FREE TOOL · CONSTRUCTION
Deck footings, piers, and fence posts all start as round holes in the ground. Enter the tube diameter, depth, and how many you’re pouring — this calculator returns the concrete volume in cubic yards and the exact number of 60- and 80-pound bags to buy.
On the default — one 12-inch tube, 4 feet deep, with a 10% waste allowance — you need 6 bags of 80-lb concrete (or 8 of 60-lb): 3.46 cubic feet, 0.13 cubic yards. Enter your own numbers below.
Enter values to see the result.
A tube is a cylinder, so the math is the cylinder volume formula: V = π × r² × h, with the radius and height in the same units. On the default: the radius is 6 inches = 0.5 ft, so V = π × 0.25 × 4 = 3.1416 cubic feet per tube. Add the 10% waste allowance → 3.46 ft³. Divide by 27 for cubic yards (0.13), or by the bag yield — 0.60 ft³ for an 80-lb bag — and round up: 3.46 ÷ 0.60 = 5.76 → 6 bags.
Bags of 80-lb mix per tube at 4 feet deep, before waste — add about 10% for spillage and topping off.
| Tube diameter | Volume (ft³) | 80-lb bags |
|---|---|---|
| 6″ | 0.79 | 2 |
| 8″ | 1.40 | 3 |
| 10″ | 2.18 | 4 |
| 12″ | 3.14 | 6 |
| 14″ | 4.28 | 8 |
| 16″ | 5.59 | 10 |
| 18″ | 7.07 | 12 |
| 24″ | 12.57 | 21 |
| 30″ | 19.63 | 33 |
| 36″ | 28.27 | 48 |
| Output | Formula | What it means |
|---|---|---|
| Concrete volume | π × (d ÷ 24)² × depth × tubes | 12″ × 4 ft → 3.14 ft³ per tube. |
| Cubic yards | ft³ ÷ 27 | 3.46 ft³ → 0.13 yd³ — what a ready-mix plant sells. |
| 80-lb bags | ft³ ÷ 0.60, round up | 3.46 ÷ 0.60 = 5.76 → 6 bags. |
| 60-lb bags | ft³ ÷ 0.45, round up | 3.46 ÷ 0.45 = 7.68 → 8 bags. |
| Mix water | ≈ 1 gal per 80-lb bag | 6 bags → ≈ 6 gallons of mixing water. |
The geometry is exact arithmetic. Bag yield is the moving part: 0.60 ft³ per 80-lb bag and 0.45 ft³ per 60-lb bag are typical for standard mixes, but fast-setting and high-strength mixes can differ — read your bag and it will tell you its yield. Holes dug by hand bell out wider than the tube, and that extra concrete is not in this number. This calculator sizes material only: footing depth, diameter, and rebar are structural decisions — follow your local building code and frost-line requirements.
Pouring a slab instead? The concrete calculator handles rectangular pours and walls. Reading the tape in decimals? The inches-to-fraction chart converts them back.
Bright Side Kit. "Concrete Tube Calculator: Bags per Pier" https://brightoolbox.com/concrete-tube/ Accessed October 22, 2026.A 12-inch-diameter tube 4 feet deep holds about 3.14 cubic feet — that is 6 bags of 80-lb concrete (5.24 rounded up), or 8 bags of 60-lb. Add the usual 10% waste allowance and the answer is still 6 bags of 80-lb. Enter your exact size above for any other tube.
A 4x4 post actually measures 3.5 × 3.5 inches. An 8-inch tube is the minimum most builders accept; 10 or 12 inches is the standard choice for deck posts, with 12 inches covering most residential footing requirements.
Below your local frost line — roughly 42–60 inches in the northern US, 24–30 inches in the South. Your county building department publishes the exact number; when in doubt, deeper beats wider for frost protection.
Below ground, leave it — the cardboard breaks down over the years without hurting the pier. Above ground, peel the tube off 24–48 hours after pouring for a clean exposed column; scoring it with a utility knife makes removal easy.
Yes — the yield is the same (0.60 ft³ per 80-lb bag) and it sets in 20–40 minutes, which is handy for fence posts. Skip it for large structural piers, where you need working time to place rebar and consolidate the pour.
Piers carrying a deck, porch, or roof load should have at least one vertical bar — a small cage of 2–4 bars with ties is better — to control cracking and tie the pier to its footing. Light fence or mailbox posts usually skip it. Your local code decides.
Multiply one tube by six, then add waste: six 12-inch × 4-ft tubes = 6 × 3.14 = 18.8 ft³, plus 10% = 20.7 ft³ → 35 bags of 80-lb. Easier: enter 6 in the tube count above and let the calculator do it.