Block Mortar Calculator
How many concrete blocks, how much mortar, and how many bags you need for a block wall — from one measurement.
Your wall
Materials estimate
Site-mix breakdown
| Material | Order |
|---|---|
| Portland cement (94-lb bags) | – |
| Hydrated lime (50-lb bags) | – |
| Masonry sand | – |
How it is calculated
- Block count: net wall area ÷ 0.8889 ft² per block (8×16 in nominal face = 1.125 blocks/ft²), minus openings, rounded up, plus waste.
- Mortar coverage: one 80-lb bag of premixed mortar lays about 13 standard 8×8×16 blocks (QUIKRETE Mortar Mix product data sheet No. 1102) — about 7.7 bags per 100 blocks at 3/8-in joints.
- Mortar volume: about 0.044 ft³ of wet mortar per block, from the Concrete Masonry Handbook estimating rule (1 m³ of mortar per roughly 800 blocks).
- Cross-checks: SPEC-MIX coverage (12 blocks per bag) and the trade rule "one 80-lb premix bag covers 10–12 ft² of wall" agree within about 10%.
- Joint scaling: mortar scales linearly with joint thickness from the 3/8-in baseline (face-shell bedding).
- Site mix: ASTM C270 volume proportions with a dry-to-wet consolidation factor of 0.67, calibrated against published handbook quantities within 10%. Equivalences: 94-lb cement ≈ 1 ft³, 50-lb hydrated lime ≈ 1.25 ft³, loose sand ≈ 100 lb/ft³.
Frequently asked questions
How many bags of mortar do I need for 100 concrete blocks?
About 7.7 eighty-pound bags of premixed mortar for 100 standard 8x8x16 blocks laid with 3/8-inch joints (one bag lays about 13 blocks, per the QUIKRETE Mortar Mix product data sheet). With 10% waste added and rounded up, order 9 bags. Using 60-lb bags, that is about 10.3 bags per 100 (order 12 with waste).
How much mortar does one concrete block need?
About 0.044 cubic feet of wet mortar per standard 8x8x16 block at 3/8-inch joints. This follows the Concrete Masonry Handbook estimating rule that 1 cubic meter of mortar lays roughly 800 blocks, and it includes normal waste allowances for face-shell bedding.
How many concrete blocks do I need per square foot of wall?
1.125 blocks per square foot for any block with an 8x16-inch nominal face (4, 6, 8, or 12 inches wide) — that is 112.5 blocks per 100 square feet. Measure the wall, subtract door and window openings, then multiply the net area by 1.125 and add waste for cuts and breakage.
Does block width (4, 6, 8, or 12 inch) change the mortar needed?
Practically no. With standard face-shell bedding, the mortar per block depends on the 8x16-inch face, not the width of the block, so all four widths use the same mortar per block. Block width sets the wall thickness, not the mortar quantity.
How does joint thickness change mortar use?
Mortar scales almost linearly with joint thickness for face-shell bedding: 1/4-inch joints use about one-third less mortar than 3/8-inch joints, while 1/2-inch joints use about one-third more. This calculator scales the 3/8-inch manufacturer coverage baseline by your chosen joint thickness.
What is the difference between Type N and Type S mortar?
Type N (1 part cement : 1 part lime : 6 parts sand, by volume) is the general-purpose mortar for above-grade walls. Type S (1 : 1/2 : 4 1/2) is stronger and used below grade or for structural, high-wind, and seismic walls. Coverage per block is the same either way; the mix choice only changes the site-mix material split.
How much cement, lime, and sand do I need to site-mix mortar for 100 blocks?
For Type N with 10% waste: about 1 ninety-four-pound bag of Portland cement, 1 fifty-pound bag of hydrated lime, and 5.44 cubic feet of sand (about 0.2 cubic yards). For Type S: 2 bags of cement, 1 bag of lime, and the same 5.44 cubic feet of sand. Derived from ASTM C270 volume proportions with a dry-to-wet consolidation factor calibrated against published handbook quantities.
How many blocks does an 80-lb bag of mortar lay?
About 13 standard 8x8x16 concrete blocks, per the QUIKRETE Mortar Mix product data sheet No. 1102. A 60-lb bag lays about 9.75 blocks. Coverage assumes standard 3/8-inch joints.
Should I subtract doors and windows from the wall area?
Yes. Enter the total area of all door, window, and other openings and the calculator subtracts it before counting blocks and mortar. Walls with many openings should also use a higher waste factor because every opening creates cut blocks.
How much waste should I add for mortar and blocks?
10% is a good default for cutting, breakage, and mortar dropped or left hardening in the mixer. Use 15% for walls with many openings, partial courses, or rough-textured block that drinks more mortar.
Is this the same as a concrete calculator?
No. This tool estimates mortar for laying concrete block (CMU) walls — the joints between blocks. For poured concrete volumes such as slabs, footings, sonotubes, and columns, use our concrete calculator instead.
How accurate is this estimate?
The coverage anchors come from a manufacturer product data sheet and the Concrete Masonry Handbook, cross-checked against independent coverage charts within about 10%. Real-world use varies with block texture, weather, and bedding style, so the tool rounds every order quantity up — confirm with your supplier on large jobs.
Free tool by NRTechStudio