Concrete · Footings

Sonotube volume calculator

Concrete per round form, bag count for the whole set, and the code-minimum footing depth for your frost line. Cubic yards or cubic metres. The depth isn't a number you pick — it's a number the frost line picks for you.

Detail A — Round form

Volume — raw

Per tube—

Total cubic feet—

Total cubic yards—

Total cubic metres—

To order — bags (with waste)

— 80 lb bags — with waste

Footing depth — IRC R403.1.4 check

Code-minimum depth—

This pour depth—

Ready-mix alternative

Order — yd³—

01 How the math works

A sonotube is a round column form, so the concrete inside it is a cylinder. Volume is π times the radius squared times the depth, with everything in consistent units. Multiply by the number of tubes for the set.

One tube: V = π × (D/2)² × H
Set of N tubes: V_total = N × π × (D/2)² × H

That part is standard, and every sonotube calculator does it. The number none of them ask for is how deep the pour has to be. Under IRC R403.1.4, an exterior footing has to sit at least 12 in (305 mm) below undisturbed grade, and under R403.1.4.1 it has to reach below the frost line. The depth that governs is whichever of those two is deeper. Enter your local frost line and the calculator returns that minimum, then checks the pour depth against it.

For ordering, the raw volume gets a waste factor (typically 10%) and rounds up — to the whole bag for premix, or to the supplier's half-yard step for ready-mix. Footings are almost always a bag job, so the bag count is the headline figure here.

02 Worked example

Take four deck-post footings in a region with a 36 in (914 mm) frost line, formed with 12 in (305 mm) sonotube. The frost line is deeper than the 12 in grade minimum, so it governs: each tube runs 36 in (914 mm) deep.

Volume per footing: π × (0.5 ft)² × 3 ft = 2.36 ft³ (0.067 m³). Four footings: 9.42 ft³ = 0.35 yd³ (0.265 m³). At the standard 0.60 ft³ yield per 80 lb bag, that's 16 bags before waste. Add 10% and round up and the calculator orders 18 bags — which is the number to actually buy.

03 When this calculator is wrong

The cylinder math is exact. What it can miss is everything the tube doesn't enclose, and the depth figure you feed it.

It assumes a straight tube with no flared base. A bell-base form (the Bigfoot type) snaps onto the bottom and spreads the bearing area, poured in one shot with the pier. That flare is a separate frustum of concrete this cylinder math doesn't count — a wider base than the 12 in tube adds several tenths of a cubic foot the tube volume never sees.
The frost line is a local number, and the check is only as good as the one you enter. Required frost depth runs from 0 in in the frost-free far south to more than 60 in (1,524 mm) in northern Minnesota and Maine, and the building department sets the figure for your address. The calculator can't look it up. Confirm it before you dig.
A hand-dug or augered hole rarely stays a clean cylinder below 36 in. If the form doesn't run to the bottom of the hole, concrete fills the gap around it and you use more than the tube volume. Set the tube to the hole bottom, or add for the slop.
Bag yield is a spec figure. The 0.60 ft³ per 80 lb bag is Quikrete's number under ideal mixing; real yield runs 0.55–0.60 ft³ depending on water. Subtract about 5% if you want the honest count, or lean on the waste factor to cover it.

04 What to do with the result

Buy the rounded-up bag count, not the raw one. The cost of a returned bag of premix is a few dollars; the cost of stopping a footing pour to drive back for one more bag, while the first mix goes off in the wheelbarrow, is your afternoon. On a set of footings the correct calibration is what to order, not what the math gives — round up one more bag when a number lands close to the line. The exception is a single small footing where you already have partial bags on the shelf; there, buy exactly.

Then check the depth flag before anything gets poured. If the pour depth is short of the code minimum, the fix is a deeper hole, not a wider tube — a footing above the frost line heaves, and a heaved deck racks its framing no matter how much concrete is in the pier.

05 Common questions

How much concrete does a 12-inch sonotube hold?: A 12 in tube holds 0.785 ft³ per foot of depth (0.0222 m³ per 0.3048 m). At 36 in deep that's 2.36 ft³ (0.067 m³); at 48 in deep, 3.14 ft³ (0.089 m³). One 80 lb bag yields 0.60 ft³, so a 36 in tube is about 4 bags before waste.
How deep does a sonotube footing need to be?: The bottom has to reach the greater of 12 in (305 mm) below undisturbed grade and the local frost line (IRC R403.1.4 and R403.1.4.1). In a warm climate the 12 in minimum governs; in the north the frost line does, and it can be 48–60 in or more. Your building department publishes the required frost depth.
How many bags of concrete for a sonotube?: Divide the tube volume by the bag yield and round up. At 0.60 ft³ per 80 lb bag: a 12 in × 36 in tube (2.36 ft³) takes 4 bags; four of them take 16 bags before waste, 18 with a 10% factor. Smaller bags cover less — a 60 lb bag yields 0.45 ft³, a 40 lb bag 0.30 ft³.
What size sonotube for a deck post?: Standard round forms come in 6, 8, 10, 12 in and up (to 36 in before the commercial line). For residential deck posts, 8–12 in is the common range, but the size is set by the load and the local footing spec, not by habit. The tube diameter and the required bearing area are a structural decision — size it to the load table your jurisdiction uses.
Can I pour a footing at the bottom of a sonotube?: Yes, and it's common. A flared bell base or a poured spread footing at the tube bottom widens the bearing area without widening the whole column, so it carries the load on less concrete than a fat tube would. It also adds volume this calculator's straight-cylinder math doesn't include — account for the base separately.