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Count it by the course, like the crew does.
Retaining walls are ordered by the course, not by the square foot: visible height divided by block face height gives the course count, wall length divided by block face length gives blocks per course, and one buried base course is non-negotiable. This calculator runs the full takeoff — blocks, caps, compacted base gravel, and the 12-inch drainage column behind the wall that keeps water pressure from tipping it over.
Landscapers and hardscape crews use it to order once: courses × blocks-per-course is the supplier's language, and the buried course and drainage column are the two line items homeowners always leave off the DIY count. Homeowners comparing bids use it to see whether a quote actually includes base prep and drainage — the parts that decide whether the wall is still standing in ten years.
With 12×6 blocks: 3 ft visible is 6 courses, plus 1 buried course = 7. Each course takes 240 ÷ 12 = 20 blocks, so 7 × 20 = 140, and with the 10% allowance, 154 blocks. Add roughly 0.56 yd³ of base gravel (18 in wide × 6 in deep) and 2.6 yd³ of drainage stone behind the full 3.5 ft stack.
The buried base course is the wall's anchor — it resists the sliding and overturning force of the soil behind it. Rule of thumb is at least 6 inches of embedment, or 1 inch per foot of wall height, whichever is greater. A wall that starts at grade will kick out at the base, usually within a few wet seasons.
Most jurisdictions require a permit and a stamped engineered design for walls over 4 feet measured from the bottom of the footing, and any wall over 3 feet should have geogrid reinforcement extending into the soil every other course. Walls holding a driveway, slope, or structure (a surcharge load) need engineering at any height.