Concrete Calculator

On this page

Formula

Volume = Length × Width × Thickness; Bags = ⌈Volume × (1 + Waste ÷ 100) ÷ Bag yield⌉

Length, Width, Thickness: dimensions of the pour, converted to feet (or meters)
Waste: extra allowance in percent for spillage, uneven ground and form movement
Bag yield: cubic feet of mixed concrete one bag makes (0.60 ft³ for 80 lb, 0.45 ft³ for 60 lb, 0.30 ft³ for 40 lb)

What the concrete calculator works out

Concrete is sold by volume. A ready-mix plant quotes in cubic yards (or cubic meters), and a premixed bag lists how many cubic feet it makes. This concrete calculator turns your measurements into that volume, adds a waste allowance, and converts the total into bags of 80, 60 and 40 lb premixed concrete, so you can price both options before you dig.

It handles the three shapes that cover most home projects:

Slab or footing: any rectangular pour, such as a patio, shed base, driveway apron, strip footing or poured wall. For a wall, enter the wall height as the width and its thickness as the depth.
Round column: sonotube piers, deck footings, post holes and round columns.
Steps: a solid flight of steps poured on the ground.

How to use it

Pick the shape tab.
Enter the measurements. Each field has its own unit, so you can type the length in feet and the thickness in inches without converting anything.
Set a quantity if you’re pouring several identical slabs or piers.
Keep the 10% waste allowance or change it. Use 5% for tidy forms on a well-compacted base, or more on rough ground.
Read the volume in cubic yards (or cubic meters if you entered metric lengths), and the number of bags of each size underneath.

How to calculate concrete by hand

The method is the same for every shape: get a volume in cubic feet, add waste, then divide by 27 for cubic yards or by the bag yield for bags.

A rectangular slab

Take a 12 × 10 ft shed base, 4 inches thick. Convert the thickness to feet first: 4 ÷ 12 = 0.333 ft.

Volume: 12 × 10 × 0.333 = 40 ft³
With 10% waste: 40 × 1.10 = 44 ft³
Cubic yards: 44 ÷ 27 = 1.63 yd³
80 lb bags: 44 ÷ 0.60 = 73.3, so 74 bags
60 lb bags: 44 ÷ 0.45 = 97.8, so 98 bags

A round column or pier

For an 8-inch tube set 3 ft deep, the radius is 4 in, or 0.333 ft. The volume is π × 0.333² × 3 = 1.05 ft³, which is 1.75 bags of 80 lb concrete. Round up to 2.

Steps

Treat each step as a block sitting on the one below. The bottom block is one rise tall, the next is two rises tall, and so on. For four steps 3 ft wide with a 6-inch rise and a 12-inch run: 3 × 1 × 0.5 × (1 + 2 + 3 + 4) = 15 ft³, or 0.56 yd³ before waste. The shortcut for 1 + 2 + … + n is n(n + 1) ÷ 2.

Concrete per 100 square feet of slab

This table shows the concrete for a 10 × 10 ft area at common thicknesses, calculated with the concrete calculator above. Scale it up for bigger slabs: a 300 ft² patio needs three times the figures shown.

Thickness	Cubic feet	Cubic yards	80 lb bags	60 lb bags	40 lb bags	Cubic yards +10%	80 lb bags +10%
3 in	25	0.93	42	56	84	1.02	46
4 in	33.33	1.23	56	75	112	1.36	62
5 in	41.67	1.54	70	93	139	1.7	77
6 in	50	1.85	84	112	167	2.04	92
8 in	66.67	2.47	112	149	223	2.72	123

For piers, a 12-inch tube 4 ft deep holds 3.14 ft³ (6 bags of 80 lb concrete), and an 18-inch tube of the same depth holds 7.07 ft³ (12 bags).

Bags or ready-mix?

Bag yields come from the manufacturer’s data sheet: an 80 lb bag of QUIKRETE Concrete Mix makes about 0.60 ft³, so 45 of them make one cubic yard. That’s about 3,600 lb of material to lift, mix and place. Bags make sense for post holes, a few steps or a small pad. Above roughly a cubic yard, most people find a ready-mix delivery or a rented mixer trailer easier, and the concrete calculator flags this when your total passes one cubic yard.

When you order ready-mix, give the supplier the volume in cubic yards including your waste allowance. Many plants sell in quarter- or half-yard steps, so round up to the next step they offer.

Common mistakes

Leaving the thickness in inches. Multiplying 12 × 10 × 4 gives 480, which is twelve times too much. Convert inches to feet (÷ 12) first, or let the calculator handle mixed units.
Dividing cubic feet by 9. There are 9 square feet in a square yard but 27 cubic feet in a cubic yard.
Forgetting the thickened edge. Many slabs have a deeper perimeter or a footing under the edge. Calculate it as a separate footing and add it.
Measuring the form height, not the pour. If the sub-base sits lower than planned, the slab gets thicker. An extra half inch across a 300 ft² patio adds about 12.5 ft³ (0.46 yd³).
Ordering exactly the volume. Without a margin, small variations in the base can leave you short in the middle of a pour.

Most slabs sit on a compacted gravel base, which the gravel calculator sizes in cubic yards and tons. If your patio has an irregular outline, find its area first with the square footage calculator and multiply by the thickness in feet. To convert cubic yards to liters or gallons, use the volume converter.

Glossary

Cubic yard: a cube 3 ft on each side, equal to 27 ft³ or 0.765 m³. Ready-mix is priced by the yard.
Yield: the volume of mixed concrete one bag produces.
Footing: the wider concrete base that spreads the load of a wall, post or column into the soil.
Cold joint: a weak seam where fresh concrete is placed against concrete that has already started to set.

Worked examples

A 4-inch patio slab

A 16 ft × 14 ft patio poured 4 inches thick, with 10% waste.

Inputs (Slab or footing): Length = 16; Length unit = Feet; Width = 14; Width unit = Feet; Thickness (depth) = 4; Thickness (depth) unit = Inches; Number of identical slabs or footings = 1; Waste allowance = 10 %

Volume = 16 ft × 14 ft × 4 in = 74.667 ft³ With 10% waste: 74.667 ft³ × 1.1 = 82.133 ft³ 82.133 ft³ ÷ 27 = 3.04 yd³ (2.33 m³) 80 lb bags: 82.133 ÷ 0.6 = 136.89 → 137 60 lb bags: 82.133 ÷ 0.45 = 182.52 → 183 40 lb bags: 82.133 ÷ 0.3 = 273.78 → 274

Answer: For one slab, plan on about 3.04 yd³ of concrete (2.33 m³) including 10% waste, or 137 bags of 80 lb premixed concrete (183 × 60 lb, 274 × 40 lb).

Six sonotube piers for a deck

Six 10-inch tubes, each 42 inches deep, with 5% waste.

Inputs (Round column): Diameter = 10; Diameter unit = Inches; Height or depth = 42; Height or depth unit = Inches; Number of columns = 6; Waste allowance = 5 %

Volume = π × (10 in ÷ 2)² × 42 in × 6 = 11.454 ft³ With 5% waste: 11.454 ft³ × 1.05 = 12.026 ft³ 12.026 ft³ ÷ 27 = 0.45 yd³ (0.34 m³) 80 lb bags: 12.026 ÷ 0.6 = 20.04 → 21 60 lb bags: 12.026 ÷ 0.45 = 26.73 → 27 40 lb bags: 12.026 ÷ 0.3 = 40.09 → 41

Answer: For 6 columns, plan on about 0.45 yd³ of concrete (0.34 m³) including 5% waste, or 21 bags of 80 lb premixed concrete (27 × 60 lb, 41 × 40 lb).

Three front steps

Three solid steps 4 ft wide with a 7-inch rise and 11-inch tread, with 10% waste.

Inputs (Steps): Step width = 4; Step width unit = Feet; Rise (height of each step) = 7; Rise (height of each step) unit = Inches; Run (depth of each tread) = 11; Run (depth of each tread) unit = Inches; Number of steps = 3; Waste allowance = 10 %

Volume = 4 ft × 11 in × 7 in × (3 × 4 ÷ 2) = 12.833 ft³ With 10% waste: 12.833 ft³ × 1.1 = 14.117 ft³ 14.117 ft³ ÷ 27 = 0.52 yd³ (0.4 m³) 80 lb bags: 14.117 ÷ 0.6 = 23.53 → 24 60 lb bags: 14.117 ÷ 0.45 = 31.37 → 32 40 lb bags: 14.117 ÷ 0.3 = 47.06 → 48

Answer: For a flight of 3 steps, plan on about 0.52 yd³ of concrete (0.4 m³) including 10% waste, or 24 bags of 80 lb premixed concrete (32 × 60 lb, 48 × 40 lb).

A metric footing with no waste (edge case)

A strip footing 6 m long, 0.45 m wide and 30 cm deep, entered in metric with the waste allowance set to 0.

Inputs (Slab or footing): Length = 6; Length unit = Meters; Width = 0.45; Width unit = Meters; Thickness (depth) = 30; Thickness (depth) unit = Centimeters; Waste allowance = 0 %

Volume = 6 m × 0.45 m × 30 cm = 0.81 m³ With 0% waste: 0.81 m³ × 1 = 0.81 m³ = 28.605 ft³ (1.06 yd³) 80 lb bags: 28.605 ÷ 0.6 = 47.67 → 48 60 lb bags: 28.605 ÷ 0.45 = 63.57 → 64 40 lb bags: 28.605 ÷ 0.3 = 95.35 → 96

Answer: For one slab, plan on about 0.81 m³ of concrete (1.06 yd³) including 0% waste, or 48 bags of 80 lb premixed concrete (64 × 60 lb, 96 × 40 lb).

Assumptions and limitations

This is an estimate only, not engineering advice. Footing sizes, slab thickness and reinforcement for structural work should come from local building codes or an engineer.
Round columns use π × (diameter ÷ 2)² × height. Steps are treated as solid blocks resting on the ground, so the volume is width × run × rise × n(n + 1) ÷ 2 for n steps.
Bag yields are the published figures for QUIKRETE Concrete Mix (0.30 ft³ per 40 lb bag, 0.45 ft³ per 60 lb bag, 0.60 ft³ per 80 lb bag). Other brands and specialty mixes may differ, so check the bag.
The waste allowance defaults to 10% if the field is left empty. It covers spillage, a sub-base that is not perfectly flat and forms that bow slightly. Use 5% for neat formwork on a level, compacted base.
Bag counts are rounded up to whole bags after the waste allowance is added. Volumes are shown to 2 decimal places.
US units are the default. Choose meters, centimeters or millimeters to work in metric; the headline result then switches to cubic meters.

Frequently asked questions

How many bags of concrete are in a yard?

One cubic yard is 27 cubic feet. At the published yields that is 45 bags of 80 lb concrete (27 ÷ 0.60), 60 bags of 60 lb (27 ÷ 0.45) or 90 bags of 40 lb (27 ÷ 0.30), before any waste allowance.

How do you calculate concrete for a slab?

Multiply length × width × thickness with every measurement in feet, which gives cubic feet. Divide by 27 for cubic yards. A 4-inch slab is 4 ÷ 12 = 0.333 ft thick, so a 10 × 10 ft slab needs 33.3 ft³, or 1.23 cubic yards.

How many 80 lb bags of concrete do I need for a 10x10 slab?

At 4 inches thick, a 10 × 10 ft slab is 33.3 ft³. That is 56 bags of 80 lb concrete without waste, or 62 bags with a 10% allowance. At that size, ordering ready-mix is often easier than mixing 60-odd bags by hand.

How thick should a concrete slab be?

Patios, walkways and shed floors are commonly 4 inches thick, and driveways for cars are often 5 to 6 inches. Requirements vary with soil, loads and local codes, so check with your building department for anything structural.

Should I order extra concrete?

Yes. Running short in the middle of a pour can leave a weak cold joint. A 5–10% allowance is typical; this calculator uses 10% unless you change it.

Is it cheaper to buy bags or ready-mix concrete?

For small jobs such as post holes, steps or a few cubic feet, bags are practical. Once a job needs about a cubic yard or more, a ready-mix truck usually saves time and labor, though many suppliers charge a short-load fee for small orders.

Sources

QUIKRETE Concrete Mix, Product No. 1101 (data sheet) — The QUIKRETE Companies. Checked October 12, 2026.
NIST Guide to the SI, Appendix B.9: Factors for units listed by kind of quantity — National Institute of Standards and Technology. Checked October 12, 2026.