Concrete cylinder calculator

This concrete cylinder calculator works out how much concrete a round column, a form tube or a post hole needs. The volume of a concrete cylinder is V = π × r² × h: the area of the circle times the depth of the pour. Multiply by the number of forms, add a margin for waste, and convert to cubic yards or to bags of premix.

Contents

The concrete cylinder formula
Bags of concrete
Concrete per foot of form tube
Worked examples
Concrete test cylinders
Round tanks, pipes and hollow forms
Common mistakes
FAQ

One form 12 in across and 48 in deep: π × 6² × 48 = 5,428.7 in³ = 3.142 ft³. Four forms plus 10 %: 13.82 ft³ = 0.512 yd³, about 24 bags of 80 lb (0.60 ft³ each).

The calculator above takes the diameter, the depth or height, the number of identical forms and the percentage to add for waste. It returns the total volume in cubic feet, cubic yards or cubic meters, and the number of 80 lb, 60 lb and 40 lb bags, rounded up.

The concrete cylinder formula

A round pour is a cylinder, so its volume is the one given by the cylinder volume formula:

V = π × r² × h × n × (1 + waste)

r is the inside radius of the form or the hole, half the diameter.
h is the depth of concrete, not the length of the tube if part of it stays empty.
n is the number of identical cylinders.
waste is the extra you order, as a fraction: 10 % is 0.10.

Concrete is ordered by volume. In the US, ready-mix is sold by the cubic yard and bagged premix by the bag. Convert as follows: cubic inches ÷ 1,728 = cubic feet; cubic feet ÷ 27 = cubic yards; cubic meters × 1,000 = liters. The page on the cubic feet of a cylinder details these conversions.

Bags of concrete

The number of bags depends on the yield, the volume of fresh concrete one bag makes. Use the figure printed on your bag. The calculator uses the typical yield printed on the bag for common premix:

Bag	Typical yield	In liters	Bags per cubic yard
80 lb	≈ 0.60 ft³	≈ 17.0 L	45
60 lb	≈ 0.45 ft³	≈ 12.7 L	60
40 lb	≈ 0.30 ft³	≈ 8.5 L	90

Divide the volume in cubic feet by the yield and always round up: 5.24 bags means 6 bags. For a large job, also check the weight you will carry: 24 bags of 80 lb is 1,920 lb.

Concrete per foot of form tube

For round columns and piers, it helps to know the concrete per foot of height. Multiply the figures below by the depth in feet.

Inside diameter	ft³ per foot	yd³ per foot	80 lb bags per foot	60 lb bags per foot
6 in	0.1963	0.0073	0.33	0.44
8 in	0.3491	0.0129	0.58	0.78
10 in	0.5454	0.0202	0.91	1.21
12 in	0.7854	0.0291	1.31	1.75
14 in	1.0690	0.0396	1.78	2.38
16 in	1.3963	0.0517	2.33	3.10
18 in	1.7671	0.0654	2.95	3.93
20 in	2.1817	0.0808	3.64	4.85
24 in	3.1416	0.1164	5.24	6.98
30 in	4.9087	0.1818	8.18	10.91
36 in	7.0686	0.2618	11.78	15.71

In metric, a round column takes 31.4 L per meter of height at 20 cm across, 49.1 L at 25 cm, 70.7 L at 30 cm, 125.7 L at 40 cm and 196.3 L at 50 cm.

Worked examples

Four piers in 12 in form tubes, 4 ft deep

One pierr = 6 in, h = 48 in. V = π × 6² × 48 = 5,428.7 in³ ÷ 1,728 = 3.142 ft³ (0.116 yd³).
Four piers3.142 × 4 = 12.566 ft³.
Plus 10 %12.566 × 1.10 = 13.82 ft³ = 0.512 yd³.
Bags13.82 ÷ 0.60 = 23.04, so 24 bags of 80 lb; or 31 bags of 60 lb; or 47 bags of 40 lb.

Fence post hole with a 4×4 post

A post hole is 10 in across and 36 in deep, with 6 in of gravel at the bottom, so the concrete is 30 in deep. The post is a 4×4, which measures 3.5 in × 3.5 in: nominal lumber sizes are larger than the actual dressed sizes.

Hole: π × 5² × 30 = 2,356.2 in³.
Post in the concrete: 3.5 × 3.5 × 30 = 367.5 in³, which is 15.6 % of the hole.
Concrete: 2,356.2 − 367.5 = 1,988.7 in³ ÷ 1,728 = 1.151 ft³ per hole.
Bags: 1.151 ÷ 0.60 = 1.92, so 2 bags of 80 lb per hole.

For 10 posts with 10 % extra: 1.151 × 10 × 1.10 = 12.66 ft³ (0.469 yd³), or 22 bags of 80 lb. If you mix each hole separately, 2 bags per hole (20 bags) covers the net volume with a margin of about 4 %.

A 6×6 post measures 5.5 in × 5.5 in. In a 12 in hole with 36 in of concrete: π × 6² × 36 = 4,071.5 in³ (2.356 ft³) for the hole, minus 5.5 × 5.5 × 36 = 1,089 in³ (0.630 ft³) for the post, leaves 1.726 ft³, or 3 bags of 80 lb. For a round post, subtract π × (post radius)² × depth instead: a 4 in round post in the 10 in hole above leaves 1.145 ft³.

Metric: six round columns 30 cm × 2.5 m

One column: π × 0.15² × 2.5 = 0.1767 m³, or 176.7 L (6.24 ft³, 0.231 yd³). Six columns with 5 % extra: 0.1767 × 6 × 1.05 = 1.113 m³. With bags yielding about 17.0 L (0.60 ft³), one column takes 10.4 bags, so 11.

Concrete test cylinders

A « concrete cylinder » also means the sample cast on site to test the strength of a batch. Compression test cylinders are commonly 6 in across by 12 in tall, or 4 in by 8 in. Their volumes follow the same formula:

Cylinder	Volume	Cubic feet	Liters	Approx. weight at 150 lb/ft³
6 in × 12 in	339.3 in³	0.196	5.56	29 lb (13 kg)
4 in × 8 in	100.5 in³	0.058	1.65	8.7 lb (4.0 kg)

Normal-weight concrete is roughly 2,400 kg/m³, close to 150 lb/ft³; the actual density depends on the mix. To weigh any concrete cylinder from its size, use the cylinder weight calculator, which includes concrete among its materials.

Round tanks, pipes and hollow forms

The concrete in a round tank or a hollow pier is the space between two circles: π × (R² − r²) × h, explained on the hollow cylinder volume page. Once a cistern is built, the cylinder tank volume calculator gives what it holds, full or partly full. A plastic or steel pipe can serve as a permanent form; its inside diameter, not its nominal size, sets the volume, as the pipe volume calculator explains.

Common mistakes

Diameter used as the radius. The result is four times too large.
Inches and feet mixed. A 12 in tube 4 ft deep is r = 0.5 ft and h = 4 ft, or r = 6 in and h = 48 in. Never r = 6 and h = 4.
Hole wider than planned. Dug holes are rarely clean cylinders, and a hole that flares at the top or bottom takes more concrete. Measure the hole you dug, and keep a margin.
Forgetting the post. In a post hole, the post displaces part of the volume; subtract it, or accept a small surplus.
Waste counted twice or not at all. Add the margin once, to the total volume, then divide by the yield and round up.

Rule of thumb: a 12 in round form takes about 0.79 ft³ per foot of depth, close to 1⅓ bags of 80 lb per foot.

FAQ

How do you calculate concrete for a cylinder?

Use the cylinder volume formula V = π × r² × h with the inside radius of the form and the depth of the pour. With the diameter and depth in inches, divide by 1,728 for cubic feet and by 46,656 for cubic yards. A 12 in form 4 ft deep needs π × 6² × 48 = 5,428.7 in³ = 3.14 ft³ = 0.116 yd³.

How many bags of concrete do I need for a 12 inch form tube?

Each foot of a 12 in tube takes 0.785 ft³. With the typical yield printed on an 80 lb bag (about 0.60 ft³), that is 1.31 bags per foot. A tube 4 ft deep needs 3.14 ft³, so 6 bags of 80 lb, before any allowance for waste.

How much concrete goes in a fence post hole?

The volume of the hole minus the part of the post that sits in the concrete. A 10 in hole with 30 in of concrete around a 4×4 post (3.5 in × 3.5 in actual) needs π × 5² × 30 − 3.5 × 3.5 × 30 = 1,988.7 in³ = 1.15 ft³, or 2 bags of 80 lb.

How many 80 lb bags make a cubic yard?

A cubic yard is 27 ft³. At a typical yield of 0.60 ft³ per 80 lb bag, that is 27 ÷ 0.60 = 45 bags. With 60 lb bags (about 0.45 ft³) it is 60 bags, and with 40 lb bags (about 0.30 ft³) it is 90 bags.

What size is a concrete test cylinder?

Compression test cylinders are commonly cast 6 in across by 12 in tall, or 4 in by 8 in. A 6 × 12 in cylinder holds π × 3² × 12 = 339.3 in³ (0.196 ft³, 5.56 L); a 4 × 8 in cylinder holds 100.5 in³ (0.058 ft³, 1.65 L).

Updated October 12, 2026