Mortar for Block Wall Calculator
Quick answer: A 20-ft × 4-ft wall (80 sq ft) needs about 90 standard 8×8×16 blocks — 99 with a 10% waste allowance — and 5 bags of 80-lb mortar.
Laying concrete block? Enter your wall dimensions and a waste allowance to get the block count plus the number of 80-lb mortar bags you'll need. Results update as you type.
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How to use this calculator
Measure your wall's length and height in feet and enter a waste allowance — 10% is a good starting point for a simple straight wall; use more for corners, curves, or a lot of cutting.
The calculator assumes standard 8×8×16 concrete blocks laid with 3/8-in. mortar joints. Adjust the waste allowance for complex layouts.
How it works
Blocks = wall area × 1.125 (standard masonry estimating: 112.5 standard 8×8×16 blocks per 100 sq ft with 3/8-in. joints). Waste is applied to the block count. Mortar bags = ceil(blocks ÷ 20): one 80-lb bag lays approximately 20 standard blocks at 3/8-in. joints.
Worked example
20 × 4 = 80 sq ft → 80 × 1.125 = 90 blocks → 99 with 10% waste → ceil(99/20) = 5 bags of 80-lb mortar.
Reference data
| Blocks per 100 sq ft (8×8×16, 3/8-in. joints) | 112.5 |
| Blocks per 80-lb mortar bag | ≈20 |
| 80-lb bag yield | ≈0.75 cu ft |
Source: Quikrete 80-lb mortar product data (2026) and standard masonry estimating practice. See the Quikrete Mason Mix specification (bag sizes, yield, and coverage).
Method & sources
The calculator converts wall dimensions to area, applies the masonry standard of 112.5 blocks per 100 sq ft, adds your waste allowance, and divides by the per-bag block coverage (rounded up so you never come up short). Bag counts are planning estimates — verify against your supplier before ordering.
Frequently asked questions
Should I use Type N or Type S mortar?
Type N suits most above-grade walls; Type S is stronger for below-grade or structural work. The bag math is the same either way.
How much mortar does one block need?
About one-twentieth of an 80-lb bag per standard block at 3/8-in. joints.
Is waste included?
Yes — the default 10% waste allowance covers breakage and cuts; adjust it for complex layouts.
What if my joints are wider than 3/8 inch?
Wider joints use more mortar — treat the bag count as a minimum and add extra.