How Many Bags of Concrete for a Sonotube? A Practical Guide

If you’re setting fence posts, deck footings, or any vertical columns, you’ve probably come across Sonotubes — those round cardboard forms that make pouring cylindrical concrete footings simple and professional.

But the most common question builders and homeowners ask is:
“How many bags of concrete do I need for each Sonotube?”

Getting this right is important — underestimating leaves you short during a pour, while overestimating means wasted mix and cost. This guide explains the math, gives you quick reference tables for standard tube sizes, and shows you how to apply real site logic when estimating concrete for Sonotubes.

Understanding What a Sonotube Does

A Sonotube (or concrete form tube) is a round form used to shape vertical concrete columns or footings. They’re typically used for:

Deck and porch footings
Fence and sign posts
Light pole bases
Structural piers and foundation pads

Sonotubes come in standard diameters such as 6", 8", 10", 12", and 18", and lengths typically 4 feet (48 inches).

To find how much concrete each tube needs, you’ll calculate the cylindrical volume — then convert that into bag counts based on your chosen mix (40, 50, 60, or 80 lb bags).

Step 1 — Formula for Sonotube Volume

The volume of a cylinder is calculated as:

Volume = π × r² × h

Where:

π = 3.1416
r = radius in feet
h = height in feet

Since most Sonotubes are measured in inches, convert to feet first.

For example:
An 8" diameter × 4' (48") tall tube

Radius = 8 ÷ 2 = 4 in = 4 ÷ 12 = 0.333 ft
Height = 4 ft

Volume = 3.1416 × (0.333)² × 4 = 1.394 ft³

So, an 8" × 4' tube holds approximately 1.39 cubic feet of concrete.

Step 2 — Convert Volume to Bag Count

Now that you know the volume, divide it by the yield per bag for the mix you’re using.

Typical bag yields:

Bag Weight	Approx. Yield (ft³)	Bags per yd³ (27 ft³)
40 lb	0.30	90
50 lb	0.375	72
60 lb	0.45	60
80 lb	0.60	45

Formula:

Bags = Tube Volume (ft³) ÷ Bag Yield (ft³)

Example (using 80 lb bags): Bags = 1.394 ÷ 0.60 = 2.32 → round up to 3 bags

So, an 8" × 4' Sonotube requires roughly 3 bags of 80 lb concrete.

Step 3 — Quick Reference Chart

Here’s a handy chart of pre-calculated estimates for the most common Sonotube sizes (based on 4' tube height):

Tube Diameter (in)	Volume (ft³)	40 lb Bags	50 lb Bags	60 lb Bags	80 lb Bags
6"	0.785	3	2	2	2
8"	1.39	5	4	3	3
10"	2.18	8	6	5	4
12"	3.14	11	9	7	6
14"	4.28	15	12	10	7
16"	5.59	19	15	12	9
18"	7.07	24	19	16	12

These numbers already include standard rounding for real jobsite conditions. If you’re pouring multiple tubes, multiply accordingly — for instance, six 12" tubes will need roughly 6 × 6 = 36 of the 80 lb bags.

Pro Tip: Always round up to the next full bag and keep one or two extra per job. It’s better to have leftover mix than to run short in the middle of a pour.

Step 4 — Adjust for Tube Height

Not all Sonotubes are 4 feet tall. Use this adjustment formula if your tube is shorter or taller:

Adjusted Volume = (Actual Height ÷ 48) × 4-ft Volume

Example:
If your 10" diameter tube is only 3' tall:
Adjusted Volume = (36 ÷ 48) × 2.18 = 1.64 ft³

For 60 lb bags (0.45 ft³ yield):
1.64 ÷ 0.45 = 3.64 → round to 4 bags

So a 10" × 3' tube will need 4 bags of 60 lb mix.

Step 5 — Check for Concrete Type and Conditions

The type of concrete mix also affects bag count and performance.

Mix Type	Strength (psi)	Typical Use	Note
General Purpose	3000 psi	Light-duty footings, posts	Standard choice for most Sonotube pours
High-Strength	4000–5000 psi	Decks, heavy loads	Slightly lower yield (≈ 0.55 ft³/bag for 80 lb)
Fast-Setting	2500–3000 psi	Fence posts, small jobs	Quick curing; set within 30–40 minutes
Crack-Resistant	3500 psi	Exposed footings	Contains fibers to reduce cracking

If your tubes support a deck, pergola, or structural beam, choose 4000 psi or higher strength mix.
For fence or sign posts, general-purpose or fast-setting bags are sufficient.

Example — Full Job Calculation

Project: Pouring six 12" diameter × 4' tall Sonotubes for a deck.

Volume per tube: 3.14 ft³
Total volume: 3.14 × 6 = 18.84 ft³
Bag yield (80 lb): 0.60 ft³
Bags required: 18.84 ÷ 0.60 = 31.4 → round to 32 bags

Result:
You’ll need approximately 32 of the 80 lb bags to fill six 12" × 4' Sonotubes.

For extra safety, always add 5–10% more — so order 34–35 bags total.

You can confirm your calculation anytime with the Concrete Bags Calculator, which handles cylinder geometry automatically.

Field Notes and Pro Tips

Consolidate properly: After pouring each tube, rod or vibrate the mix lightly to remove trapped air.
Cut tubes to fit formwork: Sonotubes are easy to trim with a handsaw if footing heights vary.
Check for frost line: In colder climates, ensure footings extend below frost depth to avoid heaving.
Pour in layers: For tall tubes, pour halfway, rod, then finish the rest for a consistent, void-free column.
Keep tops clean: If you’re embedding post anchors, smooth the top surface and insert brackets while the mix is still wet.
Mix on-site near the forms: Moving full tubes is almost impossible once poured.

Fence Post Bags of Concrete
Footing Bags of Concrete
Slab Bags of Concrete
Understanding Concrete Bag Coverage and Yield
Visit Concrete Calculator Max for complete tools, charts, and concrete calculators.

Conclusion

Estimating concrete for Sonotubes isn’t complicated once you understand the cylinder formula and bag yields. Measure the tube, calculate its volume, and divide by the yield of your chosen bag size. Add a little extra for waste, and you’ll never come up short again.

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