# Sonotube Calculator: Concrete for Footings, Piers and Fence Posts

> Sonotube calculator: cu ft = 0.7854 × diameter² (ft) × depth (ft). Concrete per tube, footing size charts, deck footing sizing, frost depth and fence posts.

Published 2026-10-22 by Evergreen Packaging (Yantai Evergreen Packaging Co., Ltd.). Canonical: https://www.egpbags.com/blog/concrete-footings-sonotube-calculator/

**Key takeaways:** - **Tube volume (cu ft) = 0.7854 × diameter² × depth**, with diameter and depth in feet. ;; Per foot of depth: **8 in tube ≈ 0.35 cu ft, 10 in ≈ 0.55, 12 in ≈ 0.79**; a 12 in × 4 ft tube needs about **6 × 80 lb bags**. ;; Deck footing area = **load ÷ soil bearing capacity**; common code assumptions are 50 psf deck load and 1,500 psf soil. ;; Footings typically go **below the local frost line** and at least 12 in below undisturbed ground; check local code. ;; A 4x4 fence post in a 10 × 30 in hole takes about **1.15 cu ft: 2 × 80 lb or 4 × 50 lb bags**.

To calculate concrete for a sonotube (Sonotube is a trade name commonly used for round cardboard concrete form tubes), square the diameter in feet, multiply by 0.7854 and by the depth in feet: the result is cubic feet. A 12 in (1 ft) tube 4 ft deep holds 0.7854 × 1 × 4 = 3.14 cu ft, which is about six 80 lb bags of premixed concrete at a typical 0.60 cu ft per bag; a 10 in tube of the same depth holds 2.18 cu ft, about four bags. Size the footing itself from the load and your soil, and set it below the local frost depth.

> **Skip the maths:** our free [concrete bag calculator](/tools/concrete-bag-calculator/) converts post holes and footings into cubic yards and 40–90 lb or 25 kg bags.

**Key formula:** Form tube concrete (cu ft) = 0.7854 × diameter (ft)² × depth (ft). Metric: m³ = 0.7854 × diameter (m)² × depth (m). Bags = cu ft ÷ bag yield (about 0.60 cu ft for 80 lb, 0.45 for 60 lb, 0.375 for 50 lb).

0.7854 is π ÷ 4, the factor that turns a diameter squared into the area of a circle. All numbers below are typical; footing sizes and depths are set by your local building code, the deck or building design, and, where required, an engineer.

## How much concrete does a sonotube need?

| Tube diameter | Cu ft per ft of depth | Litres per m of depth | 4 ft (1.22 m) tube: cu ft | 80 lb bags | 60 lb bags | 50 lb bags |
|---|---|---|---|---|---|---|
| 6 in (150 mm) | 0.196 | 18 | 0.79 | 2 | 2 | 3 |
| 8 in (200 mm) | 0.349 | 32 | 1.40 | 3 | 4 | 4 |
| 10 in (250 mm) | 0.545 | 51 | 2.18 | 4 | 5 | 6 |
| 12 in (300 mm) | 0.785 | 73 | 3.14 | 6 | 7 | 9 |
| 14 in (350 mm) | 1.069 | 99 | 4.28 | 8 | 10 | 12 |
| 16 in (400 mm) | 1.396 | 130 | 5.59 | 10 | 13 | 15 |
| 18 in (450 mm) | 1.767 | 164 | 7.07 | 12 | 16 | 19 |
| 24 in (600 mm) | 3.142 | 292 | 12.57 | 21 | 28 | 34 |

Bag counts are rounded up per tube, before waste, using typical yields. Read the yield on your own bag, because mixes vary.

For a different depth, multiply the "per ft" column by the depth in feet. **Example — 12 in form tube, 42 in deep:** 0.785 × 3.5 = 2.75 cu ft, which is 4.6 → **5 × 80 lb bags**, or 7 × 60 lb bags.

## Worked example: deck footing calculator

Deck footings are sized so the soil under them is not overloaded.

**Footing area (sq ft) = load on the post (lb) ÷ allowable soil bearing (psf)**

Many US residential codes assume a deck load of **40 psf live + 10 psf dead = 50 psf** and, without a soil test, a presumptive soil bearing capacity of **1,500 psf**. Your local values may differ.

**Example:** posts 8 ft apart along a beam that supports joists spanning 12 ft. Each post carries half the joist span (6 ft) × the post spacing (8 ft) = **48 sq ft** of deck.

1. Load = 48 × 50 = **2,400 lb** (about 1,090 kg)
2. Required area = 2,400 ÷ 1,500 = **1.6 sq ft** (0.149 m²)
3. Round footing diameter = √(1.6 ÷ 0.7854) = 1.43 ft = 17.1 in → use an **18 in round** (1.77 sq ft) or a **16 × 16 in square** (1.78 sq ft) footing

The bearing area is at the **bottom**: a 10 in tube sitting on an 18 in spread footing or a flared footing base carries far more than a plain 10 in tube. For six such piers made of 10 in tubes 42 in deep: 0.545 × 3.5 × 6 = 11.45 cu ft, about 20 × 80 lb bags, or 21 with 10% extra, plus the concrete in any pads.

**Pier with pad:** a 24 × 24 × 8 in pad (2.67 cu ft) plus a 10 in tube 36 in tall (1.64 cu ft) is 4.30 cu ft, about **8 × 80 lb bags** per pier.

### Deck footing sizing chart (typical)

| Deck area carried by the post | Load at 50 psf | Area at 1,500 psf soil | Round footing | Square footing |
|---|---|---|---|---|
| 24 sq ft | 1,200 lb | 0.8 sq ft | 14 in | 12 × 12 in |
| 36 sq ft | 1,800 lb | 1.2 sq ft | 16 in | 14 × 14 in |
| 48 sq ft | 2,400 lb | 1.6 sq ft | 18 in | 16 × 16 in |
| 64 sq ft | 3,200 lb | 2.13 sq ft | 20 in | 18 × 18 in |
| 80 sq ft | 4,000 lb | 2.67 sq ft | 24 in | 20 × 20 in |

Sizes rounded up to common dimensions. Footing thickness for spread pads is commonly at least 6–8 in; check your local deck guide.

## Concrete footing size rule of thumb

For continuous **wall footings** under light-frame walls, a long-standing rule of thumb is: **footing width about twice the wall thickness, footing thickness about equal to the wall thickness**. US residential codes commonly set minimums of about **12 in wide and 6 in thick**, with wider footings for more stories or weaker soil.

| Foundation wall | Rule-of-thumb footing (width × thickness) | Concrete per 10 ft of footing |
|---|---|---|
| 6 in (150 mm) | 12 × 6 in (300 × 150 mm) | 5.0 cu ft (0.19 yd³) |
| 8 in (200 mm) | 16 × 8 in (400 × 200 mm) | 8.9 cu ft (0.33 yd³) |
| 10 in (250 mm) | 20 × 10 in (500 × 250 mm) | 13.9 cu ft (0.51 yd³) |
| 12 in (300 mm) | 24 × 12 in (600 × 300 mm) | 20.0 cu ft (0.74 yd³) |

Treat this footing size chart as a starting point for estimating, not a design. Actual footing dimensions depend on the building, soil bearing capacity and code tables.

**Wall footing example:** 40 ft of footing, 16 in wide × 8 in thick (12.2 m × 0.41 m × 0.20 m): 40 × 1.333 × 0.667 = **35.6 cu ft = 1.32 yd³** (1.0 m³). Add 10% → about 1.45 yd³. That is around 60–65 × 80 lb bags, so a ready-mix delivery is usually the better choice; see [how much concrete do I need](/blog/how-much-concrete-do-i-need/) for ordering ready-mix.

## How deep should footings be?

**Footing depth** is set by two things:

- **Frost depth.** The bottom of the footing typically has to sit below the local frost line so freezing soil cannot heave it. In the US this ranges from a few inches in warm southern areas to 48 in or more in the northern states. Your building department publishes the local figure.
- **Undisturbed soil.** Codes commonly require footings to bear on undisturbed soil at least **12 in (300 mm)** below grade, even where frost is not a factor.

Deeper is not automatically better: a deep, narrow tube with no spread base still bears only on its small bottom area.

## How much concrete per fence post?

Fence posts use the same round-hole formula, minus the post itself. Our [bags of concrete guide](/blog/how-many-bags-of-concrete-do-i-need/) has a full table; the common cases are:

| Hole (diameter × concrete depth) | Post | Concrete | 80 lb | 60 lb | 50 lb |
|---|---|---|---|---|---|
| 8 × 24 in | 4x4 | 0.53 cu ft | 1 | 2 | 2 |
| 10 × 24 in | 4x4 | 0.92 cu ft | 2 | 3 | 3 |
| 10 × 30 in | 4x4 | 1.15 cu ft | 2 | 3 | 4 |
| 12 × 36 in | 6x6 | 1.73 cu ft | 3 | 4 | 5 |

So the answer to **how many 50 lb bags of concrete per fence post** is typically 3–4 for a 4x4 in a 10 in hole. In weight, a 10 × 30 in hole takes about 150 lb of dry premix per post.

### Setting posts in concrete

- Dig the hole about **three times the post width** across and typically one-third to one-half of the above-ground height deep, or below frost depth where required.
- Put **4–6 in of gravel** in the bottom for drainage.
- Brace the post plumb, fill with concrete and **slope the top** away from the post so water runs off.

**Fence post cement mix:** use premixed concrete or a fast-setting post mix, not straight cement or mortar. If you mix your own, a common general-purpose ratio is about 1 part cement : 2 parts sand : 3 parts gravel by volume. **Concrete fence posts** (precast posts used with panel fencing) are typically set with roughly a quarter of their length in the ground and calculated the same way.

## Bags for concrete and post-mix producers

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## Frequently asked questions

### How many bags of concrete for a 12 inch sonotube?
A 12 in form tube holds about 0.785 cu ft per foot of depth. A 4 ft tube needs 3.14 cu ft: about six 80 lb bags, seven 60 lb bags or nine 50 lb bags at typical yields.

### What size footing do I need for a deck post?
Divide the load on the post by the soil bearing capacity. With a typical 50 psf deck load and 1,500 psf soil, a post carrying 48 sq ft of deck needs about 1.6 sq ft of footing: an 18 in round or 16 × 16 in square pad. Confirm with your local deck code.

### How deep should concrete footings be?
Typically below the local frost line and at least 12 in below undisturbed ground. Frost depth varies from a few inches to 48 in or more across the US, so check with your building department.

### How many 50 lb bags of concrete per fence post?
For a 4x4 post in a 10 in hole with 30 in of concrete, about four 50 lb bags (1.15 cu ft). An 8 in hole 24 in deep needs about two.

### What is the concrete footing size rule of thumb?
A traditional rule is footing width about twice the wall thickness and footing thickness about equal to the wall thickness, so an 8 in wall sits on a 16 × 8 in footing. Codes commonly require at least 12 in wide and 6 in thick; follow local code.

### Can I use a form tube without a footing pad?
You can for light loads where the tube's bottom area is enough for the soil, but a plain 10 in tube bears only about 0.55 sq ft. Most deck piers use a wider spread footing or flared base under the tube; check your design.

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Contact: doristeng2013@yt-evergreen.com · +86 186 6389 6305