Concrete Block Calculator
Concrete blocks, often called cinder blocks or CMU blocks, are estimated here by wall size, block size, mortar, fill, and waste. Includes block fill calculation for hollow-block grouting around reinforcement.
Results
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 isn't responsible for material over- or under-orders.
How to use this calculator
- 01Enter your wall length and height in feet (or meters in metric mode). 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’re grouting cores for rebar reinforcement. The calculator returns the cubic yards (or cubic meters) of concrete and 80-lb (36-kg) bag count needed to fill every block. Solid 4×8×16 blocks can’t be filled. The calculator flags this automatically.
- 05Read the results. Blocks Needed already includes waste and rounds up. Mortar Bags assumes 70-lb (32-kg) bags and the standard 3-bags-per-100-blocks rate. For adjustable brick or block mortar coverage, use the Mortar Calculator.
Understanding the math
Every block calculation comes down to face area. A standard 8×16 block (200×400 mm), plus its share of a 3/8″ (10 mm) mortar joint, occupies an 8″ × 16″ module on the wall. That’s 128 in² or 0.889 ft² (0.083 m²). So one square foot of wall takes 1 ÷ 0.889 = 1.125 blocks (or about 12.1 blocks per square meter):
blocks = wall area × 1.125 × (1 + waste %) · mortar bags = blocks × 3 / 100 metric: blocks = wall area (m²) × 12.1 × (1 + waste %)
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, 12-23 kg each) and core volume. Hollow blocks have voids running vertically through them: about 0.6 ft³ (0.017 m³) per 8-inch block, 0.45 ft³ (0.013 m³) per 6-inch block, and 0.85 ft³ (0.024 m³) 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 (6 m × 2.4 m) in 8×8×16 with 5% waste. Wall area = 20 × 8 = 160 ft² (14.9 m²). Blocks = 160 × 1.125 × 1.05 = 189. Mortar = 189 × 3 ÷ 100 = 5.67, rounded up to 6 bags. Weight = 189 × 38 = 7,182 lbs (3,260 kg). With fill on: 189 × 0.6 ft³ = 113.4 ft³ (3.21 m³) = 4.2 cubic yards (3.21 m³), or 189 80-lb (36-kg) bags.
Concrete block reference chart
Face area, count per 100 sq ft, weight, and fill volume for the four standard concrete block, cinder block, or 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 to calculate concrete blocks needed?
Multiply your wall length by height to get square footage, subtract any openings, then multiply by 1.125 for standard 8×8×16 concrete blocks, often called cinder blocks or CMU blocks. For a 20-foot × 8-foot wall (6 m × 2.4 m): 160 sq ft (14.9 m²) × 1.125 = 180 blocks before waste. Add waste for breakage and cuts. Different block sizes use different weights and fill volumes, and the calculator adjusts when you select a block size.
How many cinder blocks do I need?
In modern estimating, cinder block usually means a concrete block or CMU. Measure the wall area, subtract openings, choose the block size, and add waste. A standard 8×8×16 block layout uses about 1.125 blocks per sq ft before waste.
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 (0.083 m²) including its mortar joint allowance. So 100 sq ft (9.3 m²) ÷ 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″ (10 mm) joints. For 200 blocks, plan on 6 bags. This assumes 70-lb (~25 kg) bags and standard joint thickness. Wider joints or rougher block faces use more. 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 (0.017 m³) of void volume in its cores, so one cubic yard fills roughly 45 standard 8-inch blocks before waste. The calculator includes a fill toggle for hollow concrete, cinder, or CMU blocks. Turn it on if you're filling cells with grout or concrete around reinforcement. 6-inch blocks have less void (~0.45 ft³ / 0.013 m³ each), and 12-inch blocks have more (~0.85 ft³ / 0.024 m³).
What's the standard concrete block size?
The most common is 8×8×16 (≈200×200×400 mm), 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?
Structural, retaining, foundation, and tall concrete block walls often need reinforcement, but the exact rebar size, spacing, horizontal reinforcement, and grout or fill requirements depend on local code, wall height, loads, soil conditions, and engineering. Decorative, short, or non-structural partition walls may have different requirements. Check local code or a qualified professional before building.
Can I use this calculator with metric measurements?
Yes. Switch the unit selector to Metric and the inputs, results, reference values, and block labels update to metric units where applicable.
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