Concrete Calculator

Estimate cubic yards, 80 lb bags and delivery cost for slabs, footings and pads.

How the concrete estimate works

Volume is length x width x thickness, with thickness converted from inches to feet (divide by 12). That gives cubic feet; dividing by 27 converts to cubic yards, the unit ready-mix trucks use. We then add a waste allowance and convert the remainder to 80 lb bags at about 0.6 cubic feet each.

Cu ft = length x width x (thickness / 12)
Cu yards = cu ft / 27
Bags = cu ft x (1 + waste%) / 0.6

Bagged vs ready-mix

For anything under about half a cubic yard, mixing bags is reasonable for a weekend project. Beyond that, ready-mix delivered by truck is dramatically cheaper per cubic foot and saves hours of labor. As a rule of thumb, if you need more than 30-40 bags, get a truck.

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Use cases

Patio or walkway - 4 inch slab; enter the rectangle and add 10% for cuts at edges.
Driveway - 5-6 inches with rebar; the cost field shows a realistic truck quote.
Shed or garage floor - 4 inches over compacted gravel; small enough that bags may win.
Fence post footings - run the calculator per hole (treat as a small cylinder) and multiply.

Thickness quick reference

4 inches suits foot traffic and light storage. 5-6 inches is standard for vehicles. Always place concrete on a compacted gravel base and use reinforcement where loads are heavy; the calculator estimates volume, not structural capacity.

Frequently asked questions

How many 80 lb bags equal a cubic yard?

One 80 lb bag yields about 0.6 cubic feet. There are 27 cubic feet in a cubic yard, so you need roughly 45 bags per cubic yard. For most jobs, ordering ready-mix by the yard is cheaper than bagging.

How much extra concrete should I order?

Add 10% for slab jobs to absorb uneven subgrade and spillage. If the site is rocky or the forms are not perfectly square, 15% is safer.

What thickness should a concrete slab be?

Walkways and patios are typically 4 inches. Driveways and garage floors are 5-6 inches with reinforcement. Shed floors are often 4 inches on compacted gravel.

Volume estimates only. Subgrade compaction, form accuracy and local code affect the real pour. Consult a contractor for structural slabs.

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Ordering concrete without the guesswork

How to use this calculator

Measure the length and width of the pour area in feet, and the thickness in inches.

Convert thickness to feet by dividing by 12 — a 4-inch slab is 0.333 feet.

Multiply length × width × thickness to get cubic feet, then divide by 27 to convert to cubic yards.

Add a waste allowance of 5–10% so a slight over-excavation or spillage does not leave you short mid-pour.

Compare the result with ready-mix trucks (sold by the cubic yard, often with a minimum load) or bagged mix (sold by the bag, each covering a known volume).

Quick reference

Project	Typical thickness	Notes
Walkway / patio	4 in	Non-structural, foot traffic only
Driveway (cars)	4–5 in	Reinforce with rebar or mesh
Driveway (trucks/RV)	5–6 in	Thicker edge, controlled joints
Garage slab	4–6 in	Check local code for requirements
Fence post footing	Varies	Depth below frost line matters more than strength

Pro tips

Bagged concrete is practical up to roughly 1 cubic yard; beyond that, ready-mix delivery is faster and usually cheaper per yard.
Ordering from a ready-mix plant? Ask about their short-load fee — loads under 3–4 yards often carry a surcharge.
A standard 80-lb bag of concrete mix yields about 0.60 cubic feet; a 60-lb bag about 0.45 cubic feet.

Frequently asked questions

How much does a cubic yard of concrete weigh?

About 4,000 lbs (roughly two tons). This is why slab subgrade preparation and proper base compaction matter as much as the pour itself.

What PSI concrete should I use?

3,000 PSI suits most patios and walkways; 4,000 PSI is common for driveways and areas exposed to freeze-thaw or vehicle loads. Footings follow the specifications in your local building code.

How long before I can walk on new concrete?

Light foot traffic is generally fine after 24–48 hours, vehicles after about 7 days, and full design strength arrives near 28 days. Keeping the slab moist during the first week produces stronger concrete.