Retaining Wall Cap Block Calculator: Caps, Adhesive Tubes and Cost
Count the cap blocks to finish the top of a retaining wall, the tubes of landscape block adhesive from the bead size and cartridge yield, the length of the last cut cap and the cost of caps and adhesive.
How it works
Caps to cover = ⌈wall length ÷ cap coverage length⌉ · Caps to buy = caps to cover + extras · Tubes = ⌈wall length × beads per cap ÷ bead yield per tube⌉ (or ⌈caps ÷ caps per tube⌉ if you enter your own figure) · Last cap = wall length − (caps to cover − 1) × cap length
Cap blocks finish the top course of a segmental retaining wall. Each cap covers a set length of wall, usually 10 to 18 inches, so the wall length divided by that coverage, rounded up, gives the caps to lay. Add extras for cut ends, corners (each 90° corner usually takes two mitre-cut caps) and breakage. For tapered caps laid in alternating directions on a straight wall, use the average of the long and short faces as the coverage length. Caps are not pinned like the courses below, so they are glued down with landscape block adhesive. Manufacturers recommend two continuous beads about 2 inches in from the front and back edges of the cap, and a 3/8 inch bead outdoors. The calculator multiplies the wall length by the number of beads and divides by the cartridge yield from the adhesive data sheet: about 13.6 ft of 3/8 inch bead (30.6 ft of 1/4 inch) from a 10 oz tube, and 38 ft (86 ft) from a 28 oz tube. If you already know how many caps your tube covers, enter that figure instead. The result assumes caps are laid tight without gaps. Check your block maker’s installation guide, which may call for a specific overhang, adhesive or cap pattern.
Worked example
A 20 ft wall with caps that each cover 12 in needs ⌈240 ÷ 12⌉ = 20 caps; add 2 for cuts to buy 22. The 20 caps end exactly at the wall end, so no cap needs cutting. Two 3/8 in beads along 20 ft is 40 ft of bead, and a 10 oz tube gives about 13.6 ft, so 40 ÷ 13.6 = 2.94, or 3 tubes (about 6.8 caps per tube).
Sources: Loctite PL 500 Landscape Block Adhesive technical data sheet (bead yields, cap bead pattern, temperatures) · Titebond PROvantage Landscape Block adhesive data sheet (bead yields, cap application) · Inch Calculator: Retaining wall calculator (caps = columns along the wall)
Frequently asked questions
How many cap blocks do I need?
Divide the total top length of the wall by how much each cap covers and round up, then add a few for corners and cut end pieces. A 30 ft wall with 10 in caps needs 360 ÷ 10 = 36 caps before extras. Curved walls use slightly more because caps are trimmed to fit.
How many caps does one tube of adhesive glue?
It depends on the bead. A 10 oz tube gives about 13.6 ft of 3/8 in bead, which is roughly 7 caps of 12 in with two beads each. With a thinner 1/4 in bead it stretches to about 15 caps, and a 28 oz tube covers about 19 caps with a 3/8 in bead. Buy a spare tube so you are not caught short.
Do I have to glue the caps down?
Yes. Caps are not pinned or interlocked like the courses below, so they are bonded with landscape block or masonry adhesive to stop them shifting, being knocked off or lifted by frost. Run continuous beads and press each cap down firmly.
What do I do at the end of the wall or a corner?
Cut the last cap to length with a masonry saw; the calculator shows the length of that final piece. At a 90° corner, mitre two caps at 45° so they meet neatly, which is why each corner adds about two caps to the order.
Should cap blocks overhang the wall?
Many installers set caps with a small, even overhang on the face so the joint below is covered and water drips clear. Follow your block manufacturer’s guide, since some cap profiles are designed to sit flush.
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This calculator is for educational purposes. Double-check important results before acting on them.