How many retaining wall blocks do you need?

How do I calculate how many retaining wall blocks I need?

Work out the number of courses — the wall height plus the buried part, divided by the block height — and the blocks per course — the wall length divided by the block length. A 20 ft wall standing 3 ft high buries 3 inches more, so 39 inches of 4 inch blocks is 10 courses of 20 blocks: 200 blocks, 210 with 5% extra, plus 20 caps.

How deep should the first course of a retaining wall be buried?

Allan Block recommends burying 1 inch of block for every foot of wall height, on top of a 6 inch compacted gravel base in a trench about 24 inches wide. A 3 ft wall buries 3 inches, so the trench is 9 inches deep.

How much gravel do I need behind a retaining wall?

Fill 12 inches behind the blocks with clean drainage stone for the full height of the wall, with a perforated drain pipe at the bottom. For the 20 ft, 3 ft wall that is 20 × 1 × 3.25 = 65 cubic feet, about 2.4 cubic yards or 3.25 tons of stone at 1.35 tons a yard (the Minnesota DOT figure for one-size stone), plus about 0.74 cubic yards for the base.

How tall can I build a retaining wall myself?

Block makers' do-it-yourself instructions cover walls up to about 4 feet. Taller walls, walls holding back a driveway or a slope above, and walls on poor soil need geogrid reinforcement and an engineer's design — and many towns require a permit above 3 or 4 feet.

Which block size should I use?

Small 4 by 12 inch blocks suit garden borders and walls up to about 2 feet. Larger 6 by 16 and 8 by 18 inch blocks are heavier and deeper, which holds back more soil and goes up faster. Check the maker's maximum height for the block without reinforcement.

Worked example: a 30-foot wall of 6 by 16 inch blocks

The wall stands 2.5 ft above grade. Buried part: 1 inch per foot of height, so 2.5 inches. Total height to build: 30.0 + 2.5 = 32.5 inches. Divided by the 6 inch block height that is 5.42, rounded up to 6 courses. Along the wall, 30 ft ÷ 1.33 ft per block is 22.5, rounded up to 23 blocks per course. That is 23 × 6 = 138 blocks, or 145 with 5% extra for breakage and cuts, and 23 caps for the top. The trench is 8.5 inches deep: 6 inches of gravel base plus the buried part of the first course.

Gravel: the base is 2 ft wide and 6 inches deep, 30 cubic feet or 1.11 cubic yards. The drainage stone behind the wall is 1 ft wide and as high as the whole wall including the buried part, 81.3 cubic feet or 3.01 cubic yards. Perforated drain pipe runs the length of the wall, so allow 30 ft plus the run to the outlet.

Why the rows are counted this way

The first course is buried so the wall cannot slide forward at its toe, which is why the buried height is added to the visible height before dividing by the block height. The drainage stone and pipe let water leave instead of collecting behind the wall, which is why the block makers' instructions include them even for low walls. Because everything is rounded up to whole blocks, the finished height can come out a little over the height you entered; check the maker's block heights before you set a design height.

Common mistakes

• Entering the wall's total height, buried part included. The calculator adds the buried course itself, so enter only the height that will show.
• Mixing block dimensions: block height and length are entered in inches and the wall's length and height in feet.
• Leaving out the drainage stone and pipe, or using soil instead of clean crushed stone behind the blocks.
• Treating a stepped wall as one wall. If the height changes, enter each section separately so each gets its own course count and burial.
• Skipping the engineer. A taller wall, or one that holds a slope, a driveway or a building, is outside the do-it-yourself instructions.

Sources: Allan Block installation guidance (burial of 1 inch per foot of height, 6 inch compacted base, 12 inches of drainage stone with perforated pipe) and Mutual Materials' retaining wall guide for walls under 4 feet; crushed stone weight of 1.35 tons per cubic yard from the Minnesota DOT figure used in our gravel calculator. This is a block count for planning, not a design. Related tools: gravel calculator, concrete block calculator, paver calculator and stone wall calculator.