Retaining Wall Calculator

Ordering concrete is a volume problem, not a guessing game: length times width times depth, divided by 27, plus a sensible allowance for waste. This page applies that formula to retaining wall calculator for building a 20 ft long, 3 ft high retaining wall. The result on this page is an estimate, not a guarantee: coverage, pack sizes, yields and prices vary by brand, product line, surface and region, and the label on the product you buy governs. Every figure below is shown as a range for exactly that reason, and the calculator on this page asks for your measurements instead of handing you a single magic number.

Retaining wall blocks are counted by the face area of the wall, and the block size you choose decides whether you need one or two blocks per sq ft. The rest of this page explains how the calculation works, shows a full worked example with its own measurements and a range result, gives cost guidance as ranges, and finishes with the steps to measure, order and check your delivery. Nothing here is a single exact number, because a single exact number would be dishonest: subgrades are uneven, forms flex, spillage happens, and a bag that yields one volume on paper can yield a little less once it is mixed and placed.

How the calculation works

The calculator needs three inputs, and each one changes the answer. First, the length and width in feet, measured at the longest and widest points of the form — for a rectangle that is a single multiplication, and for an L-shape or a footing run it is the sum of the separate rectangles. Second, the depth in inches, converted to feet by dividing by 12: a 4 in (10.2 cm) slab is one-third of a foot deep, a 6 in (15.2 cm) slab is half a foot, and an 8 in (20.3 cm) slab is two-thirds. Third, the waste allowance: our material data uses about 10% extra for waste, spillage and uneven subgrade as the typical allowance, because forms are never perfectly true and a little concrete always stays in the mixer, the wheelbarrow or the chute.

Retaining wall blocks typically cover about 1–2 blocks per sq ft of wall face, depending on block size — our material data gives that as a range, not a single figure. A 20 by 3 ft wall has 60 sq ft of face, so the count runs 60–120 blocks before caps, corners and cuts.

The formula is short: cubic yards = length (ft) × width (ft) × depth (ft) ÷ 27, because one cubic yard holds 27 cubic feet (0.76 cu m). Bagged mix follows from the same volume: one cubic yard takes about 45 bags of 80 lb (36.3 kg) mix or about 60 bags of 60 lb (27.2 kg) mix, typical yields from our material data — an 80 lb bag yields about 0.6 cu ft and a 60 lb bag about 0.45 cu ft. Multiply the cubic yards by 45 or by 60, add the 10% allowance, and round up to whole bags. Always round up: a pour cannot wait while someone fetches three more bags, and a short load costs far more than the spare bags would have.

Worked example: a 20 ft long, 3 ft high retaining wall

Here is the full arithmetic for a 20 ft long, 3 ft high retaining wall, with every step shown so you can check it against your own project.

Step 1 — face area. 20 ft × 3 ft = 60 sq ft (5.6 sq m).

Step 2 — blocks at the low end. 60 sq ft × 1 block per sq ft = 60 blocks.

Step 3 — blocks at the high end. 60 sq ft × 2 blocks per sq ft = 120 blocks.

Step 4 — caps, corners and base. Add cap blocks for the top course, extra blocks for corners and cuts, and a gravel base plus drainage pipe behind the wall — a wall without drainage is a wall that will lean.

Result: 60–120 blocks for the wall face — buy the high end if your blocks are small, and add caps separately. Buy a few extra bags beyond the high end if the subgrade is uneven or the forms are rough, and keep the spares dry — bagged mix that gets damp in storage is wasted money.

Cost guidance

Costs on this page are ranges, labelled typical retail range — check the current shelf price before budgeting. No single national price is quoted, because ready-mix and bagged concrete prices vary by region, supplier, mix strength and delivery distance, and a price that is right at one plant can be wrong at the next. The way to budget is to multiply, not to guess: take the cubic yards from the worked example, multiply by the shelf or quoted price per cubic yard for the mix you have chosen, then add delivery, any short-load fee for small orders, reinforcement such as rebar or mesh, and formwork and finishing tools if you do not own them.

Two cost traps catch first-time buyers. The first is ordering the low end of the range and then paying for a second small delivery — or a short-load fee — for the concrete that finishes the pour. The second is forgetting the extras: reinforcement, formwork, curing compound and finishing tools together are a meaningful share of a small job's cost, and a slab that needs excavation or a gravel base costs more before a single yard is poured. Compare the total, not the per-yard price, and get the quote against the same dimensions and depth you used for the volume count. Ready-mix usually beats bagged mix on price once the job passes about a cubic yard, while bagged mix wins for small repairs where a truck cannot justify the trip.

Step by step: measure, calculate, order

1. Measure twice. Measure the length and width in feet at the longest and widest points, and the depth in inches with a tape held against the form. Forms are rarely perfectly true — measure the depth in several places and use the deepest reading for the order.

2. Choose the mix and the method. Ready-mix suits jobs over about a cubic yard; bagged mix suits small pours and repairs. Check the bag label or the supplier's ticket for the mix strength and the yield per bag before you commit.

3. Use the calculator on this page. Enter the length, width and depth, and the tool returns a range in cubic yards and in bags. Plan on the high end — a pour cannot wait while someone fetches more mix.

4. Order and check the delivery. Order the high end of the range, confirm the delivery time and the access route for the truck, and have the forms, reinforcement and finishing tools ready before the concrete arrives.

5. Before you pour. Wet the subgrade lightly, check the forms are level and braced, confirm the reinforcement is in place, and make sure you have help, wheelbarrows and finishing tools within reach — concrete waits for no one.

Common mistakes to avoid

  • Ordering the low end of the range and then paying a short-load fee or a second delivery for the concrete that finishes the pour.
  • Measuring the depth once, at the shallowest spot, and under-ordering by a tenth of a yard or more.
  • Forgetting the 10% allowance for waste, spillage and uneven subgrade — the subgrade is never as flat as it looks.
  • Pouring without reinforcement, formwork checks or a curing plan — the volume count is only half the job.

Frequently asked questions

How many retaining wall blocks do I need?

For the 20 by 3 ft wall on this page, 60–120 blocks for the face, because blocks typically cover about 1–2 per sq ft depending on block size. Add cap blocks for the top course and extra blocks for corners and cuts.

How high can I build a retaining wall without an engineer?

That depends on your local code — many areas limit unengineered walls to about 3–4 ft, and taller walls need engineering and permits. Check the local code, which governs, before you buy a single block.

What goes behind a retaining wall?

Compacted gravel backfill and a drainage pipe at the base, plus filter fabric where the manufacturer calls for it. Drainage is what keeps a wall standing — a wall that holds water will lean and fail.

Do retaining wall blocks need mortar?

Most modern segmental blocks are dry-stacked and interlock without mortar; older systems and some decorative blocks do use it. Follow the block manufacturer's instructions for your exact product.

How do I estimate the gravel base for a retaining wall?

Measure the trench length, width and depth and convert to cubic yards with the same length × width × depth ÷ 27 formula used for concrete, then add the same 10% allowance. Check the bag or bulk label for the coverage of the gravel you buy.

Related calculators

Results are estimates — verify with the product label. Coverage, pack sizes, yields and prices vary by brand, product line, surface and region; the label on the product you buy governs. Cost figures are typical retail ranges — check the current shelf price before budgeting.