Counting courses from the trench, not from the grass
A segmental wall is measured from the top of the base gravel, and the bottom course sits below the finished grade in front of the wall. The common minimum is one buried course, or one inch of burial for every eight inches of total wall height, whichever is greater. So a wall you want 32 inches tall is four courses of 8 inch block exposed plus one buried, which is five courses, 40 inches of block, sitting on 6 inches of compacted gravel in a trench 46 inches below the top of the wall.
The block count itself is simple arithmetic. A 24 foot wall is 288 inches, and at a 12 inch face that is 24 blocks per course. Five courses is 120 blocks. Caps are counted along the same 288 inches, so 24 caps at 12 inches each. Where it stops being simple is a curved wall, because on an inside curve the blocks close up and you need fewer, and on an outside curve they fan and you need more, plus the cuts. Add 10 percent on anything with a radius in it.
Setback, and what it costs you at the top
Almost every segmental block has a built-in batter, from a lip on the underside, from pins, or from a groove. Typical setback runs between half an inch and about an inch and a quarter per course. Over five courses at 3/4 of an inch that is 3 inches of lean, which means the top of the wall sits 3 inches further back than the bottom. On a straight wall the length does not change, so the block count is unaffected, but the lean matters when you are working to a property line, when the wall meets steps, or when the cap has to line up with something.
Setback is also structural. It leans the wall into the hill, and reducing it because you want a vertical face is not a free choice. If the block offers a near-vertical setting, that option usually comes with a lower height limit.
Base, stone and the pipe
The base trench is normally the block depth plus about 12 inches, so there is 6 inches of compacted gravel either side of the block. For an 8 inch deep block that is a 20 to 24 inch trench with 6 inches of compacted crushed base in it. Order gravel loose and expect it to lose roughly 20 percent of its volume when you compact it, so a 1 cubic yard hole wants about 1.2 yards delivered. Crushed base runs somewhere around 1.4 tons per cubic yard; clean 3/4 inch drainage stone is a little lighter, near 1.3 to 1.4. Both figures vary with the rock, so if you are ordering by the ton, ask the quarry for their conversion rather than using this one.
Behind the wall goes a column of clean stone, commonly 12 inches wide, running the full height, with a 4 inch perforated pipe at the bottom that has to actually go somewhere. A pipe that dead-ends behind the wall is decoration. Wrap the stone in geotextile so the soil behind does not migrate into it and clog the drainage over a few winters, which is a slow failure that looks fine right up until it does not.
Everything above assumes a wall under about 4 feet with level ground behind it and nothing heavy near the top. Add a slope above, a driveway within the wall height of the back face, or a few more courses, and the design changes from a stacking exercise into a soil mechanics problem with geogrid in it. That is the point to stop and get it engineered, and most building departments will insist anyway.
Questions people ask
How high can I build a retaining wall without an engineer?
Four feet of total height is the figure most jurisdictions use, measured from the bottom of the footing to the top of the wall rather than from the ground you are standing on, which catches people out. Below that, a well-built segmental wall on a compacted base with proper drainage is generally within reach of a competent DIY build. Above it you are into geogrid reinforcement, and any surcharge such as a slope above the wall, a driveway, or a pool, lowers the threshold. Check your local code before you order, because the number is not the same everywhere.
How many blocks do I need for a 20 foot wall 3 feet high?
Take it in courses. Three feet exposed is 36 inches, which is 5 courses of 8 inch block, plus one buried, so 6 courses. Twenty feet is 240 inches, which at a 12 inch face is 20 blocks per course. Six courses of 20 is 120 blocks, plus 20 caps. That is before any allowance for curves or cuts at the ends, and it assumes your block is genuinely 12 inches on the face rather than 11-5/8 or 11-1/2, which several of them are.
Do I really need gravel behind the wall?
Yes, on anything retaining real soil. The wall is not fighting the weight of the dirt so much as the water pressure that builds up in it, and a saturated clay backfill can generate several times the load of the same soil dry. A 12 inch column of clean stone with a perforated pipe at the base and a fabric wrap gives the water a path out. The pipe has to daylight somewhere lower than the wall or drain into something that does. This is the single most common reason walls that were built straight end up leaning.
How much base gravel do I need?
Multiply the wall length by the trench width by the gravel depth and divide by 27 for cubic yards, then add about 20 percent because gravel compacts. A 24 foot wall on a 24 inch trench with 6 inches of base is 24 x 2 x 0.5, which is 24 cubic feet, or 0.89 cubic yards compacted, so order about 1.1 yards. At roughly 1.4 tons per cubic yard that is close to a ton and a half. Confirm the tonnage conversion with the supplier, as it moves with the stone type and moisture.
What if my blocks are not 12 inches wide?
Change the field. Face widths of 11-5/8, 12, 16 and 18 are all common, some units are tapered so the front face is wider than the back, and several product lines mix two or three sizes in a random ashlar pattern where there is no single number. For a mixed-size pattern, buy by the manufacturer's square-foot coverage figure rather than by unit count, and treat this calculator as a check on the order rather than the order itself.