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Concrete Calculator

Enter the length, width and thickness of your slab or footing to get the concrete volume in cubic yards (or m³) and the number of pre-mix bags. A 12 × 10 ft slab, 4 in thick, needs about 1.63 yd³ with 10% waste, which is 74 bags of 80 lb mix.

Units

A standard patio or shed slab is 4 in (10 cm). Driveways are usually 5–6 in.

For example, 4 footings of the same size.

Results

Concrete needed (incl. waste)
1.63 yd³
Pre-mix bags to buy
74

 

How to use the concrete calculator

  1. Choose Imperial (feet and inches) or Metric (metres and centimetres). The calculator converts your numbers if you switch.
  2. Enter the length and width of the area you are pouring.
  3. Enter the thickness. Use the finished thickness, not the depth of the hole, unless you plan to fill the full excavation.
  4. If you are pouring several identical pieces, such as footings or deck piers, set the number of identical pours.
  5. Set the waste allowance. Ten percent is a sensible default for most DIY jobs.
  6. Pick the bag size you plan to buy to see how many bags you need. Use Copy result to paste the numbers into a shopping list or a message to your supplier.

Formula

Concrete volume for a rectangular slab or footing is length × width × thickness. The calculator then adds your waste allowance and divides by the yield of one bag:

V      = L × W × T × N
V_order = V × (1 + w ÷ 100)
bags   = ⌈ V_order ÷ Y ⌉
  • L, W, T: length, width and thickness, converted to the same unit (the calculator works in metres internally).
  • N: number of identical pours.
  • w: waste allowance in percent.
  • Y: yield of one bag. An 80 lb bag yields about 0.60 ft³ and a 20 kg bag about 0.0093 m³. Check the label, because yields vary by brand and mix.
  • ⌈ ⌉: round up to the next whole bag.

One cubic yard equals 27 cubic feet, or about 0.7646 m³.

Worked examples

Example 1: a backyard patio slab

A patio measures 12 ft × 10 ft and will be 4 in thick.

  • Convert the thickness to feet: 4 ÷ 12 = 0.333 ft.
  • Volume: 12 × 10 × 0.333 = 40 ft³, which is 40 ÷ 27 = 1.48 yd³.
  • Add 10% waste: 1.48 × 1.10 = 1.63 yd³ (44 ft³).
  • In 80 lb bags: 44 ÷ 0.60 = 73.3, so buy 74 bags.

At that size, ask a ready-mix supplier for a quote. A truck delivers the whole slab in one pour, which gives a stronger result than mixing 74 bags in batches.

Example 2: four post footings

You need four footings, each 2 ft × 2 ft and 12 in deep.

  • One footing: 2 × 2 × 1 = 4 ft³. Four footings: 16 ft³.
  • With 10% waste: 17.6 ft³, or 0.65 yd³.
  • In 60 lb bags (0.45 ft³ each): 17.6 ÷ 0.45 = 39.1, so buy 40 bags.

Example 3: a metric shed base

A shed base is 4 m × 3 m and 10 cm thick.

  • Volume: 4 × 3 × 0.10 = 1.20 m³.
  • With 10% waste: 1.32 m³.
  • In 20 kg bags: 1.32 ÷ 0.0093 = 141.9, so buy 142 bags.

How to interpret the results

The volume is the amount to order from a ready-mix plant. Suppliers usually sell in steps of a quarter or half cubic yard, or 0.2 to 0.5 m³, so round up to the next step they offer. The bag count is for pre-mixed concrete that only needs water. If you mix your own from cement, sand and gravel, use the volume instead and work out each ingredient from your mix ratio.

If the bag count is above about 40 to 50, a truck delivery or a hired mixer is usually worth pricing. Mixing that many bags by hand is slow, and concrete that sets between batches forms weak joints.

Tips and common mistakes

  • Order extra. Add 5–10% for spillage, uneven ground and over-excavation. Running short mid-pour leaves a weak cold joint.
  • Measure thickness, not just area. A slab that is 4 inches on paper is often 4½ inches in places. Measure the depth of the forms at several points.
  • Convert units before multiplying. Mixing feet and inches is the most common error: 4 inches is 0.333 ft, not 0.4 ft.
  • Check the bag yield. An 80 lb bag makes about 0.6 cubic feet; a 60 lb bag about 0.45. For more than about 1 cubic yard, ready-mix is usually cheaper and stronger.
  • Don’t forget the base. Gravel sub-base, rebar or mesh, and edge forms are separate from the concrete volume.

Assumptions and limitations

  • The shape is a rectangle with a uniform thickness. Thickened slab edges, steps and haunches need to be added separately.
  • Bag yields are typical label values. Your brand may differ slightly.
  • Rebar, mesh and gravel sub-base are not included. A compacted gravel base of 4 in (10 cm) under slabs is common practice but is a separate material order.
  • The calculator estimates quantities only. It does not check structural strength, reinforcement or frost depth. Follow local codes and get engineering advice for load-bearing work.

Frequently asked questions

How many 80 lb bags of concrete make a cubic yard?

An 80 lb bag of standard pre-mix yields about 0.60 cubic feet. A cubic yard is 27 cubic feet, so you need 27 ÷ 0.60 = 45 bags per cubic yard before any waste allowance.

How thick should a concrete slab be?

Four inches (about 10 cm) is the usual minimum for patios, walkways and shed bases. Driveways that carry cars are commonly 5 to 6 inches. Follow your local building code and any engineer's drawings, which take priority over rules of thumb.

How much extra concrete should I order?

Most estimators add 5 to 10 percent. Use the higher end when the ground is uneven, the formwork is hand-built or you are mixing bags yourself, because spillage and over-excavation add up quickly.

When is ready-mix cheaper than bags?

For roughly one cubic yard or more, a ready-mix truck is usually cheaper and much less work than mixing 45 or more bags by hand. Many suppliers charge a short-load fee for small orders, so compare both prices for your area.

Does the calculator work for round columns?

This version handles rectangular shapes. For a round column, calculate the volume as π × radius² × height, then enter that as a slab with the same volume, or use an equivalent square of side 0.886 × the diameter.

Why does the bag count round up?

You cannot buy part of a bag, and running out halfway through a pour leaves a cold joint that weakens the slab. The calculator always rounds up to the next whole bag.

Sources