Retaining Wall Block Calculator
A retaining wall order has three parts people forget to price together: the blocks, the cap row, and the gravel that goes under and behind the wall. Enter your wall's length and height and this calculator returns all three — blocks by the piece, gravel by the ton.
How this calculator works
Blocks per course = wall length ÷ block length; courses = (wall height + one buried course) ÷ block height. The calculator adds 5% for cuts and breakage, counts cap blocks off the top course, and estimates the gravel: a compacted base 6" deep and twice the block depth wide, plus a 12" drainage zone behind the wall — both converted to tons at 1.4 tons/cu yd.
Frequently asked questions
How many blocks do I need for a 20 ft retaining wall?
With standard 16×6 blocks and a 24" exposed height: 15 blocks per course × 5 courses (including the buried one) = 75 blocks plus 15 caps, before the 5% cut allowance. At $3–$8 per block, that wall is roughly $270–$700 in block alone.
Do I really need to bury the first course?
Yes — bury at least one full course (or 10% of wall height) below grade on a 6" compacted gravel base. Walls that lean or bulge almost always failed at the base, not the top. It feels like wasted block; it's actually the foundation.
How tall can I build a retaining wall without an engineer?
Most U.S. jurisdictions allow up to 3 or 4 ft (measured from the base) without a permit or engineering. Above that — or with a slope, driveway, or structure above the wall — you need geogrid reinforcement and usually a stamped design. Check your local code before buying block.