Concrete Calculator
Concrete volume and bags for slabs and columns.
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Work out how much concrete a job needs before you order. This calculator handles rectangular slabs and footings plus round columns, giving you cubic yards, cubic metres, and the equivalent number of bags — with a waste margin built in.
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How Much Concrete? — The Quick Answer
Concrete volume is length × width × thickness for a slab, or π × radius² × height for a column. Convert cubic feet to cubic yards by dividing by 27. Always add about 5–10% for waste.
Shortcut
Slab volume = L × W × T | Column volume = π × r² × h | Cubic yards = cubic feet ÷ 27
Quick slab volumes (4 inch thick)
| Slab size | Cubic feet | Cubic yards | 80 lb bags |
|---|---|---|---|
| 10 × 10 ft | 33.3 | 1.23 | 56 |
| 12 × 12 ft | 48.0 | 1.78 | 80 |
| 10 × 20 ft | 66.7 | 2.47 | 112 |
| 20 × 20 ft | 133.3 | 4.94 | 223 |
For anything more than about 1 cubic yard, ready-mix delivery is usually cheaper and easier than mixing dozens of bags. Bag counts here assume standard 80 lb (0.6 cu ft) or 60 lb (0.45 cu ft) bags.
How It Works
The Formulas
Volume by shape
Formula
Slab: V = L × W × T Column: V = π × r² × h Cubic yards = cubic feet ÷ 27
Variables
Length, width, thickness
Slab dimensions. Keep units consistent — this calculator takes length and width in feet and thickness in inches.
Radius, height
Column dimensions. The radius is half the diameter (in feet).
Cubic feet per cubic yard
Concrete is usually ordered by the cubic yard, and 1 cubic yard = 27 cubic feet.
Note: A standard 80 lb bag of concrete mix yields about 0.6 cubic feet; a 60 lb bag yields about 0.45 cubic feet. These are typical values — always check your bag's stated yield.
Worked Example: A 10 × 10 ft Slab, 4 Inches Thick
From dimensions to cubic yards and bags
Convert thickness to feet
4 in ÷ 12 = 0.333 ft
Multiply length × width × thickness
10 × 10 × 0.333 = 33.3 cu ft
Convert to cubic yards (÷ 27)
33.3 ÷ 27 = 1.23 cu yd
Add waste and count bags
+5% ≈ 1.3 cu yd ≈ 56 × 80 lb bags
Reference Guide
| Slab size (4 in thick) | Cubic feet | Cubic yards | 80 lb bags |
|---|---|---|---|
| 10 × 10 ft | 33.3 | 1.23 | 56 |
| 12 × 12 ft | 48.0 | 1.78 | 80 |
| 10 × 20 ft | 66.7 | 2.47 | 112 |
| 20 × 20 ft | 133.3 | 4.94 | 223 |
Ordering: Bags vs Ready-Mix
Turning the volume into a sensible order
Ground isn't perfectly level and some mix is always lost to spillage and over-filling, so add about 5–10% to the calculated volume. Running short mid-pour means a cold joint, so it's better to have a little spare.
Best for: Avoiding coming up short on the pour
A cubic yard is roughly 45 × 80 lb bags — a lot of mixing. Once a pour climbs past about a cubic yard, ready-mix delivery is usually cheaper, faster, and more consistent than bags.
Best for: Choosing between bags and a delivery
This tool works out the volume for the thickness you enter — it doesn't tell you how thick to pour. A path might be 4 inches, a driveway 5–6, and load-bearing work should follow an engineer's spec and local codes.
Best for: Separating 'how much' from 'how thick'
How to Work Out Concrete Volume
Concrete is ordered by volume, so the job is to find the volume of your slab, footing, or column and convert it to cubic yards (or cubic metres). For a slab, that's length × width × thickness, with the thickness converted to the same unit; for a round column it's π × radius² × height. Divide cubic feet by 27 to get cubic yards. Always add about 5–10% for waste, since ground is uneven and mix is lost to spillage. Below roughly a cubic yard, bags are practical (an 80 lb bag yields about 0.6 cu ft); above that, ready-mix delivery is usually the better choice. This calculator handles slabs and columns and converts to bags automatically.
Key Features
💡 Pro Tips
- →Measure twice — a small error in thickness scales across the whole slab area and changes the volume a lot.
- →Order 5–10% extra; it's far cheaper than stopping a pour to fetch more mix.
- →For a sub-base under the slab, estimate the hardcore separately with a gravel calculator.
- →Bag yields vary by product — check the stated cubic-feet-per-bag on your mix rather than assuming.
Common Mistakes
Forgetting to convert thickness to feet
Thickness is usually in inches but length and width in feet. Multiplying 10 × 10 × 4 (instead of × 0.333) overstates the volume twelvefold.
Ordering exactly the calculated amount
Uneven ground and spillage mean you need a little more than the ideal volume. Skipping the waste allowance risks running short and creating a cold joint.
Mixing dozens of bags for a large pour
Beyond about a cubic yard, bagged mix is slow and inconsistent. Ready-mix is usually cheaper and gives a more uniform result for bigger jobs.
Research & Citations
Sources and standards referenced for the factual claims on this page
- [1]
Portland Cement Association (2023). Concrete Basics and estimating resources. cement.org.
Reference on concrete proportioning, volume, and placement.
View source - [2]
American Concrete Institute (ACI) (2021). Guide to Residential Concrete Construction (ACI 332). ACI.
Guidance on residential slab thickness and construction.
View source
This calculator is a reference tool, not a substitute for professional advice. Verify important health, financial, legal, or safety decisions with a suitably qualified professional.
Last updated: October 7, 2026
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Frequently Asked Questions
How do I calculate how much concrete I need?
Multiply length × width × thickness to get the volume, then convert to cubic yards by dividing by 27 (for feet/inches measurements). A 10 × 10 ft slab, 4 inches thick, is 10 × 10 × (4/12) = 33.3 cubic feet, or about 1.23 cubic yards.
How many bags of concrete make a yard?
About 45 bags of 80 lb mix, or about 60 bags of 60 lb mix, per cubic yard. Since that's a lot of mixing, most slabs larger than a yard are poured with ready-mix delivery.
Should I add extra for waste?
Yes — a 5–10% allowance is standard. Ground isn't perfectly level and some mix is always lost to spillage and over-filling. This calculator adds your chosen percentage automatically.
How thick should a concrete slab be?
For a garden path or shed base, 4 inches is typical; for a driveway, 5–6 inches; for heavy loads, more. Thickness should always be set by a structural requirement — this calculator only works out the volume for the thickness you enter.