Retaining wall calculator
Blocks, courses and the gravel under and behind a segmental retaining wall.
Calculator
Detailed breakdown
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How to use this calculator
Measure
Enter the wall length and the height above grade in feet, then the face size of your block. The course you bury is added automatically.
Calculate
Once you hit calculate, results update as you type. Change any number to compare options instantly.
Order
Print the estimate and take it to your supplier. Quantities are rounded up so you don't come up short.
The first course goes below grade
Segmental wall blocks stack in courses, so the count is two multiplications — with one course more than you might expect. A 20 foot wall of 12 inch wide blocks takes 20 blocks per course. Two feet above grade in 6 inch blocks is 4 courses, but the first course goes below grade, so you buy 5 courses: 100 blocks. Add cap blocks, one per block in the top course, if you're capping it.
The base is where retaining walls are won or lost. That 20 foot wall needs a leveling trench 2 feet wide and deep enough to take the buried course on top of 6 inches of compacted gravel — 1.48 cubic yards here. The trench grows with the wall: a taller wall buries more courses, so don't reuse a number from a shorter one. It must be level in both directions before the first block goes down, because every error you leave in the base gets multiplied by every course above it.
Bury the first course. The rule is one full course below grade, or 10 percent of the wall height, whichever is greater. A wall built entirely above grade has nothing holding back the soil pressing against its toe, and it will lean out over a few freeze cycles.
Behind the wall you need drainage, not soil. A 12 inch column of clean gravel against the back of the blocks — about 1.48 cubic yards on this wall — plus a perforated drain pipe at the base, gives water somewhere to go. Saturated backfill is heavy, and it's the most common reason these walls fail. And note the limit: walls over about 4 feet usually need an engineered design and a permit, because at that height the soil loads are past what a dry-stacked block wall can carry on its own.
Frequently asked questions
How many blocks for a 20 foot retaining wall?
A hundred, plus caps, for a wall standing 2 feet above grade. With 12 inch wide blocks that's 20 blocks per course: 4 courses show above grade and a fifth is buried, and you have to buy that one too.
How deep should the base of a retaining wall be?
A trench about 2 feet wide and 6 inches deep, filled with compacted gravel and leveled in both directions before the first course.
How far below grade should the first course be?
One full course, or 10 percent of the wall height, whichever is greater. The buried course is what keeps the wall from leaning out.
Do I need gravel behind a retaining wall?
Yes, about 12 inches of clean drainage gravel plus a perforated pipe at the base. Saturated backfill is the leading cause of failure.
What is geogrid and do I need it?
Geogrid is a synthetic mesh laid between courses and running back into the soil behind the wall, tying the block and the earth together into one mass. Most segmental walls above roughly 3 to 4 feet need it at specified course intervals, and it is the main thing separating a stacked wall from an engineered one.
How tall can a retaining wall be without an engineer?
Roughly 4 feet in most jurisdictions, but it varies. Above that you need an engineered design, a permit, and almost always geogrid reinforcement tied back into the soil at set course intervals — which is the real difference between a stacked wall and an engineered one. Check locally before building.
Sources
Formulas and material assumptions on this page follow published industry guidance: