Concrete Calculator

By Uzair Arshad , Senior Civil and Structural Engineer

Last updated: April 27, 2026

Use this free concrete calculator to estimate cubic yards, cubic feet, and bag counts for slabs, footings, foundations, columns, and custom pours. Enter your dimensions below to get waste-adjusted volume and rounded bag totals for 40 lb, 50 lb, 60 lb, and 80 lb mix before you order.

How to use this calculator

This concrete calculator estimates cubic yards, cubic feet, and bag counts for slabs, footings, columns, and custom rectangular pours. Select a project type, enter your dimensions, and review your results before ordering.

Choose a project type. Select Slab for patios, driveways, sidewalks, and walkways. Select Footing for wall footings, post footings, and foundation footings. Select Column for round form tubes like Sonotubes. Select Custom for any other rectangular pour.
Enter your dimensions. For slabs and footings, enter length and width in feet and depth or thickness in inches. If you know the total square footage, divide by one dimension to get the other. For columns, enter diameter in inches and height in feet.
Set the number of identical pours if you have repeated footings, columns, or matching slabs. This multiplies a single pour volume by the count.
Adjust the waste factor. Use 5% for well-prepared, level subgrade with clean forms. Use 10% for most residential jobs. Use 15% for rough terrain or irregular excavation.
Click "Calculate concrete" to see order volume in cubic yards, exact volume before waste, bag counts for all sizes, and 2026 cost estimates.

Pro tips for accurate concrete estimates

Measure depth after compacting the base, not before grading. Loose gravel or soil can settle 1 to 2 inches, and measuring before compaction means your finished slab ends up thinner than planned.

Take three depth measurements across the area and average them. Uneven subgrade means actual thickness varies, and averaging prevents under-ordering. A second concrete delivery fee costs more than a few extra bags.

For repeated post footings or columns, count every hole before entering the quantity. Missing one footing is a common mistake that leads to a short pour on the last hole.

Quick reference: common concrete volumes

Project	Dimensions	Cubic yards	80 lb bags
Patio slab	10 × 12 ft, 4 in	1.48	67
Driveway slab	20 × 20 ft, 6 in	7.41	334
Wall footing	30 × 1.33 ft, 8 in	0.99	45
4 columns	12 in dia × 4 ft, qty 4	0.46	21
Sidewalk	25 × 3 ft, 4 in	0.93	42
Garage slab	24 × 24 ft, 4 in	7.11	320

Values before waste factor. Add 5% to 10% for your order quantity.

How the calculation works

Rectangular pours (slab, footing, custom):
Depth (ft) = Depth (in) / 12
Volume (cu ft) = Length (ft) × Width (ft) × Depth (ft)
Total Volume (cu ft) = Volume per Pour × Number of Pours

Column pours:
Radius (ft) = (Diameter (in) / 12) / 2
Volume (cu ft) = π × Radius² × Height (ft)
Total Volume (cu ft) = Volume per Column × Number of Columns

Conversion and waste:
Cubic Yards = Total Volume (cu ft) / 27
Order Volume = Cubic Yards × (1 + Waste % / 100)

Bag count:
40 lb Bags = Order Volume (cu ft) / 0.30 (rounded up)
50 lb Bags = Order Volume (cu ft) / 0.375 (rounded up)
60 lb Bags = Order Volume (cu ft) / 0.45 (rounded up)
80 lb Bags = Order Volume (cu ft) / 0.60 (rounded up)

Length: Long dimension of the pour area in feet
Width: Short dimension of the pour area in feet
Depth: Thickness of the pour in inches, converted to feet for volume math
Diameter: Inside diameter of a round column form in inches
Height: Column height from base to top in feet
Number of Pours: Identical sections to multiply (e.g. repeated footings)
Waste %: Extra percentage for uneven subgrade, spillage, and ordering safety

This concrete calculator converts your project dimensions into cubic yards, applies a waste factor, then calculates bag counts so you can order the right amount of material.

Rectangular pours (slabs, footings, custom)

Volume (cubic feet) = Length (ft) × Width (ft) × Depth (ft)

Cubic yards = Volume (cubic feet) / 27

Order volume = Cubic yards × (1 + Waste % / 100)

Convert depth from inches to feet first by dividing by 12. So 4 inches = 0.333 feet, and 6 inches = 0.5 feet.

Column pours

Volume (cubic feet) = π × Radius² × Height

Radius (ft) = (Diameter (in) / 12) / 2

This is standard cylinder math. If you have a 12 inch diameter column, the radius is 0.5 ft.

Worked example

A 10 ft × 12 ft patio slab at 4 inches thick with 10% waste:

Depth in feet = 4 / 12 = 0.333 ft
Volume = 10 × 12 × 0.333 = 40 cu ft
Cubic yards = 40 / 27 = 1.48 cu yd
With 10% waste = 1.48 × 1.10 = 1.63 cu yd (44 cu ft)
80 lb bags = 44 / 0.60 = 74 bags (rounded up)
60 lb bags = 44 / 0.45 = 98 bags (rounded up)

Bag yield comparison

Use this table to compare bag sizes before buying. An 80 lb bag yields 33% more per bag than a 60 lb bag, so you need fewer bags for the same pour.

Bag size	Yield per bag	Typical 2026 price	Best for
40 lb	~0.30 cu ft	$4.00 to $5.50	Small repairs and patching
50 lb	~0.375 cu ft	$4.50 to $6.50	Medium repairs
60 lb	~0.45 cu ft	$5.00 to $7.50	Small footings and post holes
80 lb	~0.60 cu ft	$6.50 to $9.00	Most DIY projects
Ready-mix	Sold by cu yd	$170 to $220/yd	Pours over 1 cu yd

Assumptions and limitations

This concrete calculator assumes uniform depth, square forms, and a compacted base. Real jobsite conditions such as over-excavation, sloped subgrade, and form flex can increase actual volume by 5% to 15% beyond the calculated amount.

Bag yields (0.60 cu ft per 80 lb bag, 0.45 cu ft per 60 lb bag) are based on manufacturer specs at standard water ratios. Mixing too wet reduces yield and weakens the pour.

Ready-mix pricing varies by region and delivery distance. Many batch plants have a minimum order of 1 to 3 cubic yards, plus short-load fees for smaller orders. Call your local plant for exact pricing.

Common concrete calculator mistakes

Measuring depth in inches and entering it as feet is the most frequent error. A 4 inch slab entered as 4 feet produces 12 times the correct volume. Always check the unit label beside each field.

Forgetting the quantity field causes trouble on multi-hole footing jobs. If you have 8 identical post footings, enter 8 in the quantity field instead of calculating one hole and multiplying later.

Ordering the exact calculated volume with zero waste is risky. I ordered exactly 2.5 cubic yards for a driveway apron and came up short by two wheelbarrow loads after the forms flexed slightly outward. A 5% to 10% buffer is cheaper than a second trip to the store or a short-load delivery fee.

Frequently Asked Questions

How do you calculate concrete?

Multiply length × width × thickness to get cubic feet, then divide by 27 for cubic yards. For a 10 ft × 12 ft slab at 4 inches thick, use 10 × 12 × 0.333 = 39.96 cubic feet, or about 1.48 cubic yards before waste. Add 5% to 10% extra for uneven subgrade.

How do I calculate yards of concrete?

Convert all dimensions to feet, multiply length × width × depth, then divide by 27. A 20 ft × 10 ft patio at 4 inches deep is 66.6 cubic feet. Divide by 27 to get 2.47 cubic yards, then add 10% waste if the base is uneven or forms are imperfect.

How do you calculate concrete yardage?

Concrete yardage is volume in cubic yards. Use length × width × depth in feet / 27. A footing that is 30 ft long, 16 inches wide, and 8 inches deep needs 30 × 1.333 × 0.667 / 27, or about 0.99 cubic yards before waste.

How do you calculate concrete yards?

Use cubic yards = cubic feet / 27 after converting inches to feet. For a 6 inch slab, depth is 0.5 ft. A 12 ft × 12 ft slab is 72 cubic feet, so 72 / 27 = 2.67 cubic yards before adding a safety margin for waste.

How do you calculate how many 80 lb bags of concrete you need?

Calculate total cubic feet with waste first. Each 80 lb bag yields about 0.60 cubic feet. Divide your waste-adjusted volume by 0.60 and round up. A project needing 34 cubic feet requires 34 / 0.60 = 57 bags. Always round up because partial bags are not practical.