Foundation Block Count Calculator
Count the concrete blocks and mortar bags for a foundation or basement wall from its length, height and courses.
How it works
blocks = wall area × 1.125 × (1 + waste) · courses = height×12 ÷ 8 · blocks per course = length×12 ÷ 16 · mortar ≈ 2.5 bags per 100 blocks
A standard concrete masonry unit is 8×8×16 inches, and with its 3/8-inch mortar joints one block covers about 0.89 sq ft of wall face — so it takes 1.125 blocks per square foot. Multiply the wall face area (length × height) by that and add a few percent for breakage and cuts. Height in 8-inch courses and length in 16-inch blocks give the coursing, and mortar runs about 2.5 to 3 bags per hundred blocks.
Worked example
A 120 ft perimeter, 8 ft tall: face area = 960 sq ft × 1.125 = 1,080 blocks, +5% ≈ 1,134. That is 12 courses (96 ÷ 8) of 90 blocks each (1,440 ÷ 16), needing roughly 28 bags of mortar.
Frequently asked questions
Why 1.125 blocks per square foot?
An 8×16-inch block face is 128 sq in, and 144 ÷ 128 = 1.125, so it takes 1.125 blocks to cover a square foot. The mortar joint is already baked into the nominal 8×16 dimension, so you do not subtract for it.
Do I subtract for openings?
For a rough order you can skip small openings and let the waste percentage absorb them, but deduct large ones — a garage or walkout — at roughly 1.125 blocks per square foot of opening. This tool figures the solid wall; subtract big openings yourself.
How much mortar will I need?
Plan on about 2.5 to 3 bags of mortar mix per 100 blocks, plus sand if you mix your own. Cold weather, wide joints and a slow first-time mason all push consumption toward the high end, so round up.
Related calculators
Estimates only — add for waste and confirm every load-bearing decision against local building codes and a design professional.