How Many Bags of Concrete for a Footing/Foundation?
Quick answer
A 20 ft strip footing (2 ft wide × 12 in deep) takes roughly 74 × 80 lb bags. A single 3×3 ft pad footing at 1 ft deep takes about 15 bags. A round pier footing, 12 in across and 4 ft deep, takes about 6 bags. The exact number depends entirely on which of the three footing shapes you’re pouring — get the shape right first, then the math is simple.
Not sure which numbers to plug in for your own footing? The Bags of Concrete Calculator handles strip, pad, and round footings and gives you the exact bag count in seconds. This article walks through the three formulas so you understand where that number comes from — and can catch a mistake if one slips in.
Three footing types, three formulas
“Footing” isn’t one shape — it’s whichever concrete base sits under a wall, a post, or a column. Which formula you need depends on which one you’re pouring.
| Footing type | What it supports | Formula |
|---|---|---|
| Strip footing | A continuous wall, running the full length of a foundation | length × width × depth |
| Pad / spread footing | A single column or post, an isolated square or rectangular base | length × width × depth (× number of pads) |
| Round pier footing | A deck post, fence post, or column base | 3.14 × radius² × depth (× number of piers) |
Notice the pad and pier formulas both need multiplying by how many you’re pouring — a strip footing is usually just one continuous run, but pads and piers are almost always poured in a set.
Why footings deserve extra care
A patio slab that’s slightly undersized is a cosmetic annoyance. A footing that’s undersized is a structural problem — it’s the part of the building that transfers the entire weight of a wall, post, or column into the ground. Get the volume wrong and you either run short mid-pour (leaving a weak cold joint right where the load needs to be strongest) or end up with a footing that doesn’t reach the depth your local code actually requires.
Footing depth vs. frost depth — don’t mix these up
These sound like the same thing, but they’re not, and confusing them is one of the most common footing mistakes.
| Term | What it means |
|---|---|
| Footing thickness | How thick the concrete pad itself is — typically 6–12 inches for most residential footings. |
| Frost depth | How far below ground level the bottom of the footing must sit, set by your local building code, so the ground beneath it doesn’t freeze and heave. |
Frost depth varies a lot by region — it can be as shallow as a few inches in warm climates or several feet in cold ones. Check your local code before you dig, since it directly affects how much concrete a pier or pad footing needs.
The formulas
Then, as always:
What to measure first
- Which footing type you’re pouringStrip, pad, or pier — this decides your formula.
- Dimensions in feetLength and width for strip/pad; diameter (then halve it for radius) for piers.
- DepthConfirm both the footing thickness and, separately, the required frost depth for your area.
- How manyPad and pier footings are almost always more than one — count them all.
Worked examples — one for each footing type
Example 1 — strip footing, 20 ft × 2 ft wide × 12 in deep, 80 lb bags
- Convert depth12 ÷ 12 = 1 ft
- Volume20 × 2 × 1 = 40 cu ft
- Add 10% waste44 cu ft
- Divide by yield (0.60)44 ÷ 0.60 = 73.3
- Round up74 × 80 lb bags
Example 2 — pad footing, 3 × 3 ft × 1 ft deep, 4 pads, 60 lb bags
- Volume per pad3 × 3 × 1 = 9 cu ft
- Four pads9 × 4 = 36 cu ft
- Add 10% waste39.6 cu ft
- Divide by yield (0.45)39.6 ÷ 0.45 = 88
- Result88 × 60 lb bags
Example 3 — round pier footings, 12 in diameter × 4 ft deep, 6 piers, 80 lb bags
- Radius12 in ÷ 12 ÷ 2 = 0.5 ft
- Volume per pier3.14 × 0.5² × 4 = 3.14 cu ft
- Six piers3.14 × 6 = 18.84 cu ft
- Add 10% waste20.72 cu ft
- Divide by yield (0.60)20.72 ÷ 0.60 = 34.5
- Round up35 × 80 lb bags
The 4 ft depth in Example 3 isn’t arbitrary — that’s a typical frost-depth requirement in a cold climate. Always check yours before setting the pier depth.
Skip the manual math.
Enter your footing type and dimensions and get an instant bag count, no matter which shape you’re pouring.
Use the free Bags of Concrete CalculatorDon’t skip the waste allowance
| Situation | Add |
|---|---|
| Formed, clean-sided trench or hole | 5–10% |
| Hand-dug or slightly over-excavated | 10–15% |
| Rocky, uneven, or collapsing soil | 15–20% |
Footings almost always come up a little over-excavated compared to the plan — hand-digging a straight-sided trench or hole is genuinely hard. Budget for it rather than being surprised by it.
Footings almost always need reinforcement
Unlike a simple patio slab, a footing is carrying real structural load, so rebar isn’t optional in most cases. Rebar itself takes up very little volume — under 1% of the pour — so it won’t meaningfully change your bag count. But skipping it can mean a footing that cracks and shifts under load, no matter how correctly you calculated the concrete. Check your local code or a structural reference for the right rebar size and spacing for your footing type and load.
7 mistakes that ruin a footing estimate
- Using the wrong footing formulaA round pier isn’t a rectangle — using length × width for a circular hole overstates the volume by roughly a third.
- Forgetting to multiply by the number of footingsOne pad footing’s volume isn’t the whole job if you’re pouring four.
- Confusing footing depth with frost depthBoth matter, and they’re not the same number — check your local code for frost depth specifically.
- Skipping the waste allowanceHand-dug footings rarely come out perfectly sized — budget 10-15% extra.
- Assuming rebar isn’t neededFootings carry real structural load; skipping reinforcement risks cracking regardless of correct volume.
- Rounding the bag count down73.3 bags means 74, not 73.
- Not checking local code before diggingFrost depth and footing size requirements vary by region — a number that’s right elsewhere might not be right for you.
When to switch from bags to ready-mix
A handful of pad or pier footings — a deck, a fence line, a small shed foundation — is a comfortable bag job. Once your total crosses roughly one cubic yard (about 45 × 80 lb bags), which happens quickly on a full strip footing or a foundation with many pads, ready-mix delivery usually becomes the more practical and cost-effective choice.
Foundation footings for an actual house or addition are almost always a ready-mix job — at that scale, hand-mixing bag by bag isn’t just slow, it risks the footing setting in disconnected batches instead of one solid pour.
Quick reference: common footings
| Footing | Dimensions | 80 lb bags |
|---|---|---|
| Single deck pier | 12″ dia × 3′ deep | 5 |
| 6 deck piers | 12″ dia × 4′ deep | 35 |
| Single pad footing | 3′ × 3′ × 1′ | 17 |
| 4 pad footings | 3′ × 3′ × 1′ each | 66 |
| Short strip footing | 10′ × 2′ × 1′ | 37 |
| Standard strip footing | 20′ × 2′ × 1′ | 74 |
What can change the number
- Local frost depthColder regions require deeper footings, which directly increases volume.
- Soil conditionLoose or rocky soil means more over-excavation and more waste allowance.
- Footing widthHeavier loads (like a two-story wall) often need a wider footing than a single-story one.
- Number of pads or piersEasy to undercount on a project with many footings — list them all before calculating.
Calculate it in five steps
- Identify the footing type. Strip, pad, or round pier — each has its own formula.
- Check your local frost depth and footing thickness requirement before finalizing the depth.
- Find the volume using the matching formula, multiplied by how many footings you’re pouring.
- Add 10–15% waste for over-excavation.
- Divide by bag yield and round up. If the total tops 45 bags, price out ready-mix.
Frequently asked questions
How many bags of concrete for a 20 ft strip footing?
At 2 ft wide and 12 in deep, a 20 ft strip footing needs about 74 × 80 lb bags, including a 10% waste allowance.
What’s the difference between a footing and a foundation?
A footing is the widened concrete base at the bottom that spreads a wall or column’s load into the soil. The foundation is the larger structure — walls, slab, or piers — that sits on top of the footings.
How deep should a footing be?
Footing thickness is usually 6–12 inches, but the required depth below grade depends on your local frost line, which can range from a few inches to several feet depending on your climate. Check your local building code.
Do footings need rebar?
Yes, in almost all cases. Footings carry structural load, and rebar helps prevent cracking and shifting under that weight.
How many bags for a fence post footing?
A typical 12″ wide, 30″ deep post hole takes about 2 bags of 80 lb concrete once the post’s own volume is subtracted. For post-specific numbers, see our Fence Post Concrete Calculator.
Should I use bags or order ready-mix for a foundation?
For a handful of footings, bags work fine. Once your total volume passes roughly one cubic yard (about 45 bags) — which happens fast on a full foundation — ready-mix delivery is almost always the better choice.
Related calculators
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Enter your footing type and dimensions and get an instant, accurate bag count — strip, pad, or pier.
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