Concrete Block Chart: Coverage, Waste & Mortar Bags

What does 124 blocks mean?

It means the wall holds 112.50 blocks, and you order 124 of them: the spares cover cuts, breakage and the half-blocks at corners and openings, and the 11 bags of 80-lb mortar lay them at 12.00 blocks per bag. The wall itself is 100 sq ft of face — length times height, minus doors and windows — and each standard block covers 0.89 sq ft of that face once its mortar joint is counted, which is why the rate is 1.13 blocks per sq ft rather than one block per 0.89 sq ft worked the other way. Order the with-waste quantity, not the base count: the base count is what the finished wall holds, and the spares are what the work eats.

Concrete block coverage chart — blocks to order with 10% waste (80-lb mortar bags)

Block counts are identical across widths because every standard width carries the same 8x16 in face; only the mortar bags change, because joint volume scales with width. Base blocks before waste are 56.25 for 50 sq ft, 112.50 for 100 sq ft, 225.00 for 200 sq ft and 450.00 for 400 sq ft.

Three things to check before you trust the number

The joint is already inside the rate. A block labelled 8x8x16 actually measures 7-5/8 x 7-5/8 x 15-5/8 in, and the missing 3/8 in on each side is the mortar joint. Add the joint back and each block occupies an 8x16 in module — 128 sq in, or 0.89 sq ft — and 144 ÷ 128 is 1.13. The common mistake is adding a second joint on top of the nominal size, which double-counts the mortar and over-orders. If your blocks are a different face size, the rate changes: an 8x8 half-block covers 0.44 sq ft and needs 2.25 per sq ft, and an 8x12 block covers 0.67 sq ft and needs 1.50 per sq ft.

Width changes the mortar, never the count. A 4-in block and a 12-in block cover the same wall face, so both need 112.50 base blocks for 100 sq ft — but the 12-in wall needs 16 bags of mortar against 6 for the 4-in wall, because each joint holds three times the mortar. The table above shows this directly: every row orders the same blocks, and the bags climb from 3 to 8 across widths at 50 sq ft, and from 21 to 62 at 400 sq ft. Ordering by width without adjusting the mortar is how a job runs short of mortar with blocks to spare.

Subtract the openings before you multiply. A standard 3 ft x 7 ft door removes 21 sq ft of wall — about 24 blocks — and a 3 ft x 4 ft window removes 12 sq ft, about 14 blocks. The calculator takes net wall area, so measure length times height and then subtract every opening; feeding it the gross rectangle over-orders by the openings' share, which on a wall with two doors and three windows can be a full pallet. Corners, lintels and bond beams need special blocks on top of the count, not instead of it.

Worked example

On the defaults: 100 sq ft wall, standard 8x8x16 blocks, 10% waste.

1. The rate: 144 sq in per sq ft ÷ 128 sq in per block face is 1.13 blocks per sq ft.
2. The base blocks: 100 × 1.125 is 112.50 blocks.
3. The order quantity: 112.50 × 1.10 is 123.75, rounded up to 124 blocks.
4. The mortar bags: 124 ÷ 12 is 10.33, rounded up to 11 bags of 80-lb mortar.
5. The check: 124 blocks at 1.125 per sq ft cover 110.22 sq ft — the wall plus its waste allowance, not the wall alone.

And the other way: the wall area for exactly 200 base blocks is 177.78 sq ft, the waste percentage for a 150-block order from 100 sq ft is about 33.33% waste, and the largest whole-foot area that orders exactly 200 blocks with 10% waste is 161 sq ft.

Every branch, checked

• 50 sq ft wall instead of 100: 50 sq ft wall instead of 100: 50 sq ft wall, 8-in blocks, 10% waste: 56.25 base blocks, 62 blocks to order, 6 bags of 80-lb mortar (12.00 blocks per bag).
• 200 sq ft wall instead of 100: 200 sq ft wall instead of 100: 200 sq ft wall, 8-in blocks, 10% waste: 225.00 base blocks, 248 blocks to order, 21 bags of 80-lb mortar (12.00 blocks per bag).
• 0% waste instead of 10%: 0% waste instead of 10%: 100 sq ft wall, 8-in blocks, 0% waste: 112.50 base blocks, 113 blocks to order, 10 bags of 80-lb mortar (12.00 blocks per bag).
• 15% waste instead of 10%: 15% waste instead of 10%: 100 sq ft wall, 8-in blocks, 15% waste: 112.50 base blocks, 130 blocks to order, 11 bags of 80-lb mortar (12.00 blocks per bag).
• 4-in blocks instead of 8-in: 4-in blocks instead of 8-in: 100 sq ft wall, 4-in blocks, 10% waste: 112.50 base blocks, 124 blocks to order, 6 bags of 80-lb mortar (24.00 blocks per bag).
• 12-in blocks instead of 8-in: 12-in blocks instead of 8-in: 100 sq ft wall, 12-in blocks, 10% waste: 112.50 base blocks, 124 blocks to order, 16 bags of 80-lb mortar (8.00 blocks per bag).
• 250 sq ft wall instead of 100: 250 sq ft wall instead of 100: 250 sq ft wall, 8-in blocks, 10% waste: 281.25 base blocks, 310 blocks to order, 26 bags of 80-lb mortar (12.00 blocks per bag).
• 20% waste instead of 10%: 20% waste instead of 10%: 100 sq ft wall, 8-in blocks, 20% waste: 112.50 base blocks, 135 blocks to order, 12 bags of 80-lb mortar (12.00 blocks per bag).

The reverses, worked backwards

• Wall area for exactly 200 base blocks: 177.78 sq ft — at 177.78 sq ft the figures are 177.78 sq ft wall, 8-in blocks, 10% waste: 200.00 base blocks, 221 blocks to order, 19 bags of 80-lb mortar (12.00 blocks per bag).; at the lower neighbor the figures are 176.78 sq ft wall, 8-in blocks, 10% waste: 198.88 base blocks, 219 blocks to order, 19 bags of 80-lb mortar (12.00 blocks per bag).; at the upper neighbor the figures are 178.78 sq ft wall, 8-in blocks, 10% waste: 201.13 base blocks, 222 blocks to order, 19 bags of 80-lb mortar (12.00 blocks per bag). — verdict: STEP.
• Waste percentage for a 150-block order from 100 sq ft: 33.33% waste — at 33.33% waste the figures are 100 sq ft wall, 8-in blocks, 33.33% waste: 112.50 base blocks, 150 blocks to order, 13 bags of 80-lb mortar (12.00 blocks per bag).; at the lower neighbor the figures are 100 sq ft wall, 8-in blocks, 32.33% waste: 112.50 base blocks, 149 blocks to order, 13 bags of 80-lb mortar (12.00 blocks per bag).; at the upper neighbor the figures are 100 sq ft wall, 8-in blocks, 34.33% waste: 112.50 base blocks, 152 blocks to order, 13 bags of 80-lb mortar (12.00 blocks per bag). — verdict: STEP.
• Largest whole-foot area that orders exactly 200 blocks with 10% waste: 161 sq ft — at 161 sq ft the figures are 161 sq ft wall, 8-in blocks, 10% waste: 181.13 base blocks, 200 blocks to order, 17 bags of 80-lb mortar (12.00 blocks per bag).; at the lower neighbor the figures are 160 sq ft wall, 8-in blocks, 10% waste: 180.00 base blocks, 198 blocks to order, 17 bags of 80-lb mortar (12.00 blocks per bag).; at the upper neighbor the figures are 162 sq ft wall, 8-in blocks, 10% waste: 182.25 base blocks, 201 blocks to order, 17 bags of 80-lb mortar (12.00 blocks per bag). — verdict: STEP.

What the defaults assume

The defaults are a 100 sq ft net wall area, standard 8x8x16 blocks and 10% waste. Net means after openings are subtracted; gross rectangles over-order by the openings' share. The rate assumes the standard 3/8-in mortar joint and the running-bond pattern, where each course is offset by half a block — the rate is a face-area rate, so it does not change with bond pattern, but half-blocks at the ends still come out of the waste allowance. Mortar bags assume premixed 80-lb bags at 12 blocks per bag for 8-in blocks, scaled with width in proportion to joint volume; site-mixed mortar and different bag weights change the yield, so check the bag's printed coverage. The order quantity is rounded up to a whole block and the bags to a whole bag, because neither can be bought in fractions. The area reverse is checked at 1 sq ft either side, because a square foot moves the base count by 1.125 blocks; the waste reverse is checked at 1% either side; the whole-foot order reverse is checked at 1 sq ft either side, and the exact boundary sits just past 161.61 sq ft, so 161 sq ft is the largest whole-foot area that still orders 200. The model does not count footings, grout fill, rebar, lintels, bond beams, caps or waterproofing — all of which sit alongside the block count in a real wall — and it does not judge whether a wall needs engineering, which taller and retaining walls usually do.

These notes repeat in every explainer section because this page is built to be read start to finish: the joint is already inside the rate; width changes the mortar, never the count; and subtract the openings before you multiply.

Output · formula · meaning

Who uses this

• A DIY builder pricing a garden wall — 100 sq ft at 10% waste is 124 blocks and 11 bags: the shopping list, before the first course is laid.
• A contractor ordering for a job — 400 sq ft at 10% waste is 495 blocks, with 42 bags for an 8-in wall and 62 for a 12-in wall: the order split by width, not one guess for both.
• A homeowner checking a quote — 200 sq ft should be 248 blocks with 10% waste: a quote for 300 blocks is over-ordering by a fifth, and the table shows it.
• A student learning masonry math — one worked example from face area to rate to base count to order quantity to mortar bags, and the branches that show how area, width and waste each move the answer.

Accuracy & limitations

Figures are the model's own arithmetic on the numbers you type, computed from unrounded values: base blocks are wall area times 1.125, the order quantity is base blocks times one plus the waste percentage rounded up to a whole block, blocks per bag are 12 times 8 divided by the block width, and mortar bags are the order quantity divided by blocks per bag rounded up to a whole bag. The displayed figures are rounded to 2dp for rates and counts that are not whole. The main limitations are the assumptions the page states: the standard 3/8-in joint and running bond, premixed 80-lb bags at the stated yield, and net wall area after openings. Real yields vary with the mortar brand, joint profile and bedding technique, so check the bag's printed coverage; half-blocks, corners, lintels and bond beams need special units on top of the count; and structural walls, retaining walls and walls over 4 ft tall usually need engineering review and reinforcement that this page does not count. This page is a calculation tool, not a structural design.

Cite this calculator

On a 100 sq ft wall of standard 8x8x16 blocks at 1.125 blocks per sq ft, the wall needs 112.50 blocks before waste, 124 blocks to order with 10% waste, and 11 bags of 80-lb mortar at 12 blocks per bag. Source: Bright Side Kit, brightoolbox.com.

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