Cardboard concrete forms are cylinders. Enter the inside diameter you are pouring and the filled height. This returns mix volume and an optional bag count from typical dry-mix yields. No account, no network.
A tube form is a right cylinder. Volume uses the inside diameter — the circle the concrete actually fills. The printed size on a Sonotube-style form is that ID. The outside of the cardboard is larger; do not measure OD and pour to that number.
V = π r² h with r = ID / 2
Convert ID and height to feet for cubic feet. Cubic yards = cu ft ÷ 27. Litres = cu ft × 28.3168.
With quantity n and waste w%: Vtotal = n × π r² h × (1 + w/100)
Bag count, when turned on, is ceil(Vtotal ÷ yield). Yields used here match a public QUIKRETE Concrete Mix (No. 1101) data sheet: 80 lb ≈ 0.60 cu ft, 60 lb ≈ 0.45 cu ft, 40 lb ≈ 0.30 cu ft. Those figures are typical of packaged Portland-cement dry mix, labelled as approximate on the sheet. Read the bag in the cart. This page is not selling mix.
| ID × height | Volume | 80 lb bags |
|---|---|---|
| 12 in × 4 ft | 3.14 cu ft (0.116 cu yd) | 6 |
| 8 in × 3 ft | 1.05 cu ft (0.039 cu yd) | 2 |
| 10 in × 4 ft | 2.18 cu ft (0.081 cu yd) | 4 |
| 16 in × 4 ft | 5.59 cu ft (0.207 cu yd) | 10 |
| 300 mm × 1.2 m | 84.8 L (3.00 cu ft) | 5 |
Those five rows were computed independently in Python (V = π r² h, bags rounded up) and match this page. A 12 in ID by 4 ft pier is π cubic feet — the usual shop check. At 0.60 cu ft per 80 lb bag that is just over five bags, so you buy six.
Inside. The mix fills the hollow of the form. Cardboard tube walls add roughly 1/8–1/4 in per side depending on the product, so the OD is larger than the pour. Use the size printed on the tube, or measure the open circle with a tape across the inside.
3.14 cubic feet, or 0.116 cubic yards. That is six 80 lb bags or seven 60 lb bags at the typical yields above, before waste. Four identical piers are 12.57 cu ft — still under half a yard, so bags usually beat a short-load truck.
If the hole is over-dug, the tube is flared at the bottom, or you are mixing in a wheelbarrow, add 5–10%. A clean tube sitting on a footing form can run closer to zero. Waste is a jobsite judgment, not a code factor. This field is optional and defaults to none.
When the takeoff climbs toward half a cubic yard or you need a consistent slump across many piers. Bags are fine for a handful of deck posts. Ready-mix is sold by the cubic yard; this page shows that number so you can call a plant without converting by hand.
Barely, for residential cages. A few #4 bars in a 12 in pier displace well under a tenth of a cubic foot. Ignore it for bag counts. If you are filling a large column with a heavy cage, subtract bar volume separately — this tool does not.
Use the unit you measured. Imperial here is ID in inches and height in feet, because that is how US tube sizes and frost depths are spoken. Metric is ID in millimetres and height in metres. Mixing a 12 on the ID with a 1200 on the height will invent a silo. The result always shows cubic feet, cubic yards, and litres.