Concrete Block Calculator

Use this concrete block calculator to find how many blocks a wall needs before you call the supplier. Enter each wall, take off the doors and windows, and it gives you the blocks to order with a waste allowance, the mortar to lay them, and the grout to fill the cores if your plans call for it. Every quantity comes from a published manufacturer or industry figure, and each one is named on this page.

About the concrete block calculator

Most block estimates go wrong in one of three places: the wrong block size, openings left in the area, or no allowance for breakage. The calculator handles all three, and it shows its working so you can check it against your own numbers.

• Several walls at once. Add one row per straight run of wall, up to 20. A garage, a shed or a garden wall with a return all fit in one estimate.
• Doors and windows. Add each opening with its size and how many there are. They come off the wall area before the blocks are counted.
• US and metric blocks. Pick 4, 6, 8, 10 or 12 in. US blocks, a UK 440 x 215 mm block in five thicknesses, or type any block size in millimetres.
• Mortar and core fill. You get mortar bags (or cement and sand), and grout for solid-grouted walls or only the reinforced cells.
• Courses. The wall-by-wall table shows how many courses high each wall is and how many blocks go in each course.

It runs in your browser. Nothing you enter is sent anywhere or saved.

How to use the concrete block calculator

1. Choose the block. In the US, the standard is the 8 in. block, sold as 8x8x16. Pick the thickness shown on your plans. Outside the US, choose the 440 x 215 mm block in the thickness you’re buying, or enter another size.
2. Enter the walls. Measure each straight run end to end and give its height. Use decimal feet for US blocks (8 ft 6 in. is 8.5) or metres for metric blocks.
3. Add the openings. For each door, window or vent, enter the width and height of the opening in the wall, not the frame you’ll buy, and how many there are.
4. Set the extras. Leave the waste allowance at 5% for a simple wall, or raise it. Choose a core fill option if the wall will be grouted.
5. Read the result. The headline is the number of blocks to order. Below it are the net wall area, the mortar, any core fill, and a table for each wall.

Reading the concrete block calculator results

• Blocks to order is the number to give your supplier. It already includes your waste allowance.
• Blocks to lay, in the line under it, is what actually goes into the wall. The difference is your spare stock for cuts and breakage.
• Net wall area is the area left after openings. Check it against your own sum: if it looks too big, an opening is probably missing.
• Mortar shows premixed bags first, then the cement and sand to mix your own, so you can price both.
• Core fill appears only when you pick a fill option. It’s in cubic yards and in 80 lb bags.
• Wall by wall lists each wall’s area, its height in courses, and the blocks in each course.

Open “Show the working” to see every multiplication. Use “Copy estimate” to paste the whole result into a message or an order.

Concrete block sizes

Blocks are sold by nominal size, which includes one mortar joint. The block itself is a little smaller. That’s why the calculator counts each block by the area it covers once the joint is added, called the module.

Every US block in the list shares the same 8 x 16 in. face. A 12 in. block and a 4 in. block cover the same area of wall, so a wall needs the same number of either. Thickness only changes the mortar per block, the core fill and the weight you’re carrying.

The formula behind the concrete block calculator

The count is area divided by the area one block covers:

1. Wall area = length x height, added up for every wall.
2. Net area = wall area minus (opening width x height x number of openings).
3. Blocks per unit area = 1 / (module length x module height). For US blocks that’s 144 / (16 x 8) = 1.125 per square foot.
4. Blocks to lay = net area x blocks per unit area, rounded up.
5. Blocks to order = the unrounded figure x (1 + waste %), rounded up.

CEMEX, one of the largest block producers in the US, uses the same 1.125 multiplier in its own calculator. Rounding up happens once, at the end, so a fraction of a block is never lost or counted twice.

Concrete block calculator example: a 20 ft garden wall

A wall 20 ft long and 8 ft high, built from 8 in. blocks, with 5% waste.

1. Area: 20 x 8 = 160 sq ft.
2. Blocks: 160 x 1.125 = 180 blocks to lay.
3. With 5% waste: 180 x 1.05 = 189 blocks to order.
4. Courses: 8 ft is 96 in., and 96 / 8 = 12 courses of 15 blocks each.
5. Mortar: 189 / 13 = 14.5, so 15 bags of 80 lb premixed mortar. Mixing your own: 189 / 46 = 4.1, so 5 bags of masonry cement and 1,200 lb of sand.
6. Core fill, if every cell is grouted: 1.6 x 1.12 x 1.05 = 1.88 cubic yards, or about 85 bags of 80 lb grout.

Example with a door and two windows

Now make the wall 40 ft long and 8 ft high, with a 3 x 7 ft door and two 4 x 4 ft windows.

Leaving the openings in would have added 60 blocks you don’t need. On the other hand, the blocks around an opening get cut, and a lintel or bond beam goes over it. That’s where the waste allowance earns its keep. If the wall has several openings, use 10%.

How much mortar for concrete blocks

US hollow blocks are usually laid with mortar on the face shells only, the two long outer edges. The concrete block calculator uses two Quikrete data sheets for this, both stated for standard 8 in. blocks:

Treat these as starting points. The CEMEX calculator allows more, 30 bags of masonry cement per 1,000 blocks, or about 33 blocks a bag. Thicker blocks, fuller bed joints and a less experienced hand all use more. If you’re laying your first wall, buy one or two extra bags.

Which type? Quikrete Mason Mix is a Type S mortar, and its masonry cement makes Type N. Type N is the general-purpose mortar for above-grade walls. Type S is stronger and is often specified for foundations, retaining walls and walls below grade. Your plans or local code decide.

Metric blocks. Solid UK blocks are laid on a full bed of mortar. The calculator works out the volume of the joints: the bed under each block plus the joint at its end, 10 mm thick and as deep as the block. Tarmac’s Blue Circle data sheet says one cubic metre of mortar lays 1,500 blocks of 100 mm, 2,000 of 75 mm and 1,000 of 150 mm. The joint geometry gives 1,504, 2,005 and 1,003, so it’s used for every thickness. Bags are counted at 0.0135 m³ per 20 kg bag of ready-to-use mortar.

Concrete block fill: grouting the cores

Reinforced block walls have steel bars in some cells, and those cells are filled with grout. Some walls, such as retaining walls and basement walls, may be grouted solid. This makes the concrete block calculator a block fill calculator too. It uses the grout chart published by Westbrook Concrete Block for standard two-cell blocks:

Cubic yards of grout per 100 square feet of wall.

The calculator applies the rate to your net wall area and adds your waste allowance. It converts the result to 80 lb bags at 0.60 cubic feet a bag, the same sheet’s figure. For larger walls, ask your supplier about ready-mixed grout delivered by truck instead of bags. The 4 in. block has no core fill option: the grout chart doesn’t cover it, and 4 in. blocks are mostly used for partitions and veneers.

Grout isn’t concrete. It’s mixed wetter, with a slump of 8 to 11 inches on Westbrook’s sheet, so it flows around the steel and fills the cell. NCMA’s guidance on grouting (TEK 3-2A) describes pouring in lifts; without cleanout openings, walls are commonly grouted in stages up to 5 ft high.

Example with two walls in metres

A UK garden wall uses 100 mm blocks, 10 m long and 2.4 m high, with 5% waste.

1. Area: 10 x 2.4 = 24 m².
2. Blocks: 24 x 9.88 = 237.0, so 238 blocks to lay.
3. With 5% waste: 249 blocks to order.
4. Mortar: 249 blocks x 0.000665 m³ of joint each = 0.166 m³, or 13 bags of 20 kg ready-to-use mortar.
5. Courses: 2.4 m / 0.225 m = 10.7, so the wall finishes on its 11th course, with 22.2 blocks in each.

That last line matters. A wall that isn’t a whole number of courses high gets a cut course or one course more than you measured. The calculator counts the height you enter and tells you when it isn’t a whole number of courses, so you can decide which it will be.

How much waste to allow

• 5%: a straight wall with few cuts. CEMEX suggests this figure, and it’s where the concrete block calculator starts.
• 10%: corners, piers, several openings, or blocks that get moved around site a lot.
• 15% or more: curved walls, lots of cutting, or a first project.

Half blocks and corner blocks are sold separately in the US. Ask your supplier what they stock, since a wall that ends cleanly at both ends may need half blocks in every other course.

Planning the wall in courses

Blocks go down in rows called courses. With US blocks, each course is 8 in. tall and each block covers 16 in. of length, so three courses make 2 ft and three blocks make 4 ft. Design to those numbers and you’ll cut fewer blocks:

• Wall heights in multiples of 8 in.: 4 ft is 6 courses, 6 ft is 9, 8 ft is 12.
• Wall lengths in multiples of 8 in., so each course ends on a whole or half block.
• Door and window openings sized and placed to the same grid.

The wall-by-wall table in the concrete block calculator shows the courses and blocks per course for each wall, so you can see straight away if a dimension is off the grid.

What the calculator doesn’t do

This concrete block calculator estimates materials. It doesn’t design the wall. The footing size, the steel, bond beams, wall height limits and whether you need a permit come from your plans, an engineer and your local building code. Retaining walls in particular need engineering once they get tall.

It also doesn’t price the job, because block and mortar prices differ a lot by region. For a quote, use the quantities here with your supplier’s prices in our job estimate calculator. If a receipt shows a total with tax included, the reverse sales tax calculator splits out the tax. To plan the build, the business days calculator counts working days between delivery and completion.

Tips before you order

1. Measure twice, per run. Measure each straight run separately and enter it as its own wall.
2. Check the block on the plans. An 8 in. and a 12 in. block need the same count, but the mortar and grout differ a lot.
3. Ask how blocks are sold. Many suppliers sell by the pallet. Ask how many blocks a pallet holds before you round your order.
4. Keep the working. Copy the estimate and keep it with your order, so you can check the delivery.

Working on paving or a driveway as well? Our bitumen calculator works out the asphalt, aggregate and binder for a road or driveway surface, the same way this concrete block calculator works out a wall.

Sources

• CEMEX USA block calculator: 1.125 blocks per square foot, 5% waste allowance, and 30 bags of masonry cement per 1,000 blocks.
• Quikrete data sheets: Mason Mix (13 blocks per 80 lb bag, Type S) and Masonry Cement Type N (46 blocks per 70 lb bag with 240 lb of sand).
• Westbrook Concrete Block, Fine Core Fill Grout sheet: grout per 100 sq ft by wall thickness and steel spacing, and 0.60 cu ft per 80 lb bag.
• Tarmac, Blue Circle Quality Assured Mortar technical information: blocks laid per cubic metre and 0.0135 m³ per 20 kg bag.
• NCMA TEK 3-2A, Grouting Concrete Masonry Walls.