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Concrete Calculator

Add slabs and post holes for the cubic yards, bag count, and full materials list — plus an honest note when ready-mix beats bagged concrete.

Measurements
Bag size

Slabs / pads

Slab 1

Post holes(optional)

Waste allowance5%

Adjust to your site — we never pad quantities.

You'll need
59 bags
of 80 lb concrete mix

33.33 cu ft (1.3 cu yd) across 1 slab, 5% waste. Consider ready-mix delivery.

Materials list

Wire mesh reinforcement1 roll
~150 sq ft per roll · overlap seams by 6 in · lift off ground before pour
Heavy gloves + rubber bootsoptional
Wet concrete is highly alkaline — protect skin and feet.

Tools you'll want

Buy-once items — skip any you already own.

Ready-mix delivery (see note)
Job is ≈ 1.3 cu yd — ready-mix delivery is typically faster and cheaper than 59 bags above 1 cu yd.
Rebaroptional
For structural slabs and footings — size to local building code.
Mixing hoe or paddle mixer
Drill paddle mixer is faster for 5+ bags.
Form boards + stakes (2×4)optional
Frame the slab perimeter; oil boards before pouring.
Bull float or mag trowel
For finishing a flat surface.

Where to buy (optional)

Happy with your list? If it's handy to order online, pick a store and your items open there. These are affiliate links that support the site at no extra cost — but shop wherever you like.

Estimates only — verify quantities before buying.

How to calculate how much concrete you need

Concrete is a volume calculation. Slabs use L × W × thickness; post holes use the cylinder formula:

slab: cu ft = length × width × (thickness_in ÷ 12)
post hole: cu ft = π × (diameter_in ÷ 24)² × depth
cu yd = total cu ft × (1 + waste%) ÷ 27
bags = cu ft with waste ÷ yield per bag

An 80 lb bag yields 0.6 cu ft; a 60 lb bag yields 0.45 cu ft. Once the job exceeds about 1 cubic yard, ordering ready-mix is almost always faster and cheaper — the calculator flags that automatically.

Common mistakes

  • Mixing dozens of bags when ready-mix is cheaper — flag appears above 1 cu yd.
  • Under-estimating thickness — a 6 in slab uses 50% more concrete than a 4 in slab.
  • Not allowing enough cure time — driving on concrete too soon cracks the surface.
  • Skipping reinforcement required by local building code for structural work.

Frequently asked questions

How much concrete do I need for a 10×10 slab?+

A 10 × 10 ft slab at 4 inches thick is about 33 cubic feet, or 1.2 cubic yards with a 5% waste allowance. That works out to approximately 56 × 80 lb bags — which is why most slabs this size are poured more easily (and cheaply) as ready-mix.

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

One 80 lb bag yields about 0.6 cubic feet. A cubic yard is 27 cubic feet, so you need about 45 bags to fill one cubic yard. A 60 lb bag yields 0.45 cu ft, so it takes about 60 bags per cubic yard.

When should I order ready-mix instead of bags?+

Once your job exceeds about 1 cubic yard (roughly 45 × 80 lb bags), ordering ready-mix from a batch plant is almost always faster, less labour-intensive, and often cheaper. The calculator flags this automatically.

How thick should a concrete slab be?+

Four inches is standard for patios, walkways and shed floors. Driveways need at least 5–6 inches; areas under heavy equipment require 6 inches or more. Always check your local building code and add the reinforcement it specifies.

How much concrete do I need for a post hole?+

For a 10 in diameter, 3 ft deep hole: volume = π × (5/12)² × 3 ≈ 1.6 cubic feet, or about 3 × 80 lb bags. Enter the diameter and depth in the post holes section and the calculator totals all holes at once.

Do I need wire mesh in a concrete slab?+

Wire mesh is recommended for slabs that carry vehicles or heavy loads, or where cracking must be minimized. For light-duty patios and walkways it is optional but inexpensive insurance. Rebar is required by code in many structural applications.

How long does concrete take to cure?+

Concrete reaches walking strength in about 24 hours and driving strength in 7 days. Full structural strength develops over 28 days. Keep the slab moist (cover with burlap or plastic) for the first 7 days — it cures stronger when it dries slowly.

Related calculators

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Reviewed by the RenoSheets team for calculation accuracy. Method: volume from slab and cylinder geometry, bag counts from standard yields (80 lb = 0.6 cu ft, 60 lb = 0.45 cu ft), waste-adjusted. Last updated 2026-05-23. Estimates only — verify before purchasing.