BCBuildCalc
BuildCalc Project Guide

Concrete Quantity Guide

How to estimate concrete for slabs, footings, post holes and bagged mixes, with formulas, waste allowances and worked examples

In this guide
  1. Start with volume
  2. Allow for real site conditions
  3. Footings and strips
  4. Post holes
  5. Bagged concrete
  6. Regional and ordering checks

Start with volume

Concrete is normally estimated by volume. For a rectangular pour, multiply finished length by width by depth using compatible units.

volume = length × width × depth

A 5 m × 4 m slab at 100 mm thick is 5 × 4 × 0.1 = 2.00 m³ before allowance.

Allow for real site conditions

Excavations are rarely perfectly rectangular. Uneven formation, over-dig, spillage and level corrections can increase the quantity placed. A contingency is therefore useful, but it should reflect the job rather than being applied blindly.

Worked example: 2.00 m³ plus 7.5% = 2.15 m³. If ordering ready-mix, discuss practical order increments and minimum loads with the supplier.

Footings and strips

Strip footings use the same rectangular-volume relationship. Measure the actual trench length, average width and intended concrete depth. Where trench width varies, split the footing into sections and add their volumes.

Post holes

For round post holes, calculate the cylinder volume using π × radius² × depth. Where useful, subtract the portion occupied by the post. Irregular augered or hand-dug holes may require more concrete than ideal geometry suggests.

concrete = hole volume − displaced post volume

Bagged concrete

Bag count depends on the yield of the exact dry mix. Divide required concrete volume by the manufacturer-stated yield per bag, then round up to a whole bag. Do not assume every 20 kg, 25 kg, 60 lb or 80 lb product has the same yield.

Regional and ordering checks

BuildCalc supports the United Kingdom, United States, Canada, Australia, New Zealand, Ireland and South Africa. The geometry is universal, but terminology, units, pack sizes and common product dimensions differ. Use the dimensions printed on the product you intend to buy rather than assuming a regional default is exact.

Before ordering, confirm excavation dimensions, reinforcement requirements, access, placing method and whether the supplier quotes wet volume, dry ingredients or product yield.

Use the related calculators

Use the calculator for your measurements, then return to this guide to check assumptions, allowances and ordering considerations.

Concrete slab calculatorFootings calculatorPost-hole concrete calculatorConcrete bags calculator
Important: BuildCalc provides indicative quantity estimates, not structural, regulatory or professional design advice. Check manufacturer data, site conditions and applicable local requirements before ordering or construction.