MathBench

Concrete Calculator

Find out how much concrete you need (in cubic yards and in 60 lb or 80 lb bags) for a slab, footing, or round column, including a realistic waste allowance.

You need

—cubic yards (incl. waste)
—cubic feet (incl. waste)
—60 lb bags
—80 lb bags

How this calculator works

Pick the shape you're pouring (a rectangular slab, a trench footing, or round columns in tube forms) and enter its dimensions. The calculator converts everything to feet, multiplies out the volume, adds your waste allowance, and reports the total three ways: cubic yards (how ready-mix is sold), cubic feet, and the number of 60 lb or 80 lb bags of pre-mixed concrete it would take.

Thickness and small cross-section dimensions are entered in inches because that's how plans and suppliers describe them. The tool divides by 12 before multiplying, which is the step most people slip on when estimating by hand. Bag counts always round up to whole bags, because that's how the store sells them.

The formula

Volume, by shape (all dimensions converted to feet first):

Slab:     V = length × width × (thickness ÷ 12)
Footing:  V = length × (width ÷ 12) × (depth ÷ 12)
Column:   V = π × (diameter ÷ 24)² × height × count

Then the waste allowance and unit conversions:

Total ft³ = V × (1 + waste% ÷ 100)
Cubic yards = total ft³ ÷ 27
60 lb bags = total ft³ ÷ 0.45   (rounded up)
80 lb bags = total ft³ ÷ 0.60   (rounded up)

The bag yields (0.45 ft³ per 60 lb bag and 0.60 ft³ per 80 lb bag) are the approximate yields printed on standard pre-mixed concrete bags (e.g., QUIKRETE Concrete Mix product data). Different products vary slightly. Check your bag if the pour is tight.

Worked example

Say you're pouring a 10 ft × 10 ft patio slab, 4 inches thick, with the default 10% waste allowance:

  1. Convert thickness to feet: 4 ÷ 12 = 0.333 ft
  2. Volume: 10 × 10 × 0.333 = 33.3 ft³
  3. Add 10% waste: 33.3 × 1.10 = 36.7 ft³
  4. Cubic yards: 36.7 ÷ 27 = 1.36 yd³
  5. Bags: 36.7 ÷ 0.45 = 81.5 → 82 sixty-pound bags, or 36.7 ÷ 0.60 = 61.1 → 62 eighty-pound bags

At that many bags, this pour is better ordered as ready-mix: you'd ask the plant for 1.5 yd³ and be done in an hour.

Assumptions & tips

  • Waste isn't optional. Slabs on grade almost always take more concrete than the math says, because the subgrade is never perfectly flat: a low spot ½ inch deep across a 10×10 slab swallows an extra 4 ft³. Use 10% for slabs and footings, 5% for tube forms.
  • Running short is the expensive mistake. A second ready-mix delivery means a second delivery fee and a cold joint in your slab. Extra concrete, by contrast, costs a few dollars and fills a fence-post hole.
  • Check your subgrade depth, not your plan. Measure the actual depth from your form boards to the compacted base in several spots and use the deepest common value as your thickness.
  • Bags are for small pours. Past roughly a cubic yard, hand-mixing gets slow enough that early batches start setting before late ones go in. For anything bigger, price a ready-mix truck and ask about short-load fees for orders under 3–4 yards.
  • Footings may be spec'd by code. Width and depth for structural footings come from your local building department, not from a calculator. This tool tells you how much concrete the spec needs, not what the spec should be.

Frequently asked questions

How many bags of concrete make a cubic yard?

A cubic yard is 27 cubic feet. At the yields printed on the bag (about 0.45 cubic feet per 60 lb bag and 0.60 cubic feet per 80 lb bag), that works out to sixty 60 lb bags or forty-five 80 lb bags per cubic yard.

How thick should a concrete slab be?

For walkways and patios, 4 inches is the common standard. Driveways that carry passenger vehicles are usually 5 to 6 inches, and slabs for heavy equipment or trucks often run 6 inches or more with reinforcement. Local building codes and soil conditions can change these numbers, so check before you pour anything structural.

Should I mix bags or order a ready-mix truck?

A good rule of thumb is one cubic yard. Below that, bags are usually cheaper and simpler. Above a yard (roughly 45 or more 80 lb bags), mixing by hand becomes slow enough that cold joints between batches are a real risk, and a ready-mix truck is worth the delivery fee. Many suppliers charge a short-load fee for orders under 3 to 4 yards. Ask when you call.

What waste percentage should I use?

Ten percent is the standard allowance for slabs on grade, where an uneven subgrade absorbs more concrete than the math predicts. For pours into rigid forms with well-defined dimensions, such as tube forms, 5 percent is usually enough. Running out mid-pour is far more expensive than a little extra, so round up.

Sources

  1. Concrete Mix, Product No. 1101 — Product Data Sheet. The QUIKRETE Companies, revised October 2022. quikrete.comThe bag yields this calculator divides by: 0.45 ft³ per 60 lb bag and 0.60 ft³ per 80 lb bag.
  2. CIP 31 — Ordering Ready Mixed Concrete. National Ready Mixed Concrete Association. nrmca.orgAdvises ordering 4 to 10 percent more than the estimate from plan dimensions (the basis for the waste allowance) and gives the cubic-yard ordering unit and the 8 to 12 yd³ truck-mixer capacity behind the bags-versus-ready-mix guidance.
  3. CIP 8 — Discrepancies in Yield. National Ready Mixed Concrete Association. nrmca.orgWhy a pour swallows more concrete than its plan dimensions predict (irregular subgrade, over-excavation, spreading forms) and the worked case that a ⅛-inch error in a 4-inch slab is a 3 percent shortfall, which is the reasoning behind the subgrade-depth tip.
  4. ACI CODE-332-20: Code Requirements for Residential Concrete and Commentary. American Concrete Institute, 2020. concrete.orgThe residential standard covering footings (Chapter 8) and slabs-on-ground including thickness (Chapter 13): the specifications this calculator sizes concrete for.

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