Concrete Calculator

How to use the concrete calculator

1. Choose Metric (m, cm, m³, kg) or Imperial (ft, in, cu yd, lb).
2. Under What are you pouring?, pick the shape: - Slab (patio, shed base, walkway): length, width and Thickness; - Footing or strip foundation: length, Footing width and Depth; - Round column or post hole (tube form): Diameter and Height or depth; - Stairs (solid steps): Stair width, Rise (height of one step), Run (depth of one tread) and Number of steps.
3. Set the Number of identical pieces (for example 6 deck post holes) and the Extra for waste and spillage (%).
4. Pick a Bag size. The Yield per bag is filled in from the manufacturer’s data; choose Other bag (enter its yield) or type over it for another product.
5. Read the Total volume, the Volume with extra, the Bags to buy (rounded up) and the Weight of the bags. The steps under the results show each multiplication. Reset defaults restores the example values.

All calculations run in your browser.

Concrete volume formulas

• Slab or footing: V = length × width × thickness.
• Round column: V = π × (diameter ÷ 2)² × height.
• Solid stairs: each step sits on the ground and is as tall as all the risers below it, so V = width × run × rise × n(n + 1) ÷ 2 for n steps. A landing or a thicker base is not included: add it as a slab.

Then bags = volume × (1 + extra %) ÷ yield per bag, rounded up.

Example: a 3 × 3 m slab, 10 cm thick, is 0.9 m³. With 10% extra, 0.99 m³; with 30 kg bags yielding about 0.0147 m³ each, that is 67.2, so 68 bags, about 2 tonnes to carry. In imperial units, a 10 × 10 ft slab, 4 in thick, is 33.3 cu ft (1.23 cu yd); with 10% extra and 80 lb bags yielding 0.60 cu ft, you need 62 bags. Four 10 in tube forms, 4 ft deep, hold 8.7 cu ft: 15 bags of 80 lb without any extra.

Bag yields used by this tool

The presets come from manufacturer data sheets for general-purpose concrete mix:

• QUIKRETE Concrete Mix (No. 1101) product data sheet: a 40 lb bag yields about 0.30 cu ft (8.5 L), a 60 lb bag about 0.45 cu ft (12.7 L), and an 80 lb bag about 0.60 cu ft (17 L).
• QUIKRETE Canada Concrete Mix (Target Products): a 30 kg (66 lb) bag yields 0.52 cu ft, about 14.7 L or 0.0147 m³.

As a rule of thumb, dry bagged concrete makes roughly 1 m³ per 2,000 to 2,200 kg of mix. Other brands and products (fast-setting, high-strength, 25 kg European bags) have different yields: read the bag and use Other bag (enter its yield).

Bags or ready-mix?

Mixing by hand or with a small mixer is practical up to about 1 cubic yard (0.76 m³, or 45 bags of 80 lb). From there on, the tool shows a reminder: ready-mix delivered by truck is usually cheaper, stronger and more uniform because it is poured in one go. Ready-mix is ordered by the cubic metre or cubic yard, often with a minimum load: give the supplier the Volume with extra.

Buying and pouring tips

• Keep 5–10% extra: excavations are rarely flat, forms bulge and some concrete stays in the mixer.
• Buy a few bags more than needed and return unopened ones; running short mid-pour leaves a weak cold joint.
• Store bags off the ground and dry. Check the weight you can safely carry and transport: 68 bags of 30 kg exceed the payload of many vehicles.
• Plan the gravel base, forms, reinforcement (mesh or rebar) and curing: they are not included here.
• Slab thickness, footing size and depth below the frost line are set by local building codes and, for structural work, by an engineer. The manufacturer’s instructions on water, mixing and curing prevail over this estimate.