Concrete Footings: Calculating the Right Number of Bags

When it comes to supporting any structure — from decks and sheds to small buildings — footings form the critical base that transfers load safely to the ground.

A footing that’s too small can lead to settlement or cracking, while one that’s oversized wastes time, labor, and money. Knowing how many concrete bags you’ll need helps ensure your project is both cost-effective and structurally sound.

This guide explains footing calculations step by step, complete with field-tested examples, volume formulas, and quick reference tables for common footing sizes.

Understanding Footings and Why Volume Matters

A concrete footing is typically a rectangular or square pad placed below a column, wall, or foundation wall. Its role is to spread the load from the structure over a wider area of soil.

Footing dimensions depend on the load, soil bearing capacity, and column or wall width, but in residential and light commercial work, common sizes are:

Isolated (pad) footings: 2 ft × 2 ft to 4 ft × 4 ft
Continuous (strip) footings: 1 ft wide and 1 ft deep along wall lines
Pier or column footings: 18" to 36" square

To estimate the number of concrete bags, you must first find the footing’s volume, then convert that volume into bag counts based on mix yields.

Step 1 — Calculate the Footing Volume

The basic formula for rectangular or square footings is:

Volume = Length × Width × Thickness

Make sure all dimensions are in feet for consistent units.

Example:

A footing measuring 3 ft × 3 ft × 1 ft has:

Volume = 3 × 3 × 1 = 9 ft³

If you have several footings of the same size, multiply by the number of units.

Step 2 — Convert Volume to Cubic Yards

Concrete bag yields are usually based on cubic yards.
Since 1 cubic yard = 27 cubic feet, convert as:

Volume (yd³) = Volume (ft³) ÷ 27

Continuing the above example:

Volume = 9 ÷ 27 = 0.333 yd³

Step 3 — Convert Cubic Yards to Bags

Once you have the volume in cubic yards, multiply it by the number of bags per yard for your chosen bag size.

Bag Size	Yield (ft³)	Bags per Cubic Yard
40 lb	0.30	90
50 lb	0.375	72
60 lb	0.45	60
80 lb	0.60	45

Example — 3×3×1 ft footing using 80 lb bags:

Bags = 0.333 × 45 = 15 bags

So, one 3×3×1 ft footing requires about 15 bags of 80 lb concrete.

If you’re using 60 lb bags:

Bags = 0.333 × 60 = 20 bags

Round up to the next full bag for practical field use.

Step 4 — Adjust for Multiple Footings

For projects with multiple columns or piers, multiply by the total number of footings.

Example: You’re pouring 6 footings, each 3×3×1 ft, using 80 lb bags.

Bags = 15 × 6 = 90 bags

For a more precise check, use the Concrete Bags Calculator — it automates the same formulas and includes waste buffers.

Quick Reference Table — Bags per Footing

Footing Size (ft)	Volume (ft³)	Volume (yd³)	40 lb Bags	60 lb Bags	80 lb Bags
2×2×1	4	0.15	14	9	7
3×3×1	9	0.33	30	20	15
3×3×1.5	13.5	0.50	45	30	23
4×4×1	16	0.59	53	36	27
4×4×1.5	24	0.89	80	54	40
5×5×1	25	0.93	84	56	42
5×5×1.5	37.5	1.39	125	83	63

Tip: Always add 5–10% extra for waste, irregular excavation, and leveling. Concrete settles slightly when vibrated or tamped.

Step 5 — Accounting for Continuous Footings

For wall foundations or grade beams, the same logic applies — only the footing extends in one direction.

Formula:

Volume = Length × Width × Thickness

Example:

A continuous footing 40 ft long, 1.5 ft wide, and 1 ft deep:

Volume = 40 × 1.5 × 1 = 60 ft³ Volume (yd³) = 60 ÷ 27 = 2.22 yd³

If using 80 lb bags:

Bags = 2.22 × 45 = 99.9 → 100 bags

That’s about 100 of the 80 lb bags for a 40 ft footing.

Pro Tips from the Field

Compacted subgrade: Always prepare a solid, compacted soil base before pouring. A soft base can cause uneven settlement.
Rebar placement: Keep rebar 3 inches clear from all edges using chairs or spacers.
Weather conditions: Avoid pouring during freezing temperatures or heavy rain; use curing blankets if needed.
Batching: For large footings (over 1 yd³), consider a small ready-mix truck — it’s more efficient than mixing 100+ bags on site.
Pour continuously: Don’t stop midway. Continuous pouring prevents cold joints and ensures a monolithic footing.
Recheck dimensions: Excavations often widen at the bottom — measure before you start mixing.

Mix Type Recommendations

Mix Type	Strength (psi)	Suitable For	Notes
General Purpose	3000–3500	Small residential footings	Standard all-purpose mix
High-Strength	4000–5000	Structural or heavy loads	Preferred for columns, decks
Fast-Setting	2500–3000	Small isolated footings	Saves time, sets in under an hour
Crack-Resistant	3500–4000	Footings in freeze–thaw zones	Added fibers reduce surface cracks

Always check the bag label for yield and strength rating. For structural footings, use at least 3000 psi mix and proper reinforcement.

Slab Bags of Concrete
Sonotube Bags of Concrete
Understanding Concrete Bag Coverage and Yield
Reference Guide — Calculating Bags of Concrete for Any Project
Visit Concrete Calculator Max for full calculators and estimation tools.

Conclusion

Calculating the number of concrete bags for footings is straightforward once you know the dimensions.
Measure the length, width, and depth, find the volume, convert to cubic yards, and multiply by the bags per yard for your chosen bag size. Always round up and keep a few extras on site.

With a bit of math and a reliable reference table, you’ll pour solid, accurate footings every time.

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