HowMuchIt / Concrete
How much concrete do I need?
Concrete is bought by volume: bags for small jobs, cubic yards from a truck for big ones. Measure the shape, pick the thickness, and the calculator turns it into bags or yards with a little extra for waste.
Work out your concrete
| Bag of ready-mix | Yield | Bags for one cubic yard (27 cu ft) |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 40 lb | 0.30 cu ft | 90 |
Slab volume in cu ft = length x width x thickness in inches / 12. Add about 10 percent.
Bags or a truck?
An 80 lb bag fills a little more than half a cubic foot, so a cubic yard takes about 45 of them, mixed one at a time. That is fine for a few post holes or a small landing, and hard work for a patio. Past a cubic yard or so, many people compare it with a ready-mix delivery, sold by the cubic yard. The calculator gives both numbers so you can compare at your own prices.
Thickness, the number people forget
A slab four inches thick needs a third more concrete than one three inches thick, so the thickness changes the order more than any other number. Patios and walkways are usually poured four inches thick, and so are driveways for cars; a driveway for heavy trucks needs more, and that is a job for a contractor. Bagged mix is made for pours of two inches or more. For post holes, the usual rule digs about a third of the post's length into the ground, in a hole about three times as wide as the post.
Before you pour
- Order a little extra: a base that is not perfectly flat swallows more than the drawing says.
- Check local rules for frost depth and permits before you dig footings.
- Wear gloves and eye protection: wet concrete burns skin.
Good to know
- One cubic yard equals 27 cubic feet; divide cubic feet by 27 to get cubic yards.1,2,3
- An 80 lb bag of ready-mix concrete yields about 0.60 cubic feet.4,5
- A 60 lb bag of ready-mix concrete yields about 0.45 cubic feet.4,6
- A 40 lb bag of ready-mix concrete yields about 0.30 cubic feet (about 0.011 cubic yards).4,7
- Bagged concrete mix is meant for pours at least 2 inches thick.4,5
- A typical home patio or walkway slab is 4 inches thick (walkways at least 4 inches).2,8,9,10
- A driveway for passenger cars is typically 4 inches thick; heavier vehicles call for a thicker slab.11,12,13
- Order about 10 percent extra concrete for spillage, waste and an uneven base.3,12,14
- For a fence or deck post, dig the hole about one-third of the post's overall length deep (for example 3 ft of a 9 ft post), and about 3 times the post's width across.4,15,16,17
How we checked this page
Checked on October 26, 2026. We kept a fact only when two independent publishers agree, and each one is numbered to its sources below. Where sources give different numbers, we keep only what they share. We never write a price. This page was written with an AI assistant and checked against the sources below.
Sources
- Cubic Yard - CADC Construction Glossary, Auburn University
- Concrete Calculator - Quickly Estimate How Much You Need, ConcreteNetwork.com
- How to Calculate Concrete Yardage, Vertigraph, Inc.
- Data Sheet - Concrete Mix No. 1101, Quikrete
- Sakrete High-Strength Concrete Mix 80 lb Gray, Oldcastle APG (Sakrete)
- Sakrete High-Strength Concrete Mix 60 lb Gray, Oldcastle APG (Sakrete)
- How Many Concrete Bags Do You Need? 7 Steps to Calculate, Ernest Maier
- Concrete Sidewalks and Small Slabs, Quikrete
- Video: Pouring Concrete Slabs, Quikrete
- What is the Minimum Thickness for Concrete?, Extreme How-To
- How Thick Should a Concrete Driveway Be?, ConcreteNetwork.com
- How Thick Should a Concrete Driveway Be?, ConcreteNetwork.com
- How Thick Should Your Concrete Driveway Be?, Port Aggregates, Inc.
- Concrete Volume Calculator, Concrete Supply Co.
- Fence Post Installation, This Old House
- Quikrete Quantity Calculator, Quikrete
- How Deep Should I Dig Fence Posts?, HGTV