Concrete Guide
Everything you need to estimate concrete quantities accurately — from volume math to how many bags to order.
Volume Basics
Concrete is ordered by the cubic yard. The core job is converting your project's length, width and thickness into cubic yards.
Cubic yards = (length × width × thickness) ÷ 27All three dimensions should be in the same unit, then converted to yards. A slab that is 10 ft × 12 ft × 4 in thick is 120 sq ft × 0.33 ft = 40 cu ft, or about 1.48 cubic yards.
From Volume to Bags
If you are mixing your own concrete, divide the total cubic feet by the coverage of one bag. An 80 lb bag yields roughly 0.6 cu ft; a 60 lb bag about 0.45 cu ft.
Bags = total cubic feet ÷ bag yield (cu ft per bag)Add Wastage
Concrete nearly always overruns. Add a wastage buffer before ordering ready-mix or buying bags so you do not run short mid-pour.
- Small jobs (slabs, patios): 5-10%
- Complex forms (columns, stairs, walls): 10-15%
- Always round up to the next full yard or bag when ordering.
Ready-Mix vs Bags
For slabs and footings over a cubic yard, ready-mix is usually cheaper per unit. For small pours, bags are practical and avoid delivery minimums.
Finishing Countdown
- Pour and screed
Level the surface as soon as it is placed.
- Finish with float and trowel
Wait for the bleed water to disappear, then float, and finally trowel.
- Cure
Cover with plastic or keep it damp for several days.
Related Calculators
Disclaimer: Figures on this page are general reference information. Always verify exact requirements with your project specifications and a qualified professional.