Concrete Calculator (Bags and Cubic Yards)

Enter the size of a slab, pad or footing, or round post holes, to estimate the concrete volume and the number of premixed bags.

How it works

Slab volume (cu ft) = length (ft) × width (ft) × thickness (in) ÷ 12. Round hole volume = π × (diameter ÷ 2)² × depth. Cubic yards = cubic feet ÷ 27. Bags = cubic feet with allowance ÷ bag yield, rounded up. Fence posts: concrete per post = (hole area − post area) × (hole depth − gravel − gap below ground); a square post's area is width², a round post's is π × (width ÷ 2)². Metric inputs are converted with 1 ft = 0.3048 m (NIST); 1 cu yd = 0.7646 m³.

Worked examples

A 10 × 10 ft slab 4 in thick is 10 × 10 × 4 ÷ 12 = 33.3 cu ft = 1.23 cu yd. With a 10% allowance that is 36.7 cu ft, or 62 bags of 80 lb mix (36.7 ÷ 0.60 = 61.1, rounded up).
Post holes: 8 holes, each 10 in across and 36 in (3 ft) deep. One hole is π × (10 ÷ 24)² × 3 = 1.64 cu ft, so 8 holes are 13.1 cu ft (0.48 cu yd). With 10% that is 14.4 cu ft, or 24 bags of 80 lb mix at 0.60 cu ft each. Any post volume is not subtracted, so this is on the safe side.
Form depth: a 4 × 3 ft landing formed with 2 × 4 boards is 3½ in deep, not 4 in (PS 20: a dry 2 × 4 is 1½ × 3½ in). At 3½ in it is 3.5 cu ft, 6 bags of 80 lb; entered as 4 in it would be 4.0 cu ft, 7 bags.

Bag-count tables

Bags of QUIKRETE Concrete Mix for common slab sizes, from its published yields: 40 lb ≈ 0.30 cu ft, 60 lb ≈ 0.45 cu ft, 80 lb ≈ 0.60 cu ft. No waste allowance is included, so add your own (10% is the calculator default). Counts are rounded up.

4 in thick

Slab	cu ft	cu yd	40 lb bags	60 lb bags	80 lb bags
4 × 4 ft	5.3	0.20	18	12	9
4 × 8 ft	10.7	0.40	36	24	18
6 × 6 ft	12.0	0.44	40	27	20
8 × 8 ft	21.3	0.79	72	48	36
10 × 10 ft	33.3	1.23	112	75	56
10 × 12 ft	40.0	1.48	134	89	67
12 × 12 ft	48.0	1.78	160	107	80
10 × 20 ft	66.7	2.47	223	149	112
12 × 24 ft	96.0	3.56	320	214	160
20 × 20 ft	133.3	4.94	445	297	223

6 in thick

Slab	cu ft	cu yd	40 lb bags	60 lb bags	80 lb bags
4 × 4 ft	8.0	0.30	27	18	14
4 × 8 ft	16.0	0.59	54	36	27
6 × 6 ft	18.0	0.67	60	40	30
8 × 8 ft	32.0	1.19	107	72	54
10 × 10 ft	50.0	1.85	167	112	84
10 × 12 ft	60.0	2.22	200	134	100
12 × 12 ft	72.0	2.67	240	160	120
10 × 20 ft	100.0	3.70	334	223	167
12 × 24 ft	144.0	5.33	480	320	240
20 × 20 ft	200.0	7.41	667	445	334

Fence posts: bags per post

Hole about 3 × the post width (QUIKRETE), 6 in of gravel in the bottom, concrete stopping 3 in below ground, post volume subtracted. 50 lb QUIKRETE Fast-Setting ≈ 0.375 cu ft; 80 lb Concrete Mix ≈ 0.60 cu ft. Bags per post are not rounded: round up the total for the whole job.

Post (actual size)	Hole	Hole depth	Concrete depth	cu ft per post	50 lb fast-setting	80 lb mix
4 × 4 (3.5 in)	12 in	24 in	15 in	0.88	2.3	1.5
4 × 4 (3.5 in)	12 in	30 in	21 in	1.23	3.3	2.0
4 × 4 (3.5 in)	12 in	36 in	27 in	1.58	4.2	2.6
4 × 4 (3.5 in)	12 in	42 in	33 in	1.93	5.1	3.2
6 × 6 (5.5 in)	18 in	36 in	27 in	3.50	9.3	5.8
6 × 6 (5.5 in)	18 in	48 in	39 in	5.06	13.5	8.4
2⅜ in round metal	8 in	30 in	21 in	0.56	1.5	0.9

How to measure

Measure the inside of the forms, not the outside of the boards.
Measure the length and width in two or three places. If they differ, use the largest figure.
Measure the depth from the top of the forms down to the compacted base, in several spots. Low spots in the base take extra concrete.
For round holes, measure the width across the top and the depth at the center. Holes dug wider at the bottom take more.

Common mistakes

Using the nominal lumber size for form depth: a 2 × 4 form is 3½ in deep and a 2 × 6 is 5½ in (PS 20).
Leaving depth in inches in a feet formula. 4 in is 4 ÷ 12 = 0.333 ft, not 4 ft. This calculator converts it for you.
Ordering exactly the calculated volume. Uneven base, form bulging and spillage use more, so add an allowance.
Assuming every bag yields the same. Yield depends on the product and bag size; QUIKRETE lists 0.60 cu ft for an 80 lb bag of its Concrete Mix. Use the yield on your bag.
Treating this volume as a design. Thickness, reinforcement and base are set by the project and local codes; check local codes.

Assumptions

Bag yields are QUIKRETE's published figures for its Concrete Mix (e.g. 80 lb ≈ 0.60 cu ft). Other brands and products (fast-setting, high-strength) can yield differently; check your bag.
Flat, uniform thickness and straight sides. Uneven ground, sloped sites and form bulging use more concrete.
Round holes are perfect cylinders. In round-hole mode the post volume is not subtracted; fence-post mode subtracts it.
Fence-post defaults follow QUIKRETE's post-setting guidance: hole diameter about 3 times the post width (12 in for a 4 in post), about 6 in of gravel in the bottom, and concrete filled to 3-4 in below ground. QUIKRETE gives the hole depth as 1/3 to 1/2 of the post height above ground (its examples: at least 2 ft for a 6 ft fence; its calculator uses 1/2). The 30 in default depth is an editable example, not a rule; frost depth and local codes may require more.
Post width default 3.5 in: a dry 4 × 4 is 3½ × 3½ in (PS 20). Bags are rounded up once for the whole job, since leftover mix from one hole goes in the next.
The waste allowance is an editable JobsiteCalc default, not an industry standard. Ask your supplier what extra they suggest for your job.
This is a quantity estimate only. It does not cover structural design, thickness, reinforcement or permits; check local codes and product instructions.

Frequently asked questions

How many 80 lb bags of concrete make a cubic yard?

About 45. QUIKRETE says an 80 lb bag of its Concrete Mix yields approximately 0.60 cubic feet, and a cubic yard is 27 cubic feet (27 ÷ 0.60 = 45).

How many 60 lb bags are in a yard of concrete?

About 60. QUIKRETE lists 0.45 cubic feet per 60 lb bag, and 27 ÷ 0.45 = 60.

How do I convert cubic feet to cubic yards?

Divide by 27, because a cubic yard is 3 ft × 3 ft × 3 ft = 27 cubic feet.

How many bags of concrete per fence post?

It depends on the hole. QUIKRETE's data sheet says two 50 lb bags of Fast-Setting Concrete set a 4 in post in a 10 in hole 2 ft deep. A 4 × 4 (3.5 in) in a 12 in hole 30 in deep, with 6 in of gravel and concrete stopping 3 in below ground, takes about 3.3 bags of 50 lb mix.

How many bags of concrete for a 10 × 10 slab?

At 4 in thick, a 10 × 10 ft slab is 33.3 cu ft: 56 bags of 80 lb, 75 bags of 60 lb or 112 bags of 40 lb QUIKRETE Concrete Mix, before any allowance. See the bag table below.

Related calculators

Concrete slab calculator – adds thickened edges and a ready-mix cost estimate
Cubic yards calculator – converts any volume to yards, feet and meters
Gravel calculator – for the stone base under a slab
Concrete block calculator – for block walls instead of poured walls
Asphalt calculator – tons of hot mix for a driveway or patch instead of concrete
Square footage calculator – to work out the area of odd shapes first

Sources

Formulas and reference values are from these sources. Page content last updated 2026-10-19.