JB PARCELRetaining Wall Block Calculator
Enter the wall length, the height you want above grade, and the block size. Get blocks per course, the total with the buried course and waste, cap units, and base gravel, and a flag when the wall is tall enough to need a permit and an engineer.
How this calculator works
Blocks per course is the wall length divided by the face length of one block, rounded up. A 40 foot wall is 480 inches, so 16 inch blocks take 30 per course. Courses are the exposed height divided by the block height, rounded up, plus the courses that sit below grade.
Segmental retaining walls are set partly in the ground. The buried course keeps the base of the wall from kicking out, and block manufacturers state how much to bury in their installation guides, often at least one course and more for taller walls. Count those blocks, because you pay for them even though nobody sees them.
Caps run the length of the wall once, so cap units are the wall length divided by the cap length. Add a few percent to blocks and caps for breakage and the cuts at corners and ends.
The base is a trench of compacted gravel under the first course. Its volume is wall length times trench width times depth, divided by 27 for cubic yards. Behind the wall goes drainage stone and a drain pipe, which a tall wall needs far more of than this base figure.
Worked example: a 60 foot wall along a sloped side yard
60 feet long, 30 inches exposed, 16 by 6 inch blocks, one buried course, 5 percent waste, and a 6 by 24 inch gravel base.
| Blocks per course | 45 |
|---|---|
| Courses | 6 |
| Wall blocks | 284 |
| Cap units | 48 |
| Base gravel | 2.2 cubic yards |
Measured from the bottom of the base to the top of the block, that wall is about 42 inches, under the 4 foot line. Raise the exposed height to 42 inches and it becomes 8 courses and 54 inches overall, which in most places means a permit and an engineer before a single block goes in.
Where the numbers come from
Block, cap, and base quantities are geometry. The 4 foot permit threshold is from the permit exemptions in the International Building Code, section 105.2, and the International Residential Code, section R105.2. Jurisdictions adopt and amend these, and the block manufacturer's installation guide governs buried depth, base, and drainage.
Questions investors ask
How many retaining wall blocks do I need?
Divide the wall length in inches by the block face length to get blocks per course. Divide the exposed height by the block height, round up, and add the buried course. Multiply the two and add about 5 percent for breakage and cuts.
How much of a retaining wall should be buried?
Follow the installation guide for the block you buy. Segmental block guides commonly call for at least one course below grade, and more as the wall gets taller or sits on a slope.
Does a retaining wall need a permit?
Under the permit exemptions in the International Building Code and International Residential Code, a wall 4 feet or less measured from the bottom of the footing to the top of the wall is exempt, unless it supports a surcharge such as a slope, driveway, or building. Many jurisdictions follow that line and some set their own, so ask the building department.
What goes behind a retaining wall?
Free-draining crushed stone directly behind the blocks and a perforated drain pipe at the base that lets water out. Water pressure behind a wall with no drainage is the most common reason block walls lean and fail.
How much base gravel does a retaining wall need?
Enough for a compacted trench under the first course, commonly around 6 inches deep and about twice as wide as the block is deep. Taller walls and soft soils need more, and the installation guide gives the figure for the block you are using.
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