Calculate cubic yards, cubic feet, concrete bags, and material cost for slabs, footings, columns, and post holes.
Calculator
Concrete Material Estimate
Review the main result, supporting measurements, and calculation details.
Volume Breakdown (Cubic Feet)
Calculation Details
| Metric | Value | Note |
|---|
Concrete bags per cubic yard: 40, 50, 60, 80 and 90 lb
These are QUIKRETE Concrete Mix No. 1101 yield examples, not universal bag-weight conversions. Whole bags are rounded up after the volume allowance. The 10% column is an illustrative allowance, not a project recommendation.
Loading an example replaces calculator inputs, uses US dimensions and 0% allowance, and clears optional prices. A 9 × 9 ft rectangle at 4 in is a volume identity for 1 yd³, not a slab design.
| Bag size | Approx. yield | 1 yd³, no allowance | 1 yd³ + 10% | Try 1 yd³ |
|---|---|---|---|---|
| 40 lb | 0.30 ft³ | 90 | 99 | |
| 50 lb | 0.375 ft³ | 72 | 80 | |
| 60 lb | 0.45 ft³ | 60 | 66 | |
| 80 lb | 0.60 ft³ | 45 | 50 | |
| 90 lb | 0.675 ft³ | 40 | 44 |
How much does an 80 lb bag of concrete cover?
At a labeled 0.60 ft³ yield, one bag covers approximately 1.8 ft² at 4 inches thick before allowance. Coverage is volume divided by thickness in feet; double the thickness and the same bag covers half the area.
| Example thickness | Area per 0.60 ft³ bag | Try 1 bag |
|---|---|---|
| 2 in | 3.6 ft² | |
| 3 in | 2.4 ft² | |
| 4 in | 1.8 ft² | |
| 6 in | 1.2 ft² |
Coverage = 0.60 / (thickness in inches / 12). These thicknesses are arithmetic examples, not structural specifications. Uneven depth, excavation and product-label differences change the quantity. Use the actual mix's finished-volume yield and the project's required thickness; do not add mixing-water volume to the label yield.
How much concrete do I need?
Select a rectangular slab or footing, round column, or set of post holes. The calculator converts every measurement to feet, calculates volume, adds the entered waste allowance, and reports cubic feet, cubic yards, bags, and estimated material cost.
rectangular volume = length x width x thickness; round volume = π x radius² x height x quantity
Concrete slab example
A 20 ft by 10 ft slab at 4 inches thick contains 66.67 cubic feet before waste. Adding 10% gives 73.33 cubic feet, or about 2.72 cubic yards. At an approximate 0.60 cubic foot yield, that is 123 80 lb bags after rounding up.
80 lb concrete bags per cubic yard
One cubic yard is 27 cubic feet. At an approximate yield of 0.60 cubic foot per 80 lb bag, 27 / 0.60 = 45 bags before any allowance. A 10% allowance makes the ordering volume 29.7 cubic feet: 29.7 / 0.60 = 49.5, so the purchase is 50 whole bags. Apply the allowance to volume first, then round the final bag quantity once.
Concrete bag yields are product-specific
The listed examples are from QUIKRETE Concrete Mix No. 1101: approximate cubic-foot yields are 0.30 for 40 lb, 0.375 for 50 lb, 0.45 for 60 lb, 0.60 for 80 lb and 0.675 for 90 lb. These are not universal weight-to-volume conversions or a brand recommendation. Other mixes can have different yields. Use the actual package's finished-volume yield, not dry powder weight or mixing water.
The custom yield accepts cubic feet, liters or cubic meters per bag. For example, a product labeled 0.50 cubic foot per bag needs 54 bags for one cubic yard before allowance. Purchased bag coverage and packaging excess are shown separately: rounding bags up is not another waste percentage. Approximate manufacturer liter and cubic-foot figures can be rounded independently; use the selected label value consistently.
US and metric dimensions
US mode uses feet for length, width and height, with inches for thickness and diameter. Metric mode uses meters and millimeters respectively. Changing modes converts the entered dimensions rather than reinterpreting the same number. The 20 ft by 10 ft by 4 in example becomes 6.096 m by 3.048 m by 101.6 mm and represents the same volume. Results include cubic feet, cubic yards and cubic meters. Material prices remain US dollars per bag or per cubic yard in both modes.
Round columns and post holes
The round modes calculate full cylinders with a whole quantity. A 12-inch diameter, 4-foot-deep hole has a geometric volume of about 3.14159 cubic feet. Four identical holes contain about 12.56637 cubic feet before allowance. A fractional quantity such as 2.5 holes is rejected, not rounded down. No post, pipe, gravel base or other displacement is subtracted; this is a gross hole-volume estimate, not a net surround or footing design.
Bagged mix versus ready-mix
The two cost estimates use only the prices you enter. Bag cost excludes sales tax, tools, water, labor, and delivery. Ready-mix cost excludes short-load, delivery, waiting-time, pumping, and minimum-order charges. Call local suppliers before treating either result as a quote.
Planning allowance and limits
Uneven excavation, subgrade variation, forms, spillage and leftover material can increase the required volume. Ten percent is an example, not a universal ordering rule. The tool accepts an entered allowance from 0%-100%; the project plans, supplier and measurement confidence determine an appropriate value. The chart separates measured volume from the added allowance, both in cubic feet. Dollars are kept in the cost details instead of mixed with volume on one axis.
Dimensions and custom yields must be finite and positive. Repeated columns or holes require a whole quantity from 1 through 1 million; estimates above 1 billion bags or outside the numerical range are rejected. Blank optional prices are reported as not entered, not free material; an explicit zero remains a zero-price scenario. Prices can be $0-$1 million per bag or cubic yard. These numerical limits do not establish real product availability, site suitability or structural safety.
Estimating loose aggregate instead? The gravel calculator separates measured volume from supplier-density weight and optional material cost.
Frequently asked questions
How many cubic feet are in a cubic yard?
One cubic yard equals 27 cubic feet.
Do I round concrete bags up?
Yes. Partial bags are not normally purchased, so the calculator rounds the final bag count up after waste is included.
Does this calculate structural requirements?
No. It estimates material volume only. Slab thickness, reinforcement, mix strength, joints, drainage, frost protection, and foundation design must follow the project plans and local requirements.