CMU Block & Mortar Calculator
How many concrete blocks and bags of mortar a wall needs — from its length and height, the block size, and any openings, with a waste allowance.
Feet or metric
Feet or metric
Doors/windows to deduct
1891.125/sf
Blocks
670–80 lb bags
Mortar
Standard · 12 courses · 5% waste
Assumptions & notes
- 1.125 block/sf × 160.0 sf net = 180 block; 189 with a 5% waste allowance.
- ≈ 12 courses at 8" nominal.
- Mortar ≈ 6 bags (rule of thumb: 3 bags of 70–80 lb mortar/mason mix per 100 block with 3/8" joints — varies with joint size, unit, and waste).
How block takeoff works
Concrete block is counted by wall face area. Because nominal block dimensions already include the 3/8" mortar joint, a standard unit covers a tidy 16" × 8" face — exactly 1.125 blocks per square foot. Multiply the net wall area (gross minus openings) by that factor, add a few percent for breakage and cuts, and round up.
Mortar is estimated from the block count — roughly 3 bags of 70–80 lb mortar mix per 100 block at standard joints. It's a rule of thumb, not a spec: thicker joints, rougher units, and more cutting all push it up, so buy a little extra.
This is a materials estimate, not a structural or mix design. Grout for filled cells, reinforcement, bond beams, and control joints come from your engineer and the adopted code. For the concrete in footings and bond beams, use the concrete takeoff; to size a footing under the wall, the footing calculator.
Frequently asked questions
How many blocks do I need for a wall?
For standard 8×8×16 units (a 16" × 8" face, which includes the 3/8" mortar joint) it's 1.125 blocks per square foot of wall. So a 20' × 8' wall = 160 sq ft × 1.125 = 180 blocks, before a waste allowance. Half-high (4") units are 2.25 per square foot.
How much mortar do I need per block?
A common rule of thumb is about 3 bags of 70–80 lb mortar/mason mix per 100 block laid with standard 3/8" joints — so ~180 blocks needs ~6 bags. It varies with joint size, the unit, and waste, so treat it as an estimate and buy a little extra.
Does this include grout and rebar?
No — it covers blocks and mortar. Grout for filled cells, vertical/horizontal reinforcement, bond beams, and control joints depend on your structural design and aren't included. Add them per your engineer and the adopted code.
How do I account for doors and windows?
Enter the total opening area (in square feet) and it's deducted from the gross wall area before counting blocks. For a quick estimate many masons don't deduct small openings, since cut blocks create waste — the waste allowance helps cover that.
Related tools
- Concrete TakeoffCubic yards, cubic metres, and bag counts for slabs, footings, walls, columns, and round piers — with a waste allowance.
- Footing / Pier Size by Soil BearingSize a spread footing or pier from the load and allowable soil bearing — required area, buildable size, and actual pressure.
- Drywall & Paint TakeoffEstimate drywall sheets and paint gallons for a wall or ceiling area.