How to calculate concrete for common projects
Calculating concrete comes down to one formula:
Cubic Yards = (Length × Width × Depth in inches ÷ 12) ÷ 27
A 20×20 slab is 400 square feet before you apply depth. At 4 inches that is (20 × 20 × 0.333) ÷ 27 = 4.94 cubic yards.
With 10% waste added, you would order 5.44 cubic yards. The same math works for a patio, driveway, sidewalk, or footer.
Calculate amount of concrete for your pour
To calculate amount of concrete for a slab, footing, or wall, multiply length × width × depth in feet, divide by 27 for cubic yards, then add 10% for waste. The calculator above does that math and converts yards into bag counts.
That is the formula for a rectangle. If the pour is an L-shape or has a notch, break it into rectangles, add the volumes, then add waste once at the end.
Use the tool at the top of this page when you want yards and bags without doing the arithmetic by hand.
How much cement do I need?
People often ask how much cement do I need when they really mean ready-mix or bagged concrete. Cement is the binder in the mix; bagged "concrete mix" already includes cement, sand, and aggregate.
For most DIY pours, use the calculator's cubic-yard and bag counts (60 lb or 80 lb bags) rather than buying cement alone. If a mix design calls for separate cement, follow that bag ratio from your supplier.
This tool estimates concrete volume, not a custom cement-only batch.
Concrete thickness by project type
| Project | Recommended Depth | Notes |
|---|---|---|
| Walkway / Sidewalk | 4 inches | Light foot traffic |
| Patio Slab | 4 inches | Add reinforcement mesh for larger patios |
| Garage Floor | 4–5 inches | 5" for heavier vehicles |
| Driveway | 5–6 inches | 6" if supporting large trucks or RVs |
| Foundation Footer | 8–12 inches | Varies by frost depth and load |
| Fence Post Footing | 12 inches | Diameter = 3× post width |
Order 10% extra
Concrete math assumes flat ground, uniform depth, and no spillage. Jobsites are messier. Ground is rarely level, forms flex, and some mix always stays in the chute. Industry standard is to add 10% to every order.
This matters most with ready-mix. A short load means waiting for another truck while the first pour starts to set. That cold joint is a weak point. Extra yards cost less than a second trip.
For an odd shape, treat it as a bounding rectangle, subtract obvious voids, and still add waste.
Ready-mix or bags?
Small pads often use 60 lb or 80-pound bags; larger slabs favor ready-mix. Use bags when you can mix and place at your own pace. Call a plant when the pour is more than about a cubic yard.
Ready-Mix Concrete
Pros
- ✓ Best for jobs over 1 cubic yard
- ✓ Consistent mix quality
- ✓ Faster pour on large projects
- ✓ Less physical labor
Cons
- — Minimum order fees
- — Short time window to place and finish
- — Requires truck access to site
Bagged Concrete Mix
Pros
- ✓ Great for small jobs under 1 CY
- ✓ No minimum order, no delivery
- ✓ Work at your own pace
- ✓ Easy to store leftover bags
Cons
- — Labor-intensive for large jobs
- — Manual mixing is inconsistent
- — Cold joints if batches aren't matched
Bags vs yards for small jobs
A 40 lb bag yields about 0.30 cubic feet. A 60 lb bag is about 0.45. An 80 lb bag is about 0.60. One cubic yard is 27 cubic feet, so an 80-pound bag covers only a couple of square feet at a 4-inch pour.
If the calculator shows more than about a yard, a truck is usually less work than mixing bags. Keep leftover bags dry; opened mix does not store well.
Square footings and slabs by square feet
Start with area. A 20×20 slab is 400 square feet. Multiply by depth in feet, then divide by 27 for cubic yards.
Square footings and spread footers use the same cubic-yard math with a thicker depth. A 2-foot by 2-foot square footing, 12 inches deep, is 4 cubic feet, or about 0.15 cubic yards before waste.
Run several footings as one pour: add each footing's cubic feet, divide by 27, then add 10%.
What wet concrete weighs
Wet concrete weighs roughly 150 lb per cubic foot. That is a typical planning number, not a lab test. Use it for wheelbarrow loads or truck access, not for ordering yards.
Order the amount of concrete by cubic yard from the calculator. Check weight only when you need to know what the mix will do to a walkway, a deck, or a pickup bed.
Tips for a successful concrete pour
Prep your subgrade
Compact the base material and remove soft spots. A stable, compacted subgrade prevents cracking and settling over time.
Set forms precisely
Double-check all dimensions before ordering. Your forms define the final slab. Getting them level and square saves headaches later.
Control joints are not optional
For slabs over 10 feet in any direction, cut control joints every 8–10 feet to direct cracking. Concrete will crack. Joints tell it where.
Cure properly
Keep setting concrete moist for at least seven days so strength develops. Plastic sheeting or a curing compound both work.
Check your local frost depth
Footings must extend below the frost line in your area. A slab that heaves every winter is a liability. Check local building codes before pouring.
After the pour: setting and curing
Concrete starts setting within a few hours. Keep setting concrete moist for at least seven days so strength develops. Do not let the surface dry out in wind or sun on day one.
Stay off a slab until it can take foot traffic without marking. Wait longer before parking a vehicle. If the mix is still plastic, you are still in the setting window, not the curing window.
Frequently Asked Questions
How much concrete do I need?+
Length × Width × Depth in feet, divided by 27, equals cubic yards. Add 10% for waste and spillage. Use the calculator above to convert yards into 60 lb and 80 lb bag counts and a typical regional ready-mix cost range.
How many 60 lb bags of concrete make a cubic yard?+
A 60 lb bag of concrete covers approximately 0.45 cubic feet. Since 1 cubic yard equals 27 cubic feet, you need about 60 bags per cubic yard.
How many 80 lb bags of concrete make a cubic yard?+
An 80 lb bag covers approximately 0.60 cubic feet. You need about 45 bags to fill one cubic yard of concrete.
When should I use ready-mix concrete vs bags?+
Use ready-mix for projects requiring more than 1 cubic yard — it's faster and more consistent. For smaller jobs under 1 CY (like a fence post or small pad), pre-mixed bags are cost-effective and easy to handle.
How thick should a concrete slab be?+
Most residential slabs are 4 inches thick. Driveways that will support vehicle weight should be 5–6 inches. Footings are typically 8–12 inches depending on local frost depths and load requirements.
How much does a cubic yard of concrete cost?+
Ready-mix concrete typically costs $120–$180 per cubic yard depending on your region, mix design, and current material prices. Add delivery fees and finishing labor for total project cost.
How much cement do I need?+
Cement is the binder in the mix. Bagged concrete already includes cement, sand, and aggregate. For a typical pad or footer, order cubic yards or 60 lb / 80 lb bags from the calculator above. Buy cement by itself only when a supplier mix design gives you the bag ratio.