How many bags of concrete do I need?

80 lb bag

0.60 ft³QUIKRETE Concrete Mix TDS

60 lb bag

0.45 ft³QUIKRETE Concrete Mix TDS

40 lb bag

0.30 ft³QUIKRETE Concrete Mix TDS

Bags per cubic yard

45 (80 lb)27 ft³ ÷ 0.60 ft³

Waste allowance

5–10%typical practice

Short version

• Volume in cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12.
• Bags needed = volume in cubic feet ÷ the yield printed on that specific bag.
• Yields are manufacturer figures and they vary by formulation — read the bag you are actually buying.
• 45 bags of 80 lb, 60 bags of 60 lb, or 90 bags of 40 lb make one cubic yard.
• Once you cross about a cubic yard, ready-mix is usually cheaper and far less work than bags.

How do you calculate the volume of concrete you need?

Volume comes first, bags second. Multiply length in feet by width in feet, then multiply by thickness in feet — which means dividing the thickness in inches by 12. A 10 ft × 12 ft patio at 4 inches is 10 × 12 × 0.333, or 40 cubic feet. Divide by 27 to convert to cubic yards if you want to compare against a ready-mix order: 40 ÷ 27 = 1.48 cubic yards.

Break irregular shapes into rectangles and circles and add the pieces. For a circle, area is π × radius² in feet, then multiply by thickness in feet. Measure the actual excavation rather than the plan dimensions — a subgrade that dips an inch across a patio adds real volume that the drawing never shows.

For reference, one cubic yard of concrete covers 81 ft² at 4 inches, 65 ft² at 5 inches, 54 ft² at 6 inches, and 41 ft² at 8 inches.

How many bags of concrete make a cubic yard?

Forty-five 80 lb bags make one cubic yard of standard concrete mix. That comes straight from the arithmetic: a cubic yard is 27 cubic feet, and QUIKRETE’s technical data sheet for its standard Concrete Mix puts an 80 lb bag at approximately 0.60 cubic feet. Sakrete lists the same 0.60 ft³ for its 80 lb High-Strength Concrete Mix and 0.45 ft³ for the 60 lb bag.

Yields are manufacturer data, not a standard, and they move with the formulation. Sakrete’s lightweight MAXIMIZER mix yields about 0.92 ft³ from an 80 lb bag — roughly half again as much as a conventional mix from the same bag weight. Never assume a yield from bag weight alone; the number is printed on the bag and published in the data sheet.

How many bags do you need for a fence post or a small footing?

A post hole takes fewer bags than most people expect, but the geometry has to be right. Volume of a round hole is π × radius² × depth, all in feet. A 10-inch-diameter hole 2 feet deep is 3.14 × 0.42² × 2 ≈ 1.1 cubic feet, which is two 80 lb bags per hole. Subtract the volume of the post itself if it is a large timber; for a metal post the difference is not worth chasing.

Square pad footings are simpler: length × width × thickness, all converted to feet. A 24″ × 24″ × 12″ pad is 2 × 2 × 1 = 4 cubic feet, or seven 80 lb bags. Count the holes, multiply, and round up — a job stopped halfway for a hardware run leaves a cold joint you did not plan for.

General information, not engineering advice. Structural work should be designed by a licensed engineer.

When should you order ready-mix instead of buying bags?

The practical crossover is around one cubic yard. Forty-five 80 lb bags weigh 3,600 pounds, need a mixer and a crew, and have to be batched consistently one after another. Above that, a ready-mix delivery is normally cheaper per yard, faster, and more uniform, since the whole placement comes from one batch instead of forty-five separate ones.

Bags stay the better choice for post holes, small pads, curb repairs, and anywhere a truck cannot reach or a job runs in pieces across several days. Suppliers set their own minimum loads and short-load charges — there is no national standard for either, so call two or three plants before deciding. NRMCA’s guidance on ordering ready mixed concrete covers what information the plant needs from you when you do.

Common mistakes

• Reading yield off bag weight instead of the data sheet — lightweight and high-yield formulations break the usual ratio and leave you short or long.
• Ordering the exact calculated quantity — subgrade dips, spillage, and over-excavated edges routinely eat 5 to 10 percent more than the math predicts.
• Forgetting to convert inches to feet — using 4 instead of 0.333 for a 4-inch slab overstates the volume twelvefold.
• Buying bags for a job over a cubic yard — mixing forty-five bags by hand produces uneven batches and cold joints between them.
• Mixing brands or formulations mid-pour — color, strength, and finish all shift where the two mixes meet.