Whether you’re pouring a driveway, setting footings for a deck, or laying a slab for a garage floor, getting your concrete estimate right before ordering is one of the most important steps in any project. Order too little and you’re scrambling for an emergency delivery. Order too much and you’re paying for concrete that ends up in a dumpster.

Here’s a straightforward breakdown of how contractors and serious DIYers estimate concrete — and how to get it right every time.

The Core Formula

Concrete is measured in cubic yards. The formula for a basic rectangular pour is:

(Length x Width x Depth) / 27 = Cubic Yards

The division by 27 converts cubic feet to cubic yards (since 1 cubic yard = 27 cubic feet). You need to convert all your measurements to feet first — so if your slab is 4 inches thick, that’s 0.333 feet.

Example: A 10-foot by 20-foot driveway, 4 inches thick:

10 x 20 x 0.333 = 66.6 cubic feet

66.6 / 27 = 2.47 cubic yards

Always Add a Waste Factor

Real pours never go exactly to plan. Subgrade irregularities, spillage, and slight variations in form depth all eat into your material. The industry standard is to add 10% to your calculated volume.

So for the driveway above: 2.47 x 1.10 = 2.72 cubic yards. Round up to 2.75 or 3 when ordering.

For bag concrete (smaller jobs), divide your cubic footage by the yield of your bag size:

– 60 lb bag: yields approximately 0.45 cubic feet

– 80 lb bag: yields approximately 0.60 cubic feet

So a pour requiring 20 cubic feet would need roughly 44 bags of 60 lb, or 34 bags of 80 lb.

Footings and Columns

For rectangular footings, use the same formula. For cylindrical piers or columns, the formula changes:

Volume = pi x radius squared x depth

A 12-inch diameter footing, 3 feet deep:

0.785 x 0.5 x 0.5 x 3 = 0.589 cubic feet (then divide by 27 for cubic yards)

Multiple footings? Calculate each one individually and add them together before factoring in waste.

Irregular Shapes

Most real-world projects aren’t simple rectangles. For L-shaped or irregular slabs, break the area into rectangles, calculate each section separately, and total them up. This is also why it’s worth sketching your project to scale before you start calculating — it’s much easier to catch measurement errors on paper than after you’ve already ordered.

Common Mistakes That Lead to Short Orders

Forgetting to convert inches to feet is the most common error — and it’s a costly one. A measurement entered as 4 (meaning 4 inches) instead of 0.333 feet will make your estimate 12 times too large. Double-check your units before running any calculation.

Skipping the waste factor is the second most common mistake. Even experienced contractors who know the formula sometimes skip the 10% buffer and end up short.

Use a Calculator to Double-Check

For any pour larger than a few bags, it’s worth running your numbers through a free concrete calculator before you call your supplier. A tool like the one at BuildCalc (buildcalc.launchyard.app/concrete) lets you enter length, width, and depth with a feet/inches toggle, and instantly outputs cubic yards, cubic feet, and bag counts for both 60 lb and 80 lb bags — useful for cross-checking your manual math before you commit to an order.

Final Tip: Order in Full Yards, Not Half-Yards

Ready-mix suppliers typically have a short-load fee for orders under 1 cubic yard. If your estimate comes to 0.85 cubic yards, it’s often cheaper to order 1.0 than to pay the short-load surcharge. Ask your supplier what their minimum is before finalizing your order.