Building a Retaining Wall? How Many Concrete Bags You’ll Need

Retaining walls look simple — just stacked blocks holding back soil — but their stability depends heavily on the concrete footing beneath and sometimes the grouted fill inside the blocks. Knowing how many bags of concrete you’ll need is critical before you start digging or mixing.

Whether you’re building a small garden wall or a taller structural barrier, this guide will walk you through the practical math, site-tested methods, and bag estimates to make sure your wall stands solid and straight — without last-minute material runs.

Why Accurate Bag Estimation Matters for Retaining Walls

Unlike a slab, which spreads load evenly, a retaining wall applies uneven pressure to its footing. The backfill soil creates lateral force, and the footing must resist both vertical and horizontal loads.

That’s why retaining walls often require:

A continuous concrete footing below frost depth.
Core-filled or grouted cells in block walls for stability.
Drainage provisions (perforated pipe or gravel backfill).

Each of these elements uses concrete differently — and if you underestimate, you risk weak support or poor bonding. Using the Concrete Bags Calculator ensures you have enough mix to fill both the footing and any hollow cores safely.

Step-by-Step: Calculating Bags for a Retaining Wall Footing

Let’s start with the base footing — the most concrete-intensive part of the structure.

Step 1 — Measure Footing Dimensions

Typical small retaining walls use a footing that’s twice as wide as the wall thickness and at least 8–12 inches thick.

For example, a 6-inch thick wall might use a footing:

Width = 12 in (1 ft) Thickness = 12 in (1 ft) Length = 20 ft

Convert everything into feet for easy calculation.

Step 2 — Calculate Volume

Volume = Length × Width × Thickness Volume = 20 × 1 × 1 = 20 ft³

Convert to cubic yards:

20 ÷ 27 = 0.74 yd³

Step 3 — Convert Volume to Bags

Using the yield data (80 lb bag = 0.60 ft³ or 45 bags/yd³):

Bags = 0.74 × 45 = 33.3 → round up to 34 bags

You’ll need 34 bags of 80 lb concrete for the footing.

If you’re using smaller 60 lb bags:

Bags = 0.74 × 60 = 44.4 → round up to 45 bags

Always round up — it’s easier to store an extra bag than to stop mid-pour.

Example: Filling Concrete Blocks for a 20-ft Wall

Some retaining walls use hollow core concrete blocks reinforced with rebar and filled with grout or concrete for added strength. Let’s calculate for that too.

Step 1 — Determine Wall Dimensions

Say your wall is 20 ft long, 3 ft high, using 8x8x16-inch blocks (0.667 ft high × 1.33 ft long each). You’ll have:

Blocks = 20 ÷ 1.33 = 15 blocks per course Courses = 3 ÷ 0.667 = 4.5 → 5 courses Total blocks = 15 × 5 = 75 blocks

Step 2 — Volume of Fill per Block

A typical 8x8x16-inch block has about 0.025 yd³ (0.67 ft³) of fill volume when fully grouted.

Total Volume = 75 × 0.67 = 50.25 ft³ = 1.86 yd³

Step 3 — Convert to Bags

For 80 lb bags (45 per yd³):

1.86 × 45 = 83.7 → 84 bags

So, 84 bags for block fill, plus 34 bags for the footing = 118 bags total for the project.

That’s for a small wall — about 20 ft long and 3 ft tall — with a fully grouted core and a standard footing.

Quick Reference Table — Retaining Wall Concrete Estimates

Wall Type	Wall Length	Wall Height	Typical Footing (ft)	Approx. Total Bags (80 lb)
Garden Wall (no grout)	10 ft	2 ft	10×1×1	18–20 bags
Retaining Wall (grouted)	20 ft	3 ft	20×1×1	115–120 bags
Structural Wall	30 ft	4 ft	30×1.5×1	175–190 bags
Heavy-Duty Wall (5–6 ft)	20 ft	5 ft	20×2×1	220–250 bags

These estimates include footing and grout fill with about 5–10% waste allowance.

Pro Tips from the Field

1. Add 10% for Waste and Irregular Excavation

Uneven trenches and air pockets in block cells always consume extra mix. A good rule is to multiply your final bag count by 1.10 to stay safe.

2. Use Higher-Strength Mix for Footings

Opt for 4000 PSI (≈ 28 MPa) bag mixes for walls over 3 ft high or where load-bearing conditions exist. For short garden walls, 3000 PSI is usually adequate.

3. Compact Subgrade and Level Forms

A stable base prevents settlement cracks. Tamp gravel under the footing until firm, then level form edges before pouring.

4. Batch and Mix Efficiently

For walls requiring 50+ bags, a portable mixer or tow-behind batch mixer is worth it. Hand-mixing that many bags is exhausting and inconsistent.

5. Keep a Continuous Pour

Avoid cold joints — pour the footing in one go. For grouting block cores, fill every second course as you build to maintain consistent bonding.

6. Add Drainage Behind the Wall

A 4-inch perforated pipe with gravel backfill prevents hydrostatic pressure, which can undermine even well-reinforced walls.

Adjusting Bag Counts for Different Bag Sizes

Bag Size	Yield (ft³)	Bags per Cubic Yard	Adjustment Factor
40 lb	0.30	90	×2.0
50 lb	0.375	72	×1.6
60 lb	0.45	60	×1.3
80 lb	0.60	45	Base reference

To convert between sizes, multiply or divide by the adjustment factor shown above.
For instance, if a project requires 100 of the 80 lb bags, you’d need roughly 130 of the 60 lb bags or 200 of the 40 lb bags.

Common Field Questions

How deep should my retaining wall footing be?

Generally, as deep as the wall is tall — but not less than 12 inches. Local codes or soil reports may specify more, especially on slopes or poor soil.

Can I pour concrete directly into soil without forms?

Yes, for small garden walls under 2 ft, but formwork ensures straight edges and proper thickness. For structural walls, always use forms and rebar.

What about using post-hole concrete instead?

Fast-setting post mixes (like QUIKRETE Fast-Set) are convenient for fence posts but not ideal for footings or grout because of limited work time and variable density.

How do I check if I ordered enough?

Use the Concrete Bags Calculator. It automatically calculates bag counts for custom footing or wall fill dimensions, including waste allowance and conversion between cubic feet, yards, or metres.

Fence Post Bags of Concrete
Footing Bags of Concrete
Slab Bags of Concrete
Understanding Concrete Bag Coverage and Yield
Explore all estimation tools at Concrete Calculator Max to simplify planning for any concrete project.

Conclusion

A retaining wall’s strength begins with its foundation — and that starts with the right amount of concrete. By calculating volume accurately, adding 5–10% waste, and selecting the right bag size, you’ll ensure the wall performs as designed for decades.

Using the Concrete Bags Calculator makes this process effortless. Input your wall dimensions, footing thickness, and bag type, and it’ll return a precise count in seconds — saving time, errors, and costly delays.

You can double-check your math anytime with the Concrete Bags Calculator — it saves time, errors, and guesswork on site.