Skip to content
TrueSquare
Open menu

Concrete Calculator

Enter the shape and size of your pour to get the concrete volume in cubic yards and cubic metres, plus how many pre-mixed bags it would take.

Formulas reviewed 13 November 2026 · See the formula

Concrete Calculator

Shape
ft
ft
in
Allowances
%

5–10 % is a common allowance for spillage, uneven subgrade and form deflection.

Bags
lb

Common sizes: 40, 50, 60, 80 lb (US) or 20, 25, 30 kg.

ft³

Leave blank to use a typical estimate (an 80 lb bag ≈ 0.60 ft³). The printed yield on your product is always more accurate.

Cost estimate (optional)
USD /yd³

Enter your local price to estimate cost. Prices vary by supplier and region.

USD each

Enter your local price to estimate cost. Prices vary by supplier and region.

Results

Concrete to order (incl. waste)
1.36 yd³
36.7 ft³ · 1.04 m³
Pre-mixed bags
62 bags
61.1 bags before rounding up
Net volume (no waste)
1.23 yd³33.3 ft³ · 0.94 m³
Approximate weight in placeBased on ≈2,400 kg/m³ (150 lb/ft³) for normal-weight concrete.
2.75 US tons2.49 tonnes
  • Bag yield is a typical estimate based on ≈0.60 ft³ per 80 lb (≈0.47 L per kg of dry mix). Check the yield printed on your product.

Estimates only. Prices are your own inputs in USD; material properties vary by product and supplier.

About the concrete calculator

Ordering concrete is one of the few DIY steps where a mistake is expensive in both directions: run short and you risk a cold joint in the middle of a pour; over-order and you pay for material you have to dispose of. This calculator handles the four shapes that cover most residential work — rectangular slabs, footings and walls; round columns and post holes; hollow tubes; and solid steps.

Results are shown in both imperial and metric, so the same page works whether your supplier quotes by the cubic yard (US) or the cubic metre (UK, Canada, Australia). Bag counts use the yield printed on your bag when you enter it, or a clearly labelled typical estimate when you don't.

How to use it

  1. Choose what you are pouring: slab/footing/wall, round column, tube or steps.
  2. Enter the dimensions. Use the unit toggle to switch between feet/inches and metres/centimetres at any time — your values are converted automatically.
  3. Set how many identical pours you need (for example, 8 post holes).
  4. Adjust the waste allowance. 5–10 % is typical; use more on uneven ground.
  5. Optionally enter your bag size and the yield printed on the bag, and your local prices, to see bag counts and cost.

Formula

Rectangular slab, footing or wall

V = length × width × thickness

Round column or post hole

V = π × (diameter ÷ 2)² × height

Hollow tube

V = π × height × [(outer ÷ 2)² − (inner ÷ 2)²]

Solid steps (n steps)

V = width × run × rise × n × (n + 1) ÷ 2

Order quantity

V_order = V × quantity × (1 + waste % ÷ 100)

Bags

bags = ⌈ V_order ÷ yield per bag ⌉
  • All dimensions are converted to metres first, the volume is calculated in cubic metres, then converted: 1 yd³ = 27 ft³ = 0.764555 m³ (exact by definition of the international yard).
  • Bag counts are always rounded up — you cannot buy part of a bag.
  • Weight in place uses about 2,400 kg/m³ (≈150 lb/ft³), a common figure for normal-weight concrete. It is shown for planning lifting and hauling only.

Worked example: A 10 ft × 10 ft patio slab, 4 in thick

  1. Volume = 10 ft × 10 ft × (4 ÷ 12) ft = 33.33 ft³
  2. 33.33 ft³ ÷ 27 = 1.23 yd³ (0.94 m³)
  3. Add 10 % waste: 1.23 × 1.10 = 1.36 yd³ (36.67 ft³)
  4. 80 lb bags at 0.60 ft³ each: 36.67 ÷ 0.60 = 61.1 → 62 bags

Result: Order about 1.36 yd³ of ready-mix, or buy 62 × 80 lb bags.

Assumptions and limitations

  • Forms are straight and the sub-base is level. Uneven ground can add several percent to the volume.
  • Steps are modelled as solid blocks sitting on grade; a platform or landing should be added as a separate slab.
  • Typical bag yields (40 lb ≈ 0.30 ft³, 50 lb ≈ 0.375 ft³, 60 lb ≈ 0.45 ft³, 80 lb ≈ 0.60 ft³) are commonly printed on US pre-mix, but products differ. Metric bag yields are estimated from the same ratio (≈0.47 L per kg of dry mix).
  • Costs are only as accurate as the prices you enter. Delivery fees, short-load charges and pump hire are not included.

Frequently asked questions

How many 80 lb bags make a cubic yard?

At a typical yield of 0.60 ft³ per 80 lb bag, one cubic yard (27 ft³) takes 27 ÷ 0.60 = 45 bags. Check the yield printed on your product, because some mixes differ.

How much waste should I add?

5–10 % is a common allowance for spillage, slight over-excavation and form movement. Increase it for irregular excavations or when you cannot easily get a top-up delivery.

Should I use bags or ready-mix?

Bags are practical for small jobs such as post holes, small pads and repairs. Once a pour needs more than about a cubic yard (≈0.75 m³), ready-mix delivery is usually less work and gives a more consistent pour. Compare both using your local prices.

Does this calculator size my footing or slab?

No. It calculates the volume of the dimensions you enter. Required thickness, footing size and reinforcement depend on loads, soil and your local code, and should come from your plans or a qualified professional.

Why is my supplier's number slightly different?

Suppliers often round to the nearest quarter or half yard and may include their own overage. Use this result to sanity-check the quote rather than to replace your supplier's advice.

Sources and further reading

Results are estimates for planning purposes. See our disclaimer for details.