How to Calculate the Cost of a Block Room Addition

Measure and determine the size of the proposed concrete-block addition. You may also look at some other rooms in your home to get an idea of the size that you want to add. Also, look at some books and magazines to get a better idea of room proportions; an out-of-proportion addition will look odd once it is attached.

Add the lengths of all of the walls together. For example, a 10-foot-by-12-foot room equals 44 linear feet of block walls. If there are any additional walls such as bump outs, add the lengths of those walls as well.

Calculate the amount of block in one course. Concrete blocks are 16 inches long, including the mud joint, so you will divide 44 feet by 1.33 (16 inches equals 1.33 feet). When you divide 44 feet by 1.33 feet (one block), you get 33.082. Because you can't purchase a part of a block, you will round up to 34 blocks.

Determine the amount of blocks in the entire wall, from bottom to top. Divide, for example, an 8-foot wall by 8 inches--.66 is the height of one block--in order to get the amount of block in the height of the wall. You will need 12.2 block high for a wall that is eight feet, so round that number to 13 block for the height of the wall. Now, to determine the amount of block in the entire wall, just multiply the amount of block long by the amount of block high: 13 x 44= 442 block.

Determine the amount of sand and mortar mix necessary for laying the block addition. Allow 3 bags of mortar per 100 blocks: 13 bags of mortar and 1 ton of sand per 10 bags of mortar. The cost of the block addition will depend greatly on your location. You can phone your local supplier and ask for concrete block, mortar and sand prices. Just multiply the cost of one unit by the amount of units in the job. For example, if you determine that one 8-inch concrete block costs $1.25, then multiply the total number of block by 1.25; the same goes for the sand and mortar. To get labor prices, just call some local masons and ask for their price-per-block labor cost, then multiply that number by the total number of blocks.