Retaining Wall Block Calculator (Caps and Gravel)

Columns = ceil(wall length / block length). Rows = ceil(wall height / block height), with the buried course counted in the height. Caps are one extra row on top, one per column. Gravel is length × height × the thickness of the drainage zone behind the wall.

Inputs

Units

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Results

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Calculation method

A 20 ft × 3 ft wall of 16×8 in blocks is 15 columns × 5 rows = 75 face blocks and 15 caps, plus 60 ft³ of gravel 12 in thick behind it. Five rows of 8 in stand 40 in, 4 in more than asked, because a partial row still takes whole blocks. In metric, a 6 × 1 m wall of 400×200 mm blocks is the same 75 blocks and 15 caps, with 1.8 m³ of gravel 300 mm thick.

Exact formula

Wall in feet, block and gravel thickness in inches. Face blocks fill the wall height, buried course included; caps sit on top and are taken as long as the blocks. No batter, no geogrid, no base pad, no drainage pipe.

Drainage

Gravel volume is the drainage zone behind the wall only. The compacted base under the first course and any gravel inside hollow blocks are extra; size the base with the gravel calculator. Filter fabric and a perforated drain pipe at the base are not in the count either.

Frequently asked questions

Are caps extra blocks?

Yes. They are counted separately, one per column, because they are usually a different product. The count takes caps as long as the blocks; for caps of another length, divide the wall length by the cap length and round up.

Should I add waste?

Add 5–10% on the order for cuts and breakage. The count covers a straight wall with no corners, curves or steps.

Is 3 ft a DIY height?

Many block systems allow unreinforced gravity walls up to about 3–4 ft in good soil; the manufacturer’s table for your block sets the limit. In the US, a wall over 4 ft measured from the bottom of the footing, or one holding back a slope, driveway or other load, usually needs a permit and an engineered design.