Question 1

How many bags of concrete do I need for a 12-inch sonotube?

Accepted Answer

A 12-inch diameter sonotube at 36 inches deep holds about 2.36 cubic feet of concrete, which is roughly 4 bags of 80-lb mix. For 6 footings at that size, you need about 24 bags total.

Question 2

How deep should concrete footings be?

Accepted Answer

Footings must extend below the frost line to prevent heaving. In northern climates (zones 3-4), that's typically 48 inches or more. In moderate climates (zones 5-6), 36 inches is common. Southern climates (zones 7-10) often use 24 inches. Always check your local building code.

Question 3

What diameter sonotube do I need for a deck post?

Accepted Answer

Most residential deck posts use 10 or 12 inch diameter sonotubes. A 12-inch tube is the most common for standard residential decks. Larger structures, heavier loads, or posts over 8 feet tall may require 16 or 18 inch footings.

Question 4

Should I use ready-mix or bags for footings?

Accepted Answer

For 6 footings or fewer, bags are usually more convenient. Once you need more than 1 cubic yard of concrete (about 45 bags of 80-lb mix), calling a ready-mix truck becomes cost-effective. Most ready-mix companies have a 1-yard minimum.

Question 5

How long do I need to wait before setting posts in concrete footings?

Accepted Answer

Standard concrete reaches working strength in 24-48 hours for light loads. Wait 3-7 days before applying significant load. For Quikrete Fast-Setting concrete, you can set posts after 4 hours. Full cure takes 28 days.

Question 6

Do I need rebar in concrete footings?

Accepted Answer

For small round post footings (sonotubes), rebar isn't typically required for residential decks. For strip footings and larger structural footings, local code usually requires rebar — commonly #4 bars at specific spacing. Always check your local building permit requirements.

Diameter	Vol Each (cf)	4 Footings (yd³)	6 Footings (yd³)	8 Footings (yd³)
8"	1.047	0.16	0.23	0.31
10"	1.636	0.24	0.36	0.48
12"	2.356	0.35	0.52	0.70
16"	4.189	0.62	0.93	1.24
18"	5.301	0.79	1.18	1.57

Concrete Footing Calculator

Round Sonotube Reference — 36" Depth

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