Block Calculator

By Uzair Arshad , Senior Civil and Structural Engineer

Last updated: April 28, 2026

A standard 8×8×16 CMU covers about 0.889 sq ft, so a 160 sq ft wall needs roughly 189 blocks with 5% waste. Use this free block calculator to estimate concrete blocks, cinder blocks, or CMU blocks for any wall. Enter dimensions to get a complete material list with mortar bags, sand, core fill, reinforcement, and waste allowance.

Block size

Wall length(ft)

Wall length (inches)(in)

Additional inches beyond whole feet.

Wall height(ft)

Wall height (inches)(in)

Additional inches beyond whole feet.

Block face height(in)

Measure the block itself, not including the mortar gap.

Block face length(in)

Measure the block itself, not including the mortar gap.

Mortar joint thickness(in)

Standard is 3/8 in (0.375). Range: 1/4 to 1/2 in.

Waste factor(%)

5% for straight walls, 10% for corners and cuts.

Openings deduction(sq ft)

Total area of doors, windows, and vents to subtract.

Core fill

Horizontal reinforcement

Price per block($/block)

Optional. Enter your local block price for a cost estimate.

Price per mortar bag($/bag)

Optional. 80 lb Type S premixed mortar bag.

Price per ton of sand($/ton)

Optional. Mason sand for mortar mixing.

Price per cubic yard of fill($/cu yd)

Optional. Grout or lightweight fill material.

Price per linear foot of rebar($/lin ft)

Optional. Horizontal joint reinforcement.

How to use this calculator

This block calculator estimates concrete blocks, mortar, sand, core fill, and reinforcement for a straight wall. Enter your dimensions and the calculator returns a complete material list.

Enter the wall length and height in feet and inches. For a typical basement wall at 20 ft long and 8 ft tall, that is 160 sq ft of gross wall area.
Choose a block size. Most residential and light commercial walls use the standard 8×8×16 CMU. Pick 6 inch for partitions or 12 inch for foundation and retaining walls. Select Custom if your blocks have non-standard face dimensions.
Subtract doors, windows, or vents by entering the total opening area in square feet. A standard 3 ft × 7 ft door is 21 sq ft. Forgetting to subtract openings on a garage wall can add 30+ unnecessary blocks to your order.
Set the waste percentage. Use 5% for simple straight walls with few cuts. Bump to 10% for corners, pilasters, and first-time DIY projects where breakage is more likely.
Choose core fill and reinforcement options if your wall design requires them. Grout fill is typical for structural or load-bearing walls. Lightweight fill adds insulation value.
Enter local prices in the optional cost fields to see a material cost breakdown. Keep block count, mortar, sand, fill, and reinforcement as separate line items for supplier quotes.

Common CMU block sizes and face coverage

Use this table to verify your block size selection. Face area includes a standard 3/8 in mortar joint.

Block size (nominal)	Face area with joint	Blocks per 100 sq ft	Typical use
8×8×16 (standard)	0.889 sq ft	~113 blocks	Most walls, foundations, fences
6×8×16	0.889 sq ft	~113 blocks	Interior partitions, non-load-bearing
12×8×16	0.889 sq ft	~113 blocks	Heavy foundations, retaining walls
8×4×16 (half-high)	0.499 sq ft	~201 blocks	Decorative, garden walls

How the calculation works

Wall area:
Gross Area (sq ft) = Wall Length (ft) × Wall Height (ft)
Net Area (sq ft) = Gross Area - Openings (sq ft)

Block face area:
Face Area (sq ft) = ((Actual Length + Joint) / 12) × ((Actual Height + Joint) / 12)
Standard 8×8×16: (15.625 + 0.375) / 12 × (7.625 + 0.375) / 12 = 0.889 sq ft

Block count:
Base Blocks = Net Area / Face Area (rounded up)
Waste Blocks = Base Blocks × Waste % / 100 (rounded up)
Total Blocks = Base Blocks + Waste Blocks

Supporting materials:
Mortar Bags = Total Blocks × 7 / 100 (rounded up)
Sand (tons) = Total Blocks × 0.5 / 100

Wall Length: Total horizontal length of the wall in feet and inches
Wall Height: Total vertical height of the wall in feet and inches
Actual Length: Actual horizontal face of the block in inches (15-5/8 in for standard 8×8×16 CMU)
Actual Height: Actual vertical face of the block in inches (7-5/8 in for standard 8×8×16 CMU)
Joint: Mortar joint thickness in inches (standard is 3/8 in)
Waste %: Extra percentage added for cuts, breakage, and damaged blocks
Openings: Total area of doors, windows, and vents to subtract from the wall

The block calculator uses wall area and block face area to estimate CMU quantity, then adds supporting materials based on industry ratios. You get blocks, mortar, sand, core fill, and reinforcement from one form.

Main formula

Net Wall Area (sq ft) = Wall Length (ft) × Wall Height (ft) - Openings (sq ft)

Block Face Area (sq ft) = ((Actual Block Length + Mortar Joint) / 12) × ((Actual Block Height + Mortar Joint) / 12)

A standard 8×8×16 CMU measures 7-5/8 × 15-5/8 inches. With a 3/8 in joint, each block covers (15.625 + 0.375) / 12 × (7.625 + 0.375) / 12 = 0.889 sq ft on the wall.

Base Blocks = Net Wall Area / Block Face Area (rounded up)

Total Blocks = Base Blocks + (Base Blocks × Waste % / 100, rounded up)

Example calculation

A 20 ft × 8 ft wall using standard 8×8×16 blocks with 3/8 in mortar joints and 5% waste:

Gross wall area = 20 × 8 = 160 sq ft
Block face area = (15.625 + 0.375) / 12 × (7.625 + 0.375) / 12 = 1.333 × 0.667 = 0.889 sq ft
Base blocks = 160 / 0.889 = 180 (rounded up)
Waste blocks = 180 × 5 / 100 = 9 (rounded up)
Total blocks to buy = 180 + 9 = 189 blocks
Mortar = 189 × 7 / 100 = 14 bags (rounded up)
Sand = 189 × 0.5 / 100 = 0.95 tons

Supporting material ratios

Material	Planning ratio	Notes
Mortar (Type S, 80 lb premixed)	7 bags per 100 blocks	Standard 3/8 in bed and head joints
Sand	0.5 tons per 100 blocks	Mason sand for mortar mix
Core fill (8 in block)	~0.0046 cu yd per block	Full grout fill, all cells
Core fill (6 in block)	~0.0030 cu yd per block	Full grout fill, all cells
Core fill (12 in block)	~0.0069 cu yd per block	Full grout fill, all cells

Assumptions and limitations

This block calculator assumes straight walls with uniform height. Stepped foundations, curved walls, and pilaster columns need separate takeoffs. Mortar and sand ratios assume standard 3/8 inch bed and head joints with Type S premixed mortar.

Core fill volumes assume all cells are filled. Partial grouting (filling only reinforced cells) uses less material. Always verify structural requirements with a licensed engineer before building load-bearing or retaining walls.

The 5% default waste factor covers clean straight walls. Increase to 10% for projects with corners, bond beam cuts, or if you are laying block for the first time. I ordered at 5% on a garage wall with two window openings and still returned 6 blocks, so the default is usually safe for simple layouts.

Standard CMU blocks (ASTM C90) are available at most big-box stores for $2 to $4 each in 2026. Quikrete and Sakrete both sell 80 lb Type S mortar bags in the $10 to $14 range. Get a supplier quote with your exact quantities before ordering, since bulk pricing drops significantly for pallet orders of 90+ blocks.

Frequently Asked Questions

How do you calculate concrete blocks needed for a wall?

Divide the net wall area by the face area of one block, then add waste. For a 20 ft × 8 ft wall, the gross area is 160 sq ft. A standard 8×8×16 CMU covers about 0.889 sq ft per block, so you need about 180 blocks before adding a 5% waste buffer.

How many blocks do I need for a wall?

Enter your wall length, height, block size, and waste percentage in the block calculator above. For a 10 ft × 6 ft wall (60 sq ft), a standard 8×8×16 CMU estimate is about 68 blocks before waste. Adding 5% brings the order to about 72 blocks.

How many cinder blocks do I need?

Use the same formula as concrete blocks. Cinder block is a common name for standard CMUs. Measure your wall area, subtract any openings, divide by the block face area (about 0.889 sq ft for 8×8×16), and round up. A 5% waste factor adds 5 extra blocks per 100.

How do you calculate block fill?

Block fill depends on wall area, block thickness, and whether cells are fully grouted. An 8 inch CMU has about 0.0046 cubic yards of void per block. Select grout fill or lightweight fill in the calculator to see the total cubic yards for your wall.

How do you calculate block wall quantity?

Multiply wall length × wall height, subtract doors and windows, then divide by the face area of one block. For a standard 8×8×16 CMU with 3/8 inch joints, each block covers about 0.889 sq ft. Round up the count and add 5% to 10% for cuts and breakage.