level and draining. The calculator sizes it.

How tall can a DIY retaining wall be?

Many areas allow un-engineered segmental walls only up to ~3-4 ft; taller walls, or walls holding a slope or driveway, need an engineer and permits.

What goes behind the wall?

Drainage gravel and landscape fabric, often with a perforated drain pipe, so water doesn't build up and push the wall over.