Concrete Calculator
Slab or pad volume
Results
| Volume | — |
|---|---|
| Cubic yards | — |
| Bags | — |
How to Use This Concrete Calculator
Ordering concrete is one of those jobs where a small math mistake turns into an expensive truck shortfall or a garage full of leftover bags. I have seen weekend warriors scribble length times width on a napkin, forget to convert inches to feet, and then wonder why the ready-mix driver quoted twice what they expected. This calculator takes the guesswork out of slab and pad estimates by handling unit conversions for you and returning both bulk cubic yards and bag counts in one pass.
Enter the length and width of your pour, each with its own unit selector—feet, yards, inches, meters, or centimeters. Then enter slab thickness. A typical patio sits around 4 inches; driveways often run 4 to 6 inches depending on soil and local code. Hit Calculate and you get volume in cubic feet, cubic yards, and the number of standard 80 lb premix bags assuming roughly 0.6 cubic feet per bag.
Think of the slab like a flat box: you are measuring how much space that box occupies, then translating that into what the supplier sells—either a ready-mix truck measured in cubic yards or bagged mix from the home center. If you are unsure of the footprint, measure the pad on site or run the room dimensions through our Square Footage Calculator first.
Concrete Calculator Formulas and Practical Applications
The math is straightforward volume arithmetic once every dimension sits in the same unit system internally (feet).
Step 1: Slab volume in cubic feet
V = Length × Width × Thickness
All three dimensions convert to feet before multiplying. A 10 ft × 10 ft pad at 4 in thick becomes 10 × 10 × (4 ÷ 12) = 33.33 cubic feet.
Step 2: Convert to cubic yards
Cubic Yards = Cubic Feet ÷ 27
Ready-mix suppliers in the US quote by the cubic yard. One cubic yard equals 27 cubic feet—picture a cube three feet on each side.
Step 3: Bag count for premix
Bags = Ceiling(Cubic Feet ÷ 0.6)
An 80 lb bag of premix typically yields about 0.6 cubic feet when mixed. The tool rounds up so you do not come up one bag short mid-pour. In practice, add 5–10% extra for spillage, uneven forms, and subgrade variation before you commit to a truck order.
Standard Units and Conversion Tables
Concrete volume in the US is ordered in cubic yards; bagged mix is sold by weight. Common slab thickness references:
- 4 inches: Standard patio, sidewalk, and light-duty slab.
- 5–6 inches: Residential driveway or heavier foot traffic.
- 1 cubic yard: 27 cubic feet—typical minimum ready-mix delivery on many routes.
- 80 lb bag: ~0.6 ft³ yield; roughly 45 bags per cubic yard (varies by mix design).
Length inputs accept feet, yards, inches, meters, and centimeters. Thickness accepts inches, feet, centimeters, and meters. Everything normalizes to feet before the volume formula runs.
Frequently Asked Questions
How much extra concrete should I order?
Most contractors add 5–10% for waste, over-excavation, and form irregularities. If your subgrade is uneven or the shape is not a perfect rectangle, lean toward the higher end. Ready-mix trucks often charge a short-load fee below a minimum yardage, so rounding up slightly can be cheaper than a second delivery.
When should I use bags instead of ready-mix?
Bagged mix makes sense for small pads, post holes, and repairs under about one cubic yard. Above that, a ready-mix truck saves labor and usually money. Our bag count assumes 80 lb premix at 0.6 ft³ each—check your bag label because high-strength or fiber-reinforced mixes can differ.
Does this calculator account for footings or walls?
This tool models a flat rectangular slab or pad. Footings, stem walls, and column piers need separate volume calculations for each segment. Measure each pour zone independently and add the cubic yards together for a total order.
Disclaimer. Informational only—not engineering, structural, or contractor advice. Add extra for waste, uneven subgrade, and local code requirements; verify with a qualified professional.