Whether you're building a sleek raised garden bed, looking to ditch traditional retaining walls for a modern landscape alternative, or taking on a full-scale hardscaping project, concrete blocks are one of the most durable and cost-effective materials you can use. Calculating the exact number of concrete blocks for a project comes down to one simple formula: total wall surface area divided by the face area of a single block, plus a standard 10% safety buffer for cuts and waste. First, calculate the wall's square footage by multiplying its length by its height in feet. Be sure your height measurement includes any below-ground rows, since many retaining walls require at least one buried layer to properly anchor the structure. If your wall includes doors or windows, calculate their area (width multiplied by height) and subtract that from the total wall area.