Concrete Calculator

United States · block count · not poured concrete

Concrete Block Calculator

Count how many concrete blocks cover a wall face before you buy them. This is a purchase count for standard masonry units, also called concrete masonry units or CMU. It is not the poured-concrete wall on the home page, and it does not estimate hollow-core grout or fill.

How many concrete blocks do I need for a wall?

Divide the wall face, after subtracting openings, by one block's nominal face. A usual US concrete masonry unit is 16 inches long by 8 inches high, and that size already includes a typical 3/8 inch mortar joint. Depth, such as 6, 8, or 12 inches, does not change this count. This page does not design the wall.

How do I use this concrete block calculator?

Measure the wall face, subtract each opening once, and divide by the nominal block face.

  1. Type the wall length and height in feet and inches. Inches can be 6, 3.5, 1/2, or 3 1/2.
  2. Add each door or window once, with how many of that size. Do not list an overlapping opening twice. This page cannot see where openings sit.
  3. Leave the face module at 16 in long by 8 in high unless your block’s nominal module is different. That size already includes the usual mortar joint. Do not add the joint again. Block depth is not part of this count.
  4. Leave the spare allowance at 5%, or type 0 to 20. It is added once. Then choose Calculate.

Wall and blocks

Wall size

Wall length

Wall height

Length can be up to 1,000 feet. Height can be up to 100 feet. These limits keep the count finite. They are not a recommended wall size.

Door and window openings

Optional. Add one row for each opening size. If you have three doors of the same size, enter that size once and type 3 for the quantity. Do not count an opening twice, and do not include area that overlaps another opening. This page only subtracts areas. It does not check positions.

Nominal block face

Type the nominal module, which already includes the usual 3/8 inch mortar joint. The standard US face is 16 inches long and 8 inches high. Do not type the smaller actual block, such as 15 5/8 by 7 5/8, and do not add the joint again. Depth, such as 6, 8, or 12 inches, does not change this face count.

Nominal module length, including the joint

Nominal module height, including the joint

Custom sizes on this page must also be joint-inclusive nominal modules, from 2 to 48 inches. Split-face, screen, and retaining-wall blocks often do not use a standard module. Do not assume they do.

Spare allowance

Starts at 5%. Use 0 to 20, the same range as the home page allowance. It is added once for cuts and spare blocks, then the count is rounded up once to a whole block. It is not a measured waste amount for a particular job.

Results

Areas and the whole-block count appear here after you calculate.

Blocks to order

This is the net area divided by the nominal module, plus the spare allowance once, then rounded up once.

Whole blocks to order
Not calculated

Areas

Gross wall area
Not calculated
Opening area
Not calculated
Net wall area
Not calculated
Area of one nominal module
Not calculated

Block count

Theoretical blocks, before allowance
Not calculated
Spare allowance
Not calculated
Blocks after allowance, before rounding up
Not calculated
Whole blocks to order
Not calculated

What this count means

The block count appears here after you calculate.

How is the block count measured?

The count uses the face of the wall, not the block depth. Gross area minus openings, divided by one nominal face, is the theoretical block count.

Opening Height Wall length Nominal face, joint included 16 in 8 in high Not wall depth
Gross area is the full wall. Subtract each opening’s area once. Divide the net area by one nominal face, usually 16 in long by 8 in high. That small face already includes the usual mortar joint. It is not the block’s depth, and it is not a laid-out wall.

A usual concrete masonry unit is named by its nominal size. The Concrete Masonry & Hardscapes Association describes a typical nominal face of 8 inches high by 16 inches long. The actual block is commonly about 3/8 inch smaller in each direction, so the block plus a 3/8 inch mortar joint fills that nominal module. This calculator divides by the nominal module. It does not add another joint on top of 16 by 8.

A common name for that unit is 8 by 8 by 16. In that name the first 8 is the nominal depth, the second 8 is the nominal height, and 16 is the nominal length. This count uses only height and length. A 6 inch, 8 inch, or 12 inch deep unit with the same face uses the same number of blocks. Half units, corners, and lintels are not counted on their own.

What does a 20 by 8 foot wall come to?

A 20 foot by 8 foot wall with no openings, a 16 by 8 inch module, and 5% spare comes to 189 blocks.

20 ft by 8 ft wall, no openings, 16 in by 8 in module, 5% spare

  1. Gross area is 20 × 8 = 160 square feet.
  2. One nominal module is 16/12 × 8/12 = 8/9 square feet.
  3. Theoretical blocks, before allowance, are 160 ÷ (8/9) = 180.
  4. A 5% spare allowance is added once: 180 × 1.05 = 189. That number is already whole, so the order is 189 blocks.

The same wall with one 3 ft by 7 ft opening

  1. The opening area is 3 × 7 = 21 square feet.
  2. Net area is 160 − 21 = 139 square feet.
  3. Theoretical blocks are 139 ÷ (8/9) = 156.375.
  4. After 5% once: 156.375 × 1.05 = 164.19375. Rounded up once, the order is 165 blocks.

The button fills that wall, one opening, the 16 by 8 module, and 5%.

How do the block-count formulas work?

Net wall area divided by one nominal face, times one plus your spare percent, then rounded up once, is the order count.

  • Gross area = wall length × wall height.
  • Opening area = the sum of width × height × quantity for each opening size you list.
  • Net area = gross area − opening area. If the openings cover the whole wall, the net area is zero and the count is zero.
  • Module area = nominal length × nominal height. For 16 in by 8 in, that is 8/9 square feet.
  • Theoretical blocks = net area ÷ module area. This can be a fraction.
  • Blocks after allowance = theoretical blocks × (1 + spare percent ÷ 100). The percent is added once.
  • Whole blocks = that result rounded up once. An exact whole number is not increased.

Feet and inches are turned into feet before multiplying. The screen shows areas to a few decimal places. The count uses the fuller value until the final round-up.

What do people ask about concrete blocks?

Is this the same as the concrete wall calculator?

No. The home page multiplies length, height, and thickness to estimate poured concrete volume. This page counts blocks on a wall face. Use the page that matches what you are buying.

Does this include grout or fill inside the blocks?

No. Hollow cores, grout, and mortar quantities are not estimated. The result is a block count only.

Why is the default 16 inches by 8 inches?

That is the usual nominal face module for US concrete masonry units, as described by the Concrete Masonry & Hardscapes Association. The actual block is typically about 3/8 inch smaller so the mortar joint can make up the module. Type 16 by 8, not 15 5/8 by 7 5/8.

Where does block depth go?

Nowhere in this count. A 6 inch, 8 inch, or 12 inch block can share the same 16 by 8 face. Depth matters for the wall thickness and for strength, which this page does not design.

Will this work for retaining-wall or decorative blocks?

Not unless that product uses a joint-inclusive face module you know. Screen block, split-face specialties, and retaining-wall systems often use different sizes. This page does not assume they match a standard module.

Does the count lay out courses, corners, and lintels?

No. It is an area estimate. It does not place blocks course by course, and it does not count bond, cuts at corners, half units, lintels, or other special units. The spare allowance can cover some of that only if you choose a percent that is enough. It is not a layout.

What if two openings overlap?

Subtract the overlap only once. This page cannot tell that two rectangles cover the same part of the wall. If you enter both full areas, the count will be short.

Where do the 16 by 8 inch module numbers come from?

They come from concrete masonry layout notes, not from a guess on this page. The usual nominal face is 8 inches high by 16 inches long.

Those notes also say other face sizes exist and that specialty units should be checked with the producer. This page does not copy a full design method from those publications.

What does this block calculator refuse to decide?

It counts face modules only. Wall strength, grout, and a course-by-course layout stay with you or a qualified professional.

  • The result is a block count from areas. It is not a structural design, a strength check, or a building-code check.
  • It does not estimate mortar, grout, or concrete fill.
  • It does not choose a wall height, a block depth, or a block strength.
  • The nominal module already includes the usual joint. Entering the actual block size and also expecting a joint to be added will not match this formula.
  • Openings are areas only. Positions, overlap, and fit inside a bond pattern are not checked.
  • Corners, cuts, bond, lintels, and special units are outside the count.
  • Wall length is limited to 1,000 feet and wall height to 100 feet. Each module side must be from 2 to 48 inches. The spare allowance is 0 to 20. Opening quantities are whole numbers from 1 to 100, with up to 20 sizes. Values that cannot produce a finite count are rejected.
  • A nominal order of blocks is not a promise that a supplier stocks that unit or that the wall can be built as drawn.