Concrete Calculator

How to use the concrete calculator

Measure the slab’s length and width in feet, and set the thickness for the job: 4 inches is the residential standard for patios, walkways, and shed slabs; driveways and anything that carries vehicles want 5–6 inches; footings and pads for heavy equipment go thicker still. For irregular pours, break the shape into rectangles and add the volumes — and for round piers, this rectangular model isn’t the right tool, so size those off the tube label instead.

Leave the order margin at the recommended +10% unless you have a measured, formed, perfectly level excavation. Real subgrades are never flat: a quarter-inch of extra average depth across a 10 × 10 slab swallows two cubic feet by itself, and spillage, over-dig, and form deflection all eat in the same direction. Running out of concrete mid-pour is the worst outcome in DIY masonry — a cold joint in the middle of a slab is permanent. The margin is cheap insurance; "Exact volume" exists for comparing quotes, not for ordering.

The result leads with cubic yards because that’s the unit ready-mix plants quote and deliver. The bag count converts the same volume into sacks of pre-mixed concrete using the locked yields — 0.60 cu ft per 80 lb bag, 0.45 per 60 lb, 0.30 per 40 lb. Switch sizes under Advanced to match what you can actually lift and what your store stocks.

Here’s the decision the bag count is really for: at about one cubic yard, bags stop making sense. This calculator’s default slab needs 62 eighty-pound bags — very nearly 2.5 tons of material to load, haul, lift, mix one wheelbarrow at a time, and place before the first batches start setting. Ready-mix delivery costs more per yard on paper, but past a yard (some would say half a yard with no helpers) the truck wins on price-per-finished-slab, consistency, and your spine. Bags are for post footings, small pads, repairs, and pours a truck can’t reach.

Two scheduling notes that save pours: order ready-mix for the cubic-yard number rounded UP to the supplier’s increment (usually quarter yards, often with a short-load fee under 3–4 yards), and have help on site — concrete waits for no one once the drum starts turning.

The formula

Slab volume with an order margin, then a conversion to the yards ready-mix is sold in and the bags pre-mix is sold in:

cubic feet = length × width × (thickness ÷ 12) × margin   [1.10 recommended, 1.0 exact]
cubic yards = cubic feet ÷ 27
bags = cubic feet ÷ bag yield   [80 lb = 0.60 cu ft · 60 lb = 0.45 · 40 lb = 0.30]

Worked example with the defaults: a 10 × 10 ft slab at 4 inches is 33.33 cubic feet of concrete; with the +10% margin you order for 36.67 cubic feet — 1.36 cubic yards. In 80 lb bags that’s 36.67 ÷ 0.60 = 61.1, so 62 bags. With "Exact volume" selected the same slab reads 33.33 cu ft / 1.23 yd³.

Sixty-two 80 lb bags is about 4,960 lb of dry mix. That single number is usually the whole bags-vs-truck decision.

Frequently asked questions

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