Concrete Calculator

How the math works

Concrete is sold by volume. The calculator computes the volume of your shape, adds a waste allowance, then converts to the units you actually buy in: cubic yards for ready-mix delivery, or bags for hand-mixing.

For a rectangular slab, footing, or wall, volume is simply length × width × thickness. For a round column or sonotube it's π × radius² × height. A cubic yard is 27 cubic feet — the unit ready-mix is sold in — and each bag yields a published fraction of a cubic foot (an 80 lb bag makes about 0.60 ft³, so 45 bags fill a yard).

Worked example: a 12 × 20 ft patio slab

Say you're pouring a 12 × 20 ft patio at 4 inches thick with the standard 10% waste allowance. Four inches is 4/12 = 0.333 ft, so net volume is 20 × 12 × 0.333 = 80 cubic feet. Adding 10% waste makes it 88 ft³, and 88 ÷ 27 = 3.26 cubic yards.

• Ready-mix: plants batch in quarter-yard steps, so 3.26 rounds up to a 3.5 yd order — roughly $525–630 at a typical $150–180 per yard.
• Bags: 88 ÷ 0.60 ft³ per 80 lb bag = 146.7, so 147 bags. At home-center prices near $6.50–7.50 a bag that's $950–1,100 — and almost six tons of material to move twice.
• Weight in place: 80 ft³ × 150 lb/ft³ = 12,000 lb of finished concrete.
• Rebar: a 16-inch on-center grid takes 10 bars running the 20 ft length plus 16 bars running the 12 ft width — 392 linear feet, about twenty 20-ft sticks before lap splices.

That's the pour ready-mix exists for: half the cost of bags, and a 45-minute placement instead of an all-day mix.

Bags vs. ready-mix — the real decision

The crossover is about one cubic yard. Below a yard, bags are usually cheaper and you avoid the short-load fee that ready-mix suppliers charge for partial loads ($60-150 is typical). Above a yard, you're looking at 45+ bags — that's a full day of mixing, and the bag price plus your time loses to a ready-mix truck. The calculator flags which side of the line you're on.

Bag yields by size

Yields are from Quikrete and Sakrete published data sheets — an 80 lb bag makes nowhere near a cubic foot. The last column is a 10 × 10 ft, 4-inch slab with 10% waste (36.7 ft³):

Yields assume the bagged water ratio. Over-watered mix yields slightly more volume and significantly weaker concrete.

Ordering ready-mix: margin, PSI, and the 90-minute rule

• Order the rounded-up number. Plants batch in quarter-yard increments and the calculator already rounds up. Don't shave the order — running out mid-pour leaves a cold joint you can't fix.
• Specify strength and air. IRC Table R402.2 sets residential minimums from 2,500 psi (interior, negligible weathering) up to 3,500 psi with air entrainment for driveways and garage slabs in severe freeze-thaw exposure. Tell the dispatcher what you're pouring and they'll batch to it.
• Be ready when the truck is. ASTM C94, the standard your supplier batches to, calls for discharge within 90 minutes of batching or 300 drum revolutions. Have forms, crew, and wheelbarrow paths set — plants bill standby after a short free unload window.

A mixer truck carries 8–10 cubic yards; bigger pours mean multiple trucks — stagger deliveries so the second arrives as the first runs dry.

Common mistakes

• Forgetting waste. A perfectly screeded pour on a perfectly flat base might use 3% extra. Reality is 10%. Running short mid-pour means a cold joint.
• Entering thickness in inches. A 4-inch slab is 0.33 feet, not 4 feet. Use the unit toggle and the inch hint.
• Ignoring the short-load fee. A 0.5-yard order can cost more than a full yard once the small-load surcharge lands.
• Sizing rebar by spacing alone. This tool gives grid length, not bar diameter. Structural slabs need an engineer's call on bar size and cover.
• Pouring below code-minimum thickness. IRC R506.1 sets 3.5 inches as the code floor for slab-on-ground concrete; 4 inches is the practical standard, 5–6 inches where vehicles load the slab.

When this calculator is the wrong tool

Use a structural reference for: suspended slabs and decks (loads and reinforcement governed by code), post-tensioned slabs, complex curved forms, and stamped or colored concrete (which changes the cost math entirely). This tool covers volume for flatwork and simple forms.

Sources & how we keep this current

The constants behind this calculator are few, public, and easy to verify:

• Quikrete and Sakrete product data sheets — bag yields of 0.30, 0.375, 0.45, and 0.60 ft³ for 40, 50, 60, and 80 lb bags, printed on every bag and spec sheet.
• Portland Cement Association — the 150 lb per cubic foot density for normal-weight concrete used in the weight figure.
• IRC R506.1 and Table R402.2 — the 3.5-inch minimum slab thickness and the 2,500/3,000/3,500 psi strength minimums by weathering exposure cited above.
• ASTM C94 — the ready-mixed concrete standard behind the 90-minute discharge guidance and plant batching practice.
• ACI 302.1R — the concrete floor and slab construction guide behind the subgrade and waste-allowance advice.

Volume, bag counts, and rebar grid length are plain geometry on top of those constants. We re-check the yields and density against the manufacturer sheets and the code citations against the current IRC edition; local amendments govern.