How to calculate blocks for a retaining wall
Retaining wall blocks are counted, not measured by volume, so the math comes down to how many blocks fit across the wall's length and how many rows it takes to reach the finished height:
Blocks per row = Wall length (in) ÷ Block face width (in), rounded up
Rows = Wall height (in) ÷ Block face height (in), rounded up
For example, a 20 ft long, 2 ft tall wall built with common 12×4 in blocks (12 in face width, 4 in face height) works out to:
Per row: 20 ft × 12 = 240 in ÷ 12 in = 20 blocks
Rows: 2 ft × 12 = 24 in ÷ 4 in = 6 rows, + 1 buried course = 7 rows
20 blocks × 7 rows = 140 wall blocks + 20 cap blocks = 160 blocks total
This calculator runs that same math for your dimensions and block size, and lets you toggle the buried course and cap row on or off depending on how you're building.
Why you bury the first course
The bottom row of a retaining wall does the most work — it's what the rest of the wall pushes down and out against. Setting that first course below grade, into compacted, level ground, keeps it from sliding or tipping forward and protects it from erosion and frost heave undermining the base over time.
A common rule of thumb is to bury roughly 10% of the wall's exposed height, or one full block, whichever is greater — a 2 ft wall might only need a few inches buried, while a 4 ft wall should have close to 5 inches or more below grade. Block manufacturers publish their own recommended burial depth for each product line, so treat this as a starting point and check the spec sheet for the blocks you're actually buying.
The gravel base and drainage backfill
Retaining walls sit on a compacted gravel base, not bare soil. The base gives the first course a level, well-drained surface to rest on and helps the wall settle evenly instead of shifting as the ground below it moves. A typical base is about 6 inches deep and 12 inches wide, compacted in thin layers and leveled before the first block goes down.
Separately, most walls also need a layer of clean, angular drainage gravel backfilled directly behind the blocks — usually at least 12 inches deep, running the height of the wall. This lets water drain down and out through weep holes or gaps at the base instead of pooling behind the wall, where it would add pressure the wall isn't designed to hold back. This calculator estimates the base layer only; the backfill volume depends on wall height and how far back you extend the gravel, so size it separately once you know your wall's dimensions.
Height limits: when you need a permit or an engineer
Most residential retaining wall projects fall into one of two categories: a simple gravity wall you can build yourself, or a taller, engineered wall that needs reinforcement and a permit. The dividing line is usually around 3 to 4 feetof exposed height, but the exact threshold — and whether it's measured from the footing or from grade — varies by city and county, and can be lower near property lines, slopes, or structures.
Most segmental block systems also have a maximum height they can reach without added reinforcement (like geogrid tied back into the soil), regardless of local permit rules. Check both your local building department and the block manufacturer's engineering guidance before committing to a wall taller than a couple of feet.
Estimating by wall area instead
If you'd rather work from square footage than a row-by-row count, you can estimate blocks by dividing the wall's visible area by a single block's face area:
Blocks = Wall area (sq ft) ÷ Block face area (sq ft)
For the same 20×2 ft wall with 12×4 in blocks, that's 40 sq ft of wall face divided by a 0.33 sq ft block face, or about 120 blocks. Notice that's lower than the 140 wall blocks from the row-by-row method above — the area method only accounts for the visible wall face, so you'll still need to add the buried course and any cap blocks on top of that number separately.