Retaining Wall Block Calculator
Work out how many modular blocks you need for a straight gravity retaining wall. Enter the wall length and height and pick a block size to get the course count and total blocks.
You need approximately
How this retaining wall calculator works
Courses = wall height ÷ block height, rounded up. Blocks per course = wall length ÷ block length, rounded up. Total blocks = courses × blocks per course, then increased by your waste margin to cover cuts, corners, and breakage.
| Preset | Block length | Block height |
|---|---|---|
| Standard (16" × 6") | 16" | 6" |
| Large (18" × 8") | 18" | 8" |
| Small / garden edging (12" × 4") | 12" | 4" |
Block dimensions vary by manufacturer — check your product's spec sheet and swap in the exact size if it differs from these presets.
Before you build
- Check local rules. Many areas require a permit or engineering review above a certain wall height.
- Plan for drainage. A compacted gravel base and backfill behind the wall, per your block manufacturer's install guide, prevents water pressure from pushing the wall out over time.
- Account for embedment. Taller walls often bury part of the first course below grade — measure the exposed height you want, not just the finished grade difference.
Frequently asked questions
How many blocks do I need for a retaining wall?
Divide the wall length by the block length to get blocks per course, and the wall height by the block height to get the number of courses, then multiply the two — both rounded up, since a partial block still needs a full unit. This calculator does that math and adds a waste margin for cuts and corners.
How tall can a DIY retaining wall be without engineering?
Many jurisdictions allow unengineered gravity block walls up to about 3–4 feet, but requirements vary widely by location and soil type. Always check your local building department before building a wall taller than a couple of feet, and consult an engineer for any wall holding back a slope, driveway, or structure.
Do retaining wall blocks need a gravel base?
Yes. Manufacturer installation guides (Versa-Lok, Allan Block, and similar systems) call for a compacted crushed-stone base under the first course, typically 6 inches deep and extending a few inches wider than the wall on each side, to provide drainage and a level starting point.
What is the difference between standard, large, and small block presets?
These presets reflect common face dimensions across popular modular block lines: roughly 16"×6" for standard retaining wall block, 18"×8" for larger units used on taller walls, and 12"×4" for small garden-edging block. Check your specific product's spec sheet, since exact sizes vary by manufacturer.
Formulas last reviewed on 2026-07-17. Block dimensions are typical planning figures — confirm the exact size for your chosen product. This tool provides planning estimates only, not engineering or structural advice; consult a licensed professional for walls above a couple of feet or any wall retaining a slope, driveway, or structure.