Use coverage for the exact product and unit
QUIKRETE Mason Mix #1136 says an 80-lb bag can lay up to 13 standard 8×8×16 blocks. That is the stated coverage for that product and assumption. Joint practice, unit shape and job losses can change the amount used. Read the current product data before purchasing.
For 100 blocks actually laid, 100 ÷ 13 = approximately 7.69 bags. Round up to eight bags before adding any separately selected allowance. Do not substitute this factor for cement-only products that require sand or other ingredients.
Separate spare blocks from installed work
Suppose a layout needs 200 blocks and you purchase another 10 as spares. Mortar coverage should begin with the 200 blocks you expect to lay. Buying 210 blocks does not automatically make the finished wall larger.
At a confirmed 13 blocks per bag, 200 ÷ 13 = approximately 15.38 bags. If you select a 10% mortar allowance, apply it before rounding: 200 × 1.10 ÷ 13 = approximately 16.92, or 17 bags. At an illustrative $10 per bag, that is $170 in mortar material before tax and delivery.
The mortar calculator uses an editable coverage basis and a visible allowance so you can reproduce the assumptions.
Check the wall details the shortcut cannot see
A coverage estimate is a buying guide, not a complete masonry takeoff. Special shapes, face-shell bedding, joint dimensions and workmanship affect use. CMHA's estimating guidance explains why assumptions about masonry construction matter.
Use the block calculator after checking the unit module. Deduct real openings and list special units separately. Estimate any specified core fill in the fill calculator; mortar used for joints and material filling cores are separate quantities.
Ask the supplier or mason to review the resulting shopping list when the wall differs from the product's published coverage conditions.
Sources & further reading
Examples illustrate arithmetic. Use project specifications and current local supplier quotes for a purchase.