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Concrete Calculator

Estimate concrete cubic feet, cubic yards, waste allowance, bag count, and optional ready-mix cost for rectangular slabs and footings.

  • concrete yards
  • concrete bags
  • slab concrete
  • footing concrete
  • ready mix

Choose the shape that best matches the pour.

For rectangular pours, enter length in feet.

For rectangular pours, enter width in feet.

For round columns or post holes, enter diameter in inches.

Enter slab thickness, footing depth, or post-hole depth in inches.

Optional number of identical pours.

Optional extra percentage for overage, spillage, and uneven subgrade.

Choose a common pre-mix bag size for bag estimates.

Optional ready-mix price for a rough cost estimate.

Optional bag price for a rough bagged-concrete cost estimate.

Status: initial

Results

Awaiting calculation

Explore your result

Concrete order plan

Explore the concrete calculator result visually while keeping this page's detailed guidance and examples.

Concrete order plan
Calculated2.8 yd³
With waste3.1 yd³
Ready mix3.25 yd³
Bags140 bags

Use this result well

  1. 1Verify the calculator inputs.
  2. 2Compare the key result relationships.
  3. 3Review the page guidance before acting.

Account for excavation variation, spillage, and supplier minimums.

Calculator guide

Introduction

The Concrete Calculator estimates concrete volume for rectangular slabs, pads, footings, and round post holes or columns from project dimensions.

What is a concrete calculator?

A concrete calculator converts project dimensions into cubic feet, cubic yards, bag estimates, rounded ready-mix ordering guidance, and optional ready-mix or bagged-concrete cost so users can plan before ordering materials.

Concrete formula

For a rectangular pour, multiply length by width by thickness in feet. For a round pour, use pi times radius squared times depth. Divide cubic feet by 27 to convert to cubic yards, then add waste allowance.

Variable explanations

Understand what each input and result means before calculating.

Project shape

Choose rectangular for slabs, pads, walls, and footings, or round for columns and post holes.

Length

The long side of the rectangular slab, pad, or footing.

Width

The short side or cross-section width.

Diameter

The width across a round post hole, pier, or column.

Thickness or depth

The slab thickness, footing depth, or post-hole depth in inches.

Waste

Extra allowance for subgrade, forms, spillage, and ordering margin.

Bag size

A common pre-mix bag size used for the main bag estimate.

Price per bag

Optional price used for a rough bagged-concrete cost estimate.

Reviewed by the Calculator.org.in Editorial Team

Formula behavior, validation cases, explanatory examples, and cited sources are checked before publication. This review supports educational accuracy and is not a substitute for qualified professional advice.

Last reviewed: 2026-08-02

Review process

Formula guide

See the calculation logic, variable definitions, and practical meaning.

Cubic feet

Cubic Feet = Length(ft) x Width(ft) x Thickness(ft)

  • Thickness in inches is divided by 12 first.

This estimates the volume of one rectangular pour.

Cubic yards

Cubic Yards = Cubic Feet / 27

  • One cubic yard equals 27 cubic feet.

Ready-mix concrete is commonly estimated in cubic yards.

Round post hole or column

Cubic Feet = pi x (Diameter(ft) / 2)^2 x Depth(ft)

  • Diameter and depth are converted to feet before calculation.

This estimates cylindrical concrete volume for piers, columns, or post holes.

Bag estimate

Bags = Cubic Feet with Waste / Bag Yield

  • Bag yields are approximate and depend on product and mixing.

Use this only as a planning estimate for bagged concrete.

Ready-mix order rounding

Suggested Order = cubic yards with waste rounded up to nearest 0.25 yd^3

  • Supplier rounding and minimum order rules can vary.

Rounding up provides a practical ordering number instead of only the exact calculated volume.

Worked examples

Follow realistic inputs through the calculation step by step.

1

Worked example

Small slab

  1. 1Length is 10 ft.
  2. 2Width is 10 ft.
  3. 3Thickness is 4 in.
  4. 4Volume is 1.23 cubic yards before waste.
2

Worked example

Waste allowance

  1. 1Base volume is 1.23 cubic yards.
  2. 2Waste is 10%.
  3. 3Plan for about 1.36 cubic yards.
3

Worked example

Bagged concrete

  1. 1Choose a bag size.
  2. 2Divide cubic feet with waste by approximate bag yield.
  3. 3Round up to whole bags.
4

Worked example

Round post hole

  1. 1Choose round column, pier, or post hole.
  2. 2Enter the hole diameter and depth in inches.
  3. 3Enter the number of identical holes.
  4. 4Review the bag breakdown for 40 lb, 60 lb, and 80 lb bags.
5

Worked example

Ready-mix ordering

  1. 1Enter the price per cubic yard if known.
  2. 2Review cubic yards with waste.
  3. 3Use the rounded order suggestion as a practical discussion point with the supplier.

Common mistakes

Avoid these common input and interpretation errors.

Forgetting thickness conversion

Thickness in inches must be divided by 12 before multiplying by feet.

Ordering exact volume

Concrete projects often need extra allowance for variation and waste.

Using bag estimates as exact yields

Bag yield varies by product, water, mixing, and placement.

Ignoring supplier minimums

Ready-mix suppliers may have minimum order or short-load fees.

Using rectangular dimensions for a round hole

Post holes and columns use a cylinder formula, not length times width.

Forgetting to round up bags

Bagged concrete should be rounded up to whole bags, and an extra bag may prevent a short pour.

Frequently asked questions

Quick answers to the questions users ask most often.

What does this Concrete Calculator calculate?
It estimates cubic feet, cubic yards, waste-adjusted cubic yards, bag count, rounded ready-mix order quantity, and optional ready-mix or bagged-concrete cost.
How do you calculate concrete for a slab?
Multiply length by width by thickness in feet, then divide cubic feet by 27 for cubic yards.
How many cubic feet are in a cubic yard?
There are 27 cubic feet in one cubic yard.
Why is thickness entered in inches?
Slab and footing thickness is commonly measured in inches, then converted to feet for volume.
What waste allowance should I use?
Many projects use 5% to 10% as a planning allowance, but actual needs vary.
Does this calculate post holes?
Yes. Choose the round project shape and enter the post-hole diameter and depth.
Does this calculate circular columns?
Yes. Choose the round project shape. The calculator uses pi times radius squared times depth.
How are concrete bags estimated?
The calculator divides waste-adjusted cubic feet by approximate bag yield and rounds up.
Are bag yields exact?
No. Bag yields are approximate and vary by product and mixing.
How many 80 lb bags are in a cubic yard?
Using an approximate 0.6 cubic foot yield per 80 lb bag, one cubic yard is about 45 bags before waste.
Why does the calculator show all bag sizes?
Showing 40 lb, 60 lb, and 80 lb bag counts helps compare store availability and handling weight.
Why round ready-mix to a quarter yard?
The exact calculated volume is useful, but many orders are discussed in rounded increments. Confirm the final quantity with the supplier.
Can I estimate ready-mix cost?
Yes, enter an optional price per cubic yard for a rough cost estimate.
Does this include delivery fees?
No. Delivery, short-load, pump, labor, reinforcement, and finishing costs are not included.
Can I use this for footings?
Yes for rectangular footings when length, width, and depth are known.
Can I use this for stairs?
No. Concrete stairs need a more specific shape-based calculator.
Should I order exactly the calculated amount?
Usually no. Confirm waste, supplier rounding, and job conditions before ordering.
Is this construction advice?
No. It is a planning calculator and not structural, engineering, or code advice.

References

Sources used to support the calculator guidance.

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Version history

A transparent record of calculator content updates.

Updated 2026-08-02
  • 1.0.0 · 2026-07-05

    Initial concrete slab and footing calculator release.

  • 1.1.0 · 2026-08-02

    Added round post-hole and column mode, all-size bag breakdown, ready-mix order rounding, bag cost estimates, and ordering guidance.