Concrete Calculator

Concrete Needed
1.36 yd³
36.7 cu ft incl. 10% waste · 1.04 m³

1Shape

A rectangular pad — patios, driveways, shed bases, sidewalks.

2Dimensions

4 in suits patios and walkways; use 5–6 in for driveways.

3Waste Factor

Covers uneven subgrade, over-excavation, and mixer residue. Running short mid-pour creates a weak cold joint — never order the exact volume.

Breakdown

Wet volume (exact)33.3 cu ft
Waste allowance+3.3 cu ft (10%)
Total volume to order36.7 cu ft
Cubic meters1.04 m³
80 lb bags (0.60 cu ft each)62 bags
60 lb bags (0.45 cu ft each)82 bags
Cubic yards needed1.36 yd³
Estimates only. Bag yields are for standard concrete mix (an 80 lb bag yields about 0.60 cu ft, a 60 lb bag about 0.45 cu ft); check your brand’s bag. At roughly one cubic yard and beyond, ready-mix delivery usually beats mixing bags by hand. Volumes exclude rebar, mesh, and gravel base — those are ordered separately.

What Is a Concrete Calculator?

Concrete is unforgiving arithmetic: order too little and the pour stops mid-slab, leaving a weak cold joint where the fresh batch meets half-set material; order too much and you pay to dispose of it. A concrete calculator converts your project’s dimensions — a slab, a trench footing, or round columns — into the volume you actually have to buy: cubic yards for a ready-mix truck, cubic meters for metric suppliers, and bag counts if you’re mixing by hand. The CalcFinity calculator adds a 10% waste factor by default and rounds bag counts up, because concrete is one purchase you never want to make twice in one day.

How It Works

Each shape reduces to a volume in cubic feet, with inch measurements divided by 12 to get feet first:

Volume by shape (cubic feet):

Slab = Length(ft) × Width(ft) × Thickness(in) ÷ 12
Footing = Length(ft) × Width(in)/12 × Depth(in)/12
Column = π × (Diameter(in)/24)² × Height(ft) × Count

The waste factor multiplies that by 1.10, and the total converts to ordering units — 27 cubic feet make a cubic yard. Bag counts divide the total by each bag’s yield and round up to whole bags:

Ordering quantities:

Cubic Yards = Total cu ft ÷ 27
Bags = roundup( Total cu ft ÷ 0.60 ) for 80 lb · roundup( Total cu ft ÷ 0.45 ) for 60 lb

Worked Example: A 10 × 10 ft Patio Slab, 4 Inches Thick

A 10 × 10 ft slab at 4 inches thick: 10 × 10 × (4 ÷ 12) = 33.3 cu ft of wet concrete. Adding the 10% waste factor brings it to 36.7 cu ft, which is 36.7 ÷ 27 = 1.36 cubic yards (about 1.04 m³). In bags, that’s 36.7 ÷ 0.60 = 61.1, rounded up to 62 eighty-pound bags — or 82 sixty-pound bags. These are the calculator’s default inputs, so the breakdown shows the same chain on load. Notice what 62 bags means physically: nearly 5,000 pounds of material to haul, hoist, and mix one wheelbarrow at a time.

Ready-Mix Truck or Bags?

The crossover point is right around one cubic yard. Below that — fence posts, deck piers, a small pad — bags are cheaper, simpler, and available the same afternoon. At a yard or more, call a ready-mix supplier: the truck delivers consistent, properly proportioned concrete faster than any crew can mix bags, and a continuous pour means no cold joints. Expect a short-load fee on small orders, since trucks are happiest delivering several yards at once; even with the fee, ready-mix usually wins on anything slab-sized. Our default patio at 1.36 yd³ sits firmly in truck territory — 62 bags is a full day of brutal mixing that a truck replaces in twenty minutes. Whichever route you choose, have forms, screed, and helpers ready before the concrete arrives; it won’t wait.

Common Mistakes to Avoid

The classic blunder is a units slip — multiplying 10 × 10 × 4 without converting the 4 inches to a third of a foot, which inflates the order twelvefold. The second is skipping the waste factor: subgrade is never perfectly level, forms bow, some mix stays in the mixer, and a 10% cushion is far cheaper than a same-day second delivery poured against a cold joint. Don’t trust a generic bag count either — yields vary by brand and mix type, so check the bag (0.60 cu ft for a typical 80 lb bag of standard mix). And remember the calculator prices concrete only: gravel base, rebar or mesh, forms, and expansion joints are separate line items that a slab still needs.

Frequently Asked Questions

How many bags of concrete make a cubic yard?

A cubic yard is 27 cu ft, so at 0.60 cu ft per 80 lb bag you need 45 bags — or 60 of the 60 lb bags. That’s about 3,600 pounds of material, which is why yard-scale jobs go to ready-mix trucks.

How thick should my slab be?

Four inches handles patios, walkways, and shed floors. Driveways and anything carrying vehicles should be 5–6 inches, over a compacted gravel base. Local codes may set minimums for footings and structural slabs.

Why add 10% waste — can I skip it?

You can toggle it off, but it’s rarely wise. Uneven subgrade, over-excavation, spillage, and residue left in the mixer all eat volume, and running out mid-pour creates a weak seam. Leftover concrete is a much smaller problem than missing concrete.

Does the calculator account for rebar or a gravel base?

No — it computes wet concrete volume only. Rebar displaces a negligible amount, so no adjustment is needed there, but gravel base, mesh, forms, and joints are separate materials you’ll buy on top of the concrete itself.

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