Free retaining wall block calculator: enter wall length, height and block size to get courses, blocks per course, total blocks and order count, plus caps.
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Enter your values and press Calculate.
Enter the exposed wall length and height in feet, the block length and height in inches, and a waste allowance for cuts and breakage (10 percent is a good default). The calculator returns the number of courses, blocks per course, total blocks, the order quantity with waste, and cap blocks for the top row. Measure exposed height only, and read the buried-course note below before ordering.
A wall is a grid: courses stacked vertically, blocks laid horizontally. A 20 foot wall that is 3 feet high, built from 16 × 8 inch blocks, has 36 / 8 = 4.5, rounded up to 5 courses, and 240 / 16 = 15 blocks per course, for 75 blocks in total. With 10 percent waste the order is 83 blocks. Rounding each direction up separately matters: rounding the total area instead can under-order by a full course, because a course cannot be bought in fractions.
Measure length along the face of the wall and height from the finished ground in front to the top of the wall, not from the bottom of the buried course. Enter the block face size, which is the length and height you see, not the block depth. The cap count assumes one cap per block position along the top; if your cap units are a different length, divide wall length by cap length instead and use that figure for caps.
One course should be buried, roughly the greater of one full block height or 10 percent of the total wall height, so the exposed height you enter is not the full built height. Dig a trench, compact about 6 inches of gravel base, set the first course level, and backfill behind the wall with clean drainage stone plus a perforated pipe with its holes down, sloped to daylight. Most wall failures are drainage failures, not block failures: water trapped behind a wall pushes it over, however strong the blocks are.
A 20 × 2 foot wall with 16 × 8 inch blocks needs 3 courses × 15 per course = 45 blocks, or 50 with 10 percent waste. A 20 × 3 foot wall with the same blocks needs 5 × 15 = 75 blocks, or 83 with waste. A 30 × 3 foot wall with 12 × 6 inch blocks needs 6 × 30 = 180 blocks, or 198 with waste. A 10 × 1 foot garden edge with 8 × 4 inch blocks needs 3 × 15 = 45 blocks, or 50 with waste, which matches the small-wall tables published by wall suppliers.
Garden walls up to about 3 to 4 feet are commonly built without engineering, but the exact limit depends on local rules, soil, slope above the wall, and any surcharge such as a driveway or building nearby. Above about 4 feet, or whenever there is a slope or surcharge, geogrid reinforcement and an engineered design are normally required, and a permit may be needed. If your wall is over 4 feet, treat this calculator as a materials estimate only and get a local engineer involved before building.
Segmental blocks are the common do-it-yourself choice, roughly 25 to 50 dollars per face square foot installed, and they flex slightly without cracking. Poured concrete costs more, roughly 40 to 80 dollars per face square foot, but suits tall engineered walls. Timber is cheapest, roughly 15 to 35 dollars, but rots over time. Gabion baskets, roughly 25 to 45 dollars, drain well and suit informal garden walls. For most garden walls under 4 feet, segmental block is the best balance of cost, looks and lifespan.
Multiply courses (height / block height, rounded up) by blocks per course (length / block length, rounded up), then add waste. A 20 × 3 foot wall with 16 × 8 inch blocks needs 75 blocks, or 83 with 10 percent waste.
With 16 × 8 inch blocks, 45 blocks before waste (3 courses × 15 per course), or 50 with 10 percent waste. With 8 × 4 inch blocks the same wall needs 180 blocks, so block size changes the count a lot.
Often up to 3 to 4 feet for a simple garden wall, but local rules, soil, slope and surcharge matter. Above about 4 feet, or with a slope or driveway above the wall, get an engineered design and check permit rules.
About one full course, or roughly 10 percent of the total wall height, whichever is greater. The buried course anchors the wall and keeps the first visible course from being undermined.
About 6 inches of compacted gravel under the first course, in a trench wide enough to work in. The base, not the block, is what keeps the wall level over time.
Yes. A column of clean drainage stone behind the wall, a perforated pipe with holes down, sloped to daylight, and filter fabric between stone and soil are standard. Without drainage, trapped water pressure is the most common cause of wall failure.
Geogrid is a mesh reinforcement laid into the backfill in layers, tying the wall to the soil behind it. It is normally needed above about 4 feet, on slopes, or where a surcharge sits above the wall, and it is specified by the wall design, not guessed on site.
Yes, with a stepped base: each section of base steps down with the slope so every course stays level. Stepped walls need extra blocks at each step, so add a few percent to the waste allowance and check drainage at every step.