How Much Concrete Do I Need? A Simple Guide to Calculating Cubic Yards and Cubic Metres
Quick answer: Multiply length x width x thickness to get volume. In imperial, measure in feet and divide by 27 to get cubic yards. In metric, measure in metres and the result is cubic metres (m³). Then add 5 to 10% for uneven ground and spillage. Example: a 10 ft x 10 ft slab, 4 inches thick, needs about 1.23 cubic yards (0.94 m³) of concrete before the extra margin.
Ordering the wrong amount of concrete is one of the most common and expensive mistakes on any pour. Order too little and you risk a cold joint while you wait for more. Order too much and you pay for concrete that goes to waste. This guide walks you through the math, with examples for the projects GTA homeowners and contractors pour most.
What Is the Formula for Calculating Concrete?
Volume = Length x Width x Thickness
Imperial (cubic yards):
Measure length and width in feet.
Convert thickness from inches to feet (divide by 12).
Multiply the three numbers to get cubic feet.
Divide by 27 to get cubic yards.
Metric (cubic metres):
Measure length, width, and thickness in metres (100 mm = 0.1 m).
Multiply the three numbers. The result is m³.
Converting between them:
1 cubic yard = 0.765 m³
1 m³ = 1.308 cubic yards
In Ontario, concrete is commonly ordered in cubic metres, but many homeowners measure in feet. Either works. Just tell us which you used.
How Many Yards of Concrete Do I Need? Quick Reference Chart
Concrete needed per 100 square feet of slab:
4 in (100 mm): Cubic Feet: 33.3; Cubic Yards: 1.23; Cubic Metres: 0.94
5 in (125 mm): Cubic Feet: 41.7; Cubic Yards: 1.54; Cubic Metres: 1.18
6 in (150 mm): Cubic Feet: 50.0; Cubic Yards: 1.85; Cubic Metres: 1.42
To use it: divide your total square footage by 100, then multiply by the number in the chart. A 600 sq ft slab at 4 inches = 6 x 1.23 = about 7.4 cubic yards.
Worked Examples for Common GTA Projects
Driveway
20 ft x 40 ft, 5 inches thick
20 x 40 x (5 ÷ 12) = 333.3 cubic feet
333.3 ÷ 27 = 12.35 cubic yards (9.44 m³)
With a 5 to 10% margin: about 13 to 13.6 cubic yards
Patio
12 ft x 12 ft, 4 inches thick
12 x 12 x (4 ÷ 12) = 48 cubic feet
48 ÷ 27 = 1.78 cubic yards (1.36 m³)
Sidewalk or Walkway
3 ft x 30 ft, 4 inches thick
3 x 30 x (4 ÷ 12) = 30 cubic feet
30 ÷ 27 = 1.11 cubic yards (0.85 m³)
Garage Slab
24 ft x 24 ft, 4 inches thick
24 x 24 x (4 ÷ 12) = 192 cubic feet
192 ÷ 27 = 7.11 cubic yards (5.44 m³)
Metric Example
6 m x 12 m, 125 mm thick
6 x 12 x 0.125 = 9 m³ (11.77 cubic yards)
How Do I Calculate Concrete for Round Footings or Sonotubes?
For round forms, use: Volume = 3.1416 x radius² x depth
Example: 10 deck footings, 12-inch diameter, 4 ft deep
Radius = 0.5 ft
3.1416 x 0.5 x 0.5 x 4 = 3.14 cubic feet per tube
10 tubes = 31.4 cubic feet ÷ 27 = 1.16 cubic yards (0.89 m³)
How Thick Should My Concrete Be?
Thickness has the biggest effect on volume, so get it right before you order. Common ranges for residential work are:
Walkways and patios: about 4 inches (100 mm)
Driveways: commonly 4 to 6 inches (100 to 150 mm), thicker for heavier vehicles
Garage and shed slabs: commonly 4 inches or more, depending on use
Your contractor, engineer, building permit, or municipality has the final say on thickness and reinforcement for your project. Mix strength matters too: exterior flatwork in Ontario is typically specified at 32 MPa with air entrainment. Learn more in Understanding Concrete Mix Strengths: MPa Explained.
How Much Extra Concrete Should I Order?
Most contractors add 5 to 10% to cover:
Uneven or over-excavated subgrade
Forms that bow or sit slightly deeper than planned
Spillage and material left in equipment
The deeper and less even your base, the closer to 10% you should plan. The flatter and better compacted, the closer to 5%.
Why Measuring Matters Less With Volumetric Delivery
With a traditional drum truck, concrete is batched at the plant. You commit to a volume before the truck leaves. Run short and you wait for another load. Order too much and you pay for leftovers.
Bathurst Mobile Ready Mix uses volumetric trucks that carry sand, stone, cement, and water separately and mix concrete fresh at your site. That means:
You pay only for what you use, measured as it's mixed
No guessing game on the final few yards. If you need a little more, we mix a little more
Fresh concrete for every section of the pour
Your estimate still matters for planning and quoting. It just matters less if the ground surprises you on the day.
Should I Mix It Myself or Get Concrete Delivered?
Bagged concrete makes sense for very small jobs like a single fence post. Past that, the numbers climb fast. As a rough comparison, a single cubic yard takes dozens of bags to make, plus hours of mixing by hand. For a full breakdown, read Choosing Between Hand Mixing and Machine Mixing Concrete: Key Considerations.
Frequently Asked Questions
How many cubic yards of concrete do I need for a 10x10 slab?
A 10 ft x 10 ft slab at 4 inches thick needs about 1.23 cubic yards (0.94 m³). At 6 inches thick, it needs about 1.85 cubic yards (1.42 m³). Add 5 to 10% extra.
How do I convert cubic feet to cubic yards?
Divide cubic feet by 27. For example, 54 cubic feet ÷ 27 = 2 cubic yards.
How do I convert cubic yards to cubic metres?
Multiply cubic yards by 0.765. One cubic metre equals about 1.308 cubic yards.
How much concrete do I need for a driveway?
Multiply length x width x thickness. A 20 ft x 40 ft driveway at 5 inches thick needs about 12.35 cubic yards (9.44 m³), plus 5 to 10% extra.
How much extra concrete should I order?
Plan for 5 to 10% extra to cover uneven ground, form movement, and spillage. With volumetric delivery from Bathurst Ready Mix, you pay only for the concrete actually used.
How much does one cubic yard of concrete cover?
At 4 inches thick, one cubic yard covers about 81 square feet. At 6 inches thick, it covers about 54 square feet.
Do I pay for concrete I don't use?
Not with Bathurst Mobile Ready Mix. Our volumetric trucks mix concrete fresh on site, so you pay for the concrete that goes into your forms, not for leftovers. Call 416-656-1937 for a quote.
How many bags of concrete make a cubic yard?
A 30 kg bag typically yields about 0.5 cubic feet, so one cubic yard takes roughly 54 bags and one cubic metre roughly 71 bags. Check the yield printed on the bag you're using. For anything beyond a small repair, delivered concrete is usually faster and more consistent.
How do I measure an irregular or curved area?
Break the area into simple shapes. Calculate rectangles as length x width, triangles as half of base x height, and circles as 3.1416 x radius squared. Add the areas together, then multiply by the thickness.
What information should I have ready when I order concrete?
Have your total volume (or your measurements), the slab thickness, what you're pouring (driveway, patio, footing, slab), the site address, truck access details, and your preferred pour date and time.
Need Help With Your Numbers?
Send us your measurements and we'll help you confirm the volume before pour day.
Call 416-656-1937 or email admin@bathurstreadymix.ca. Serving Toronto, Scarborough, Markham, Aurora, Oshawa, and the GTA.
Related reading: Exposed Aggregate Concrete: The Complete Guide · How Long Does Concrete Take to Cure? · Understanding Concrete Mix Strengths: MPa Explained · Choosing Between Hand Mixing and Machine Mixing Concrete