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Concrete

Sonotube Concrete Calculator

Enter tube diameter, fill height, and how many tubes you are pouring. Get cubic feet, yards, and whole 40, 60, or 80 lb bags — with the cylinder math shown so you can check it.

in
in
in

Leave blank if you fill the tube solid. Enter a round post or pier diameter to subtract its displacement from the pour.

%

Rule of thumb, not a spec. Bagged concrete spills, tubes settle, and you cannot buy a partial bag — so pros order ~10% over net volume. Adjust for your site.

Bag size

You need

5 × 80 lb bags

or 6 × 60 lb · or 9 × 40 lb

2.36 ft³ net · 2.59 ft³ (0.10 yd³) incl. 10% waste

The working

Cylinder formula

A round form tube is a cylinder. Convert every inch measurement to feet before multiplying — diameter and height both — then:

volume_ft³ = π × (diameter_in ÷ 24)² × (height_in ÷ 12)

That is the same as π × radius² × height with radius and height in feet. Skip the inch-to-foot step and the volume is twelve times too large (or worse if both dimensions stay in inches). The working panel shows gross volume per tube in cubic feet so you can match a hand takeoff.

When a round post or pier sits in the center for the full fill height, subtract a second cylinder of the same height. Net concrete per tube never goes below zero — if the void is as wide as the tube or wider, the calculator reports zero bags and flags the measurements.

Multiple-tube rounding

Bag counts round up once on the total waste-adjusted volume, not once per tube. Three tubes that each need 0.4 of an 80 lb bag are not three bags — they are two bags after you sum the cubic feet and divide. Rounding per tube over-orders every multi-pier job. Enter the tube count, let the total accumulate, then ceil.

Bagged concrete versus ready-mix

Small pier jobs — a few deck footings, mailbox bases, lamp posts — usually stay in bag territory. Quikrete publishes 0.30 / 0.45 / 0.60 ft³ for 40 / 60 / 80 lb Concrete Mix bags; divide the waste-adjusted cubic feet by the yield of the bag you are buying and round up. Once the total climbs toward a cubic yard (27 ft³, about 45 × 80 lb bags before waste), price a short-load ready-mix delivery. That threshold is a labor and logistics judgment, not a manufacturer yield. The result blade surfaces a planning notice when the waste-adjusted volume reaches one yard.

For slabs, continuous trench footings, or dug post holes that are not form tubes, use the Concrete Bag Calculator. Fence layouts with many posts are faster in the Fence Calculator.

Footing-size limitation

This tool does not select tube diameter, embedment, frost depth, or rebar. Those come from the project drawings, the deck or post product instructions, an engineer when required, and the local building department. Stock form tubes come in common diameters and cut lengths — Quikrete’s QUIK-TUBE® data sheet (No. 6922) lists typical sizes — but available SKUs are not the same as the footing your site needs. Enter the diameter and fill height your project already requires; the calculator only turns those numbers into volume and bags.

Sonotube® is a registered brand name for Sonoco’s cardboard concrete forms. Search traffic often uses “Sonotube” for any round fiber form. The math here is geometry plus Quikrete bag yields; it is not an endorsement of one brand over another, and it does not replace the form maker’s installation sheet (bracing above grade, gravel under footings, and similar details).

Sources

Grouped by manufacturer or publisher. Expand a group for the cited value, document, and verified-on date — every citation stays in the HTML either way.

Quikrete(3)
Trade conventions(1)

Frequently asked questions

How many bags of concrete do I need for a Sonotube?

It depends on the tube diameter, fill height, whether a post sits in the center, and which bag size you buy. A solid 12-inch tube filled 36 inches deep is about 2.36 ft³ net; with 10% overage that is roughly five 80 lb bags (Quikrete yield 0.60 ft³). Enter your actual sizes above — the working shows each step.

How do you calculate concrete for a cylindrical footing?

Convert diameter and height to feet, then volume = π × radius² × height. If a round post or pier occupies the center, subtract that cylinder’s volume and never go below zero. Multiply by the number of tubes, add overage, divide by the bag yield, and round whole bags up once on the total.

Do I subtract the post from a Sonotube pour?

Only when the post or pier stays in place and displaces concrete for the full fill height. Leave the optional center-void field blank for a solid pour. Enter the round-post diameter when you are filling around a post that remains in the tube.

How many 60 lb bags fill an 8-inch by 4-foot form tube?

An 8-inch tube filled 48 inches (4 ft) is about 1.75 ft³. With 10% overage (~1.92 ft³) that is five 60 lb bags at Quikrete’s 0.45 ft³ yield, or four 80 lb bags at 0.60 ft³. Confirm the height you actually fill — stock tube length is not always the pour height.

When should I use ready-mix instead of bagged concrete for form tubes?

As a planning rule of thumb — not a code rule — once the waste-adjusted total approaches or passes one cubic yard (27 ft³, roughly 45 × 80 lb bags before waste), price a short-load ready-mix delivery. Mixing labor and water haul often lose to a truck at that scale.

Does this calculator choose Sonotube diameter or footing depth?

No. Footing diameter and depth come from your project requirements, engineer, product instructions, frost depth, and local permits. This tool only converts the sizes you enter into volume and bag counts.