Concrete Calculator – How Much Concrete You Need and What to Buy
Ordering concrete is one of the few DIY jobs where a small arithmetic mistake is expensive either way. Too little and you are left with a half-finished slab and a cold joint; too much and you are paying for a truck load you can't use. This concrete calculator works out the volume of slabs, footings, round and square columns, solid stairs and curb & gutter, adds a waste allowance, and then turns the total into something you can actually buy: a number of pre-mix bags or a ready-mix order in cubic yards or cubic metres.
How Concrete Volume Is Measured
Concrete is sold by volume, so every job comes down to length × width × depth. A slab 10 ft long, 10 ft wide and 4 in thick is 10 × 10 × 0.333 = 33.3 ft³, or about 1.23 yd³. The main trap is mixed units: plans give lengths in feet and thicknesses in inches. The calculator lets you pick a unit for every single measurement and converts everything with exact factors (1 in = 25.4 mm), so you never have to divide by 12 in your head.
Other shapes follow the same idea. A round pier is π × (d/2)² × height. Solid stairs poured on grade are a stack of blocks, one per riser, plus an optional landing at the top. A curb and gutter is an L-shaped cross-section (the curb plus the gutter flag) multiplied by its length. Add each part of the job as its own pour, use the quantity field for identical pieces such as six deck piers, and the tool adds them up.
Why Concrete Is Sold by the Cubic Yard (or Cubic Metre)
In the United States, ready-mix plants batch and price concrete by the cubic yard: a cube 3 ft on each side, or 27 ft³. Most other countries use the cubic metre, which is about 1.31 yd³. Suppliers usually sell in steps such as a quarter of a yard, and many charge a short-load fee for small orders. The calculator rounds your total up to the order increment you enter, so the number you read out on the phone is the number you'll be billed for.
How Much Waste to Allow
No pour uses exactly the theoretical volume. Subgrade is never perfectly flat, trenches get over-dug, forms bulge under the weight of wet concrete, and some always ends up on the ground or left in the mixer. A waste allowance of 5–10% is typical, and this calculator defaults to 10%. Much more than 20% usually means a measurement is wrong rather than a genuinely messy job.
Bags or Ready-Mix?
Pre-mix bags are convenient for post holes, steps and small pads. A typical 80 lb bag of standard concrete mix yields about 0.60 ft³, so a single cubic yard takes 45 of them, which is over 3,500 lb of material to lift and mix by hand. As a rule of thumb, above roughly a cubic yard most people either rent a mixer or order ready-mix. Enter your local prices, including any delivery or short-load fee, and the calculator compares the options side by side. It rounds only once, on the final total, so several small pours never add up to extra bags you don't need.
Measuring Tips for an Accurate Estimate
- Measure slab depth in several places. Check the corners and the middle of the excavation; a slab that is nominally 4 in thick can easily average 4.5 in.
- Measure inside the forms, not the outside of the boards.
- Count risers, not treads, for stairs, and include any top landing.
- Check bag labels. Yields vary by product, especially for fast-setting and high-strength mixes; use the custom bag option with the yield printed on your bag.