How Much Concrete Do I Need? Formulas for Slabs, Footings & Columns
The master volume guide for estimating concrete. Learn the core formula, convert inches to feet, and run real examples for slabs, footings, and Sonotube columns.
Ordering concrete is one of the few jobs where guessing costs you twice. Come up short and you scramble for a second truck or a cold joint in your pour; over-order and you pay for material that goes back as waste. The good news is that the math is simple once you lock in one habit: work in feet, find the volume in cubic feet, then divide by 27 to get cubic yards.
This guide walks through the single formula that covers slabs, footings, walls, and round columns, plus the πr²h variation for Sonotubes. We will run three worked examples with real numbers, show you when bagged mix beats a ready-mix truck, and cover the waste factor and measuring mistakes that trip up even experienced crews.
Every example uses US units and 2026 pricing. Round generously, order a little extra, and you will never be the one holding up a pour.
The Core Formula: Length × Width × Depth
Concrete is sold by the cubic yard, but you measure your project in feet and inches. The whole job comes down to one equation: Length (ft) × Width (ft) × Depth (ft) = cubic feet. Then divide cubic feet by 27 to get cubic yards, because one cubic yard is 3 ft × 3 ft × 3 ft = 27 cubic feet.
The catch is depth. Slabs and footings are almost always specified in inches, so you must convert inches to feet by dividing by 12. A 4-inch slab is 4 ÷ 12 = 0.333 ft deep. A 6-inch slab is 0.5 ft. Skip this step and you will overshoot your order by a factor of twelve, which is the single most common rookie error in concrete math.
Worked Example: A Concrete Slab
Say you are pouring a 20 ft × 12 ft patio at 4 inches thick. First convert depth: 4 ÷ 12 = 0.333 ft. Then multiply: 20 × 12 × 0.333 = 80 cubic feet. Divide by 27: 80 ÷ 27 = 2.96 cubic yards.
Now add waste. A standard 10 percent factor brings you to 3.26 cubic yards, so you would order 3.5 yards to be safe (suppliers sell in quarter- or half-yard increments). At a 2026 ready-mix price of roughly $145 to $170 per cubic yard, that slab runs about $510 to $600 in concrete, before any short-load fee for ordering under the typical 4- to 5-yard minimum.
Worked Example: A Footing
Footings are just long, narrow slabs, so the same formula applies. Suppose you have a continuous footing 40 ft long, 16 inches wide, and 8 inches deep. Convert both the width and depth to feet: 16 ÷ 12 = 1.333 ft, and 8 ÷ 12 = 0.667 ft.
Multiply: 40 × 1.333 × 0.667 = 35.6 cubic feet. Divide by 27: 35.6 ÷ 27 = 1.32 cubic yards. Add 10 percent for waste and over-dig (trenches rarely hold an exact width) and you land near 1.45 yards, so order 1.5 yards. Trench footings often need extra because the soil walls bulge, so do not shave this number.
Worked Example: A Round Column or Sonotube
Round forms use the cylinder formula: Volume = π × r² × h, where r is the radius (half the diameter) and h is the height, both in feet. For a 12-inch Sonotube standing 9 ft tall, the radius is 6 inches = 0.5 ft.
Calculate: 3.1416 × (0.5)² × 9 = 3.1416 × 0.25 × 9 = 7.07 cubic feet. Divide by 27: 7.07 ÷ 27 = 0.26 cubic yards per tube. For four columns that is 1.05 cubic yards, plus 10 percent waste equals about 1.16 yards. Round columns are where bagged mix often makes sense, since the totals stay small.
Bags vs. Ready-Mix and the Break-Even Point
Bagged concrete is convenient for small pours. An 80 lb bag yields about 0.6 cubic feet, and a 60 lb bag yields about 0.45 cubic feet. It takes roughly 45 of the 80 lb bags to fill one cubic yard (27 ÷ 0.6 = 45).
At a 2026 price near $6 per 80 lb bag, a full cubic yard of bagged mix costs about $270 in material alone, plus the labor of mixing 45 bags by hand. A ready-mix truck delivers the same yard for $145 to $170. The break-even point usually lands around 1 cubic yard, or roughly 40 bags. Below that, bags win on convenience; above it, ready-mix wins on cost and your back.
Waste Factor and Common Measuring Mistakes
- Always add 10 percent for waste. Spillage, uneven subgrade, form deflection, and slab thickness creep all eat into your order. For rough or hand-dug footings, bump it to 15 percent.
- Never mix units. Convert every inch dimension to feet before multiplying, and double-check the depth conversion first.
- Account for thickness creep. A slab poured slightly thick across a large area adds up fast. A 20 × 30 slab poured at 4.5 inches instead of 4 needs nearly 12 percent more concrete.
- Measure actual dimensions, not the plan. Trenches over-dig and forms bow outward under the weight of wet concrete.
- Round up, never down. Concrete trucks sell in quarter-yard steps. It is always cheaper to order a little extra than to pay a return-trip or short-load fee.
Frequently Asked Questions
- How many 80 lb bags of concrete are in a cubic yard?
- About 45 bags. Each 80 lb bag yields roughly 0.6 cubic feet of mixed concrete, and a cubic yard holds 27 cubic feet, so 27 ÷ 0.6 equals 45 bags. For 60 lb bags, which yield about 0.45 cubic feet, you need closer to 60 bags per cubic yard. Always add 10 percent for waste.
- Why do I divide by 27 to get cubic yards?
- Because one cubic yard is a cube measuring 3 feet on each side, and 3 × 3 × 3 equals 27 cubic feet. Concrete suppliers price and sell by the cubic yard, but you measure projects in feet, so you first find the volume in cubic feet and then divide by 27 to convert to the yards you actually order.
- How much extra concrete should I order for waste?
- Add 10 percent as a standard waste factor for most slabs and formed work. This covers spillage, uneven subgrade, and minor thickness variation. For hand-dug or rough trench footings where the soil walls bulge, bump it to 15 percent. Concrete is sold in quarter-yard increments, so round the final figure up rather than down.
- Is it cheaper to use bagged concrete or a ready-mix truck?
- For pours under about one cubic yard, bags are usually more convenient and competitive. Above that, ready-mix wins. A full yard of bagged mix costs around $270 in material plus heavy mixing labor, while a ready-mix truck delivers the same yard for $145 to $170. The break-even point sits near 1 cubic yard, or roughly 40 bags.