Concrete Block Calculator
Calculate how many concrete blocks you need for a wall, plus mortar bags and total weight. Includes block fill calculation for hollow-block grouting with rebar reinforcement.
Results
Estimates only. Verify measurements on site, account for openings (doors, windows) by subtracting their area, and order with a 5–10% buffer.
How to use this calculator
- 01Enter your wall length and height in feet. The wall area drives the block count, so measure the actual wall footprint and subtract any large openings (doors, windows) afterwards.
- 02Pick a block size. 8×8×16 is the structural standard. 4×8×16 is a solid partition block. 6×8×16 is lighter; 12×8×16 is heavy retaining-wall stock. All four sizes cover the same face area, so block count stays the same — only weight and fill volume change.
- 03Set your mortar joint and waste factor. The default 3/8″ joint is the industry standard. 5% waste covers normal breakage and corner cuts; bump it higher for unusual layouts or lots of openings.
- 04Turn on Block Fill if you are grouting cores for rebar reinforcement. The calculator returns the cubic yards of concrete and 80-lb bag count needed to fill every block. Solid 4×8×16 blocks cannot be filled — the calculator flags this automatically.
- 05Read the results. Blocks Needed already includes waste and rounds up. Mortar Bags assumes 70-lb bags and the standard 3-bags-per-100-blocks rate.
Understanding the math
Every block calculation comes down to face area. A standard 8×16 block, plus its share of a 3/8″ mortar joint, occupies an 8″ × 16″ module on the wall — that is 128 in² or 0.889 ft². So one square foot of wall takes 1 ÷ 0.889 = 1.125 blocks:
blocks = wall area × 1.125 × (1 + waste %) · mortar bags = blocks × 3 / 100
All four block sizes share the same 8×16 face, so they cover the same wall area per block. The differences are weight (26–50 lbs each) and core volume. Hollow blocks have voids running vertically through them — about 0.6 ft³ per 8-inch block, 0.45 ft³ per 6-inch block, and 0.85 ft³ per 12-inch block. Filling those cores with concrete (typically around vertical rebar) is what the Include Block Fill toggle calculates.
Worked example: a 20 × 8 ft wall in 8×8×16 with 5% waste. Wall area = 20 × 8 = 160 ft². Blocks = 160 × 1.125 × 1.05 = 189. Mortar = 189 × 3 ÷ 100 = 5.67, rounded up to 6 bags. Weight = 189 × 38 = 7,182 lbs. If you turn on fill: 189 × 0.6 ft³ = 113.4 ft³ = 4.2 cubic yards, or 189 80-lb bags.
Concrete block reference chart
Face area, count per 100 sq ft, weight, and fill volume for the four standard CMU sizes. Use this to sanity-check the calculator output or to estimate at a glance.
| Block Size | Face Area | Blocks per 100 sq ft | Weight | Fill Volume |
|---|---|---|---|---|
| 8×8×16 (std) | 0.89 ft² | 113 | 38 lbs | 0.60 ft³ |
| 4×8×16 | 0.89 ft² | 113 | 26 lbs | solid (0) |
| 6×8×16 | 0.89 ft² | 113 | 30 lbs | 0.45 ft³ |
| 12×8×16 | 0.89 ft² | 113 | 50 lbs | 0.85 ft³ |
Frequently asked questions
How many concrete blocks do I need for a wall?
Multiply your wall length by height to get square footage, then multiply by 1.125 (the blocks-per-square-foot factor for standard 8×8×16 blocks with 3/8″ mortar joints). For a 20-foot × 8-foot wall: 160 sq ft × 1.125 = 180 blocks. Add 5% for waste from breakage and cuts at corners. Different block sizes use different factors — the calculator above adjusts automatically when you select a block size.
How many concrete blocks are in 100 square feet?
About 113 blocks for standard 8×8×16 (the most common size used for foundations and structural walls). The math: each block face covers ~0.89 sq ft including its mortar joint allowance. So 100 ÷ 0.89 = ~113 blocks. Smaller wall thicknesses (4-inch, 6-inch blocks) cover the same face area, so the count is the same — only the block weight and wall thickness change.
How much mortar do I need for concrete blocks?
The standard rule is 3 bags of type-N or type-S mortar per 100 blocks for a typical 8×8×16 block wall with 3/8″ joints. For 200 blocks, plan on 6 bags. This assumes 70-lb bags and standard joint thickness — wider joints or rougher block faces consume more. Always order one extra bag as insurance against partial bags drying out.
How much concrete do I need to fill concrete blocks?
Each standard 8×8×16 hollow block has about 0.6 cubic feet of void volume in its cores. So 100 blocks need 60 cubic feet (2.22 cubic yards) of concrete fill. The calculator above includes a fill toggle that does this automatically — turn it on if you're filling cells with grout or concrete for reinforcement around rebar. 6-inch blocks have less void (~0.45 ft³ each); 12-inch blocks have more (~0.85 ft³).
What's the standard concrete block size?
The most common is 8×8×16 — the "8-inch block" used for residential foundations, retaining walls, and structural walls. Other common sizes: 4×8×16 (partition walls, non-structural), 6×8×16 (lighter walls), and 12×8×16 (load-bearing or heavy retaining walls). All four sizes have the same 8×16 face area, so they cover the same wall area per block — they differ in thickness and weight.
Do I need rebar in concrete block walls?
For most structural walls and retaining walls, yes. Building codes typically require vertical rebar every 32-48 inches with horizontal rebar every 16-24 inches, with the cells filled with grout or concrete. Decorative or partition walls may not need reinforcement. Check local codes for your specific application — requirements vary by jurisdiction, soil conditions, and wall height.
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Estimates only. Verify wall measurements on site, subtract openings (doors, windows) from your wall area, and check local building codes for required reinforcement and fill specifications. TakeoffCalc is not responsible for material over- or under-orders.