Retaining Wall Blocks Calculator
A retaining wall is priced and ordered by the block, so before you visit the yard you need a reliable count. This calculator works from the wall's face: the flat surface you see once the wall is built. You enter the wall length and height, the exposed face area of a single block, and a waste allowance for cuts and breakage, and it returns the number of blocks rounded up to a whole unit. The method is straightforward area division. Length times height gives the total face area, dividing by the area one block covers gives the base count, and the waste percentage adds a margin for end cuts, corners and any damaged pieces. Curved or stepped walls cut more blocks than a straight run, so the waste figure is left fully editable to match your design. Buying a small surplus is sensible, because blocks come from production batches that can differ slightly in color, and a mid-job shortfall means a second trip and a possible mismatch. Cap blocks that finish the top course are usually ordered separately by length. Every figure here is computed deterministically from your measurements, and the worked example below reconciles exactly to the calculator so you can check each step.
Block count divides the wall face area by one block's face area, then adds waste: blocks = (length x height / face) x (1 + waste). A 20 ft by 3 ft wall with 0.50 sq ft face blocks and 5% waste needs 126 blocks, rounded up.
Retaining wall blocks formula
blocks = ceil( (L x H / f) x (1 + w) )
L = wall length
H = wall height
f = exposed face area of one block (same units)
w = waste allowance as a decimal
Length times height is the total face area. Dividing by a single block's face area gives the base count, the waste fraction adds margin for cuts, and rounding up means you never run short on the last course.
Worked example
A wall 20 feet long and 3 feet high is built from blocks showing 0.50 square feet of face, with a 5 percent waste allowance.
- Face area: 20 x 3 = 60 square feet
- Base count: 60 / 0.50 = 120 blocks
- Add waste: 120 x (1 + 0.05) = 126 blocks
- Round up: ceil(126) = 126 blocks
You should buy 126 blocks. These are the calculator's default inputs, so the result above matches the widget exactly.
Common block face areas
Face area is the visible width times the visible height of one block.
| Face size | Square inches | Square feet |
|---|---|---|
| 12 in x 4 in | 48 | 0.333 |
| 12 in x 6 in | 72 | 0.50 |
| 16 in x 6 in | 96 | 0.667 |
| 18 in x 8 in | 144 | 1.00 |
Measurement and unit guidance: US Food and Drug Administration (FDA).
Retaining wall blocks calculator: frequently asked questions
How do I calculate retaining wall blocks?
Find the wall face area by multiplying length by height, then divide by the exposed face area of a single block. Multiply by one plus a waste allowance and round up. Blocks needed = (length x height) divided by block face area, times one plus waste. A 20 foot long, 3 foot high wall has 60 square feet of face.
What is the exposed face area of a block?
It is the part of the block you see once the wall is built, its visible width times its visible height. A common segmental block shows 12 inches wide by 6 inches high, which is 72 square inches, or 0.50 square feet. Use the face dimensions from the manufacturer, not the full block depth.
How much waste should I allow for a retaining wall?
A straight wall usually needs about 5 percent extra for cuts at the ends and the occasional damaged block. Curved or stepped walls and walls with corners need more, often 10 percent, because more blocks are cut to fit. This calculator defaults to 5 percent and the figure is editable.
Do I need to count cap blocks separately?
Yes. This calculator counts the structural face blocks. Cap or coping blocks that finish the top course are usually sold by linear foot of wall, so estimate them separately as wall length divided by cap length. Buried base courses below grade are part of the face area if you include their height.
Should I round the block count up?
Always round up. Blocks are sold whole, and a shortage stops the job and risks a color difference between production batches. A small surplus also leaves spares for future repairs. This tool rounds the final count up to the next whole block automatically.
Official sources
- Consumer measurement and product guidance: US Food and Drug Administration (FDA). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.