BuildFigure

Concrete Calculator

Estimate concrete for slabs, footings, columns and stairs.

Shape

Depth of the slab or footing.

Number of identical elements (e.g. 4 columns).

The primary result reflects this choice.

Concrete can’t be re-ordered mid-pour. 10% is recommended for small jobs; 5% for large form-controlled pours.

Enter your dimensions to see how much concrete you need.

Shop 80 lb concrete mix →

BuildFigure — Concrete estimate
·

Estimate only — verify quantities before ordering.

How much concrete do I need?

To figure out how much concrete you need, multiply length × width × thickness (converted to feet) to get cubic feet, divide by 27 for cubic yards, then add ~10% for waste. For example, a 10 ft × 10 ft × 4 in slab needs about 1.36 cubic yards. That works out to roughly 62 × 80 lb bags — but nobody hand-mixes that many: above about 1 cubic yard, ready-mix delivery is almost always cheaper and easier than bagged mix. Bagged mix makes sense for small pours (a few dozen bags at most). Enter your dimensions above for an instant, waste-adjusted estimate — in bags and ready-mix yards — that you can print or share.

How we calculate concrete

  • Slab / footing: volume (cu ft) = length × width × thickness — all in feet
  • Column / round: volume (cu ft) = π × (diameter ÷ 2)² × height
  • Stairs: volume (cu ft) = width × run × rise × [ n(n+1) ÷ 2 ]
  • Cubic yards = cubic feet ÷ 27
  • With waste = volume × (1 + waste%) · Bags = ⌈ volume(cu ft) ÷ bag yield ⌉
  • Bag yields: 80 lb = 0.60 cu ft, 60 lb = 0.45 cu ft, 40 lb = 0.30 cu ft

How to use this calculator

  1. Pick a shape. Choose slab, footing, column or stairs.
  2. Enter dimensions. Type length, width and thickness and pick each unit (ft, in, yd, m).
  3. Set waste. Keep the default 10% waste, or adjust it for your pour.
  4. Read your estimate. See cubic yards, 40/60/80 lb bag counts and the ready-mix order, then print or share it.

Tips & real-world notes

  • Above ~1 cubic yard, order ready-mix — hand-mixing dozens of bags isn’t realistic. Bagged mix is for small pours only.
  • Over-order for uneven or soft subgrade — a slab on a rutted base uses more than its nominal thickness.
  • Ready-mix has minimum loads and short-load fees; below ~1 cu yd, bagged mix is often cheaper.
  • Bags are heavy: 45 × 80 lb bags ≈ 3,600 lb per cubic yard — check DIY feasibility.
  • Add rebar or wire mesh and plan control joints — not part of the volume, but part of the job.

Frequently asked questions

How many bags of concrete are in a cubic yard?
One cubic yard equals about 45 × 80 lb bags, 60 × 60 lb bags, or 90 × 40 lb bags of pre-mixed concrete. That many bags is also why anything over about a yard is usually a ready-mix order.
How much concrete do I need for a 10×10 slab?
A 10 ft × 10 ft slab at 4 inches thick is about 1.23 cubic yards, or 1.36 cubic yards with 10% waste. That’s roughly 62 × 80 lb bags — enough that ready-mix delivery (order 1.5 cu yd) is the practical choice.
How much concrete for a 24×24 garage slab?
A 24 ft × 24 ft slab at 4 inches is about 7.1 cubic yards, or 7.8 cubic yards with 10% waste — order about 8 cubic yards of ready-mix. At that size bagged mix (350+ bags) isn’t realistic; this is a ready-mix pour, often with a thicker 5–6 in edge.
How much concrete for a 20×20 slab?
A 20 ft × 20 ft slab at 4 inches is about 4.94 cubic yards, or 5.43 with 10% waste — order roughly 5.5 cubic yards of ready-mix. Go up to 5–6 inches for driveways or heavy loads.
How much concrete per fence or deck post?
Use the Column / round shape with your hole diameter and depth — that also covers sonotube and post-hole (tube form) pours. A typical 8 in diameter hole 2 ft deep is about 0.7 cu ft, so one 80 lb bag nearly fills it; buy one to two bags per post.
What waste factor should I use for concrete?
Use 10% for small residential jobs and uneven subgrade, and 5% only for large, form-controlled pours. Concrete can’t be re-ordered mid-pour, so it’s safer to slightly over-order.
Should I use bags or ready-mix?
Below about 1 cubic yard, bagged mix is usually cheaper and easier. Above that, ready-mix delivery saves a lot of mixing — watch for minimum-load and short-load fees.
How thick should a concrete slab be?
Patios and walkways are typically 4 inches; driveways and heavier loads often 5–6 inches. Thicker slabs use proportionally more concrete.
How much does a bag of concrete cover?
An 80 lb bag yields about 0.60 cu ft, a 60 lb bag about 0.45 cu ft, and a 40 lb bag about 0.30 cu ft of mixed concrete.

Shop materials

BuildFigure may earn a commission from these links, at no cost to you.