How much concrete does a new patio really need? Order too little and you get a cold joint in the middle of the pour. The Concrete Slab Calculator turns slab size into cubic yards, bags, cost, and weight.
It is built for homeowners and small contractors planning a patio, shed base, or walkway. You enter a few numbers in feet and inches, and the results update as you type.
Table of Contents
- What the Calculator Does
- How to Use It, Step by Step
- The Concrete Slab Formula
- Worked Example
- How to Read the Results
- Thickness, Waste, and Ordering Tips
- Limitations and Assumptions
- FAQs About the Concrete Slab Calculator
What the Calculator Does
The tool handles one flat, rectangular slab. Five inputs feed five results, and two buttons help you save or restart.
- Required Concrete Volume (Inc. Waste): the main result, in cubic yards with two decimals.
- 80lb Premix Bags Needed: the bag count if you mix the concrete yourself.
- Estimated Ready-Mix Cost: cubic yards times your price per yard.
- Slab Surface Area: length times width, in square feet, with up to two decimals.
- Total Wet Concrete Weight: the estimated weight of the poured concrete in pounds.
- Copy Slab Estimate copies a text summary, and Reset restores the sample values.
How to Use It, Step by Step
- Enter Slab Length (Feet). The sample value is 20 feet.
- Enter Slab Width (Feet). The sample value is 15 feet.
- Enter Slab Thickness (Inches). Use inches here, not feet, such as 4 or 6.
- Enter Waste Allowance (%). The sample uses 10% for spillage and uneven ground.
- Enter Concrete Price Per Cu Yd ($) from your local ready-mix supplier.
- Read the volume first, then the four result cards below it.
- Press Copy Slab Estimate to paste the numbers, including your waste percentage, into a note.
The Concrete Slab Formula
The tool finds the volume in cubic feet, converts it to cubic yards, and adds waste. One cubic yard holds 27 cubic feet.
Length × Width×Thickness ÷ 12÷27×(1 + Waste %)=Cubic Yards
- Area in square feet = length × width.
- Cubic yards = area × (thickness ÷ 12) ÷ 27 × (1 + waste ÷ 100).
- Bags = total cubic feet ÷ 0.60, rounded up. An 80 lb bag yields about 0.6 cubic feet.
- Cost = cubic yards × price per cubic yard.
- Weight = cubic yards × 4,050 lbs, which is about 150 lbs per cubic foot.
Worked Example
Devon plans a backyard patio that is 20 feet long and 12 feet wide. He wants a 4 inch slab with 10% waste. His supplier quotes $150 per cubic yard.
| Result | Calculation | Output |
|---|
| Slab Surface Area | 20 × 12 | 240 sq ft |
| Exact Volume | 240 × (4 ÷ 12) ÷ 27 | 2.96 cu yd |
| Volume With Waste | 2.963 × 1.10 | 3.26 cu yd |
| 80lb Bags | 88 cu ft ÷ 0.60, rounded up | 147 Bags |
| Ready-Mix Cost | 3.259 × $150 | $488.89 |
| Wet Weight | 3.259 × 4,050 | 13,200 lbs |
If Devon thickens the same patio to 6 inches, the tool shows 4.89 cubic yards. The bag count rises to 220, and the ready-mix cost climbs to $733.33.
How to Read the Results
Required Concrete Volume is the number to give your supplier. It already includes the waste allowance you entered.
80lb Premix Bags Needed uses that same volume with waste included. At 147 bags, hand mixing is a big job, so ready-mix is often easier.
Estimated Ready-Mix Cost covers the concrete only. Delivery fees, pump fees, forms, and labor are not part of this number.
Total Wet Concrete Weight helps you plan truck access and wheelbarrow trips.
Thickness, Waste, and Ordering Tips
Thickness changes the volume more than most people expect. Going from 4 to 6 inches adds half again as much concrete. Many patios and walkways use 4 inches, while driveways are often thicker.
Many builders add 5% to 10% extra for uneven subgrade and spills. Running short mid-pour is worse than a little leftover.
A compacted gravel base helps keep the depth even. The gravel calculator can size that layer.
For footings, columns, or other shapes, try the general concrete volume calculator. If you mix from raw materials, the cement calculator covers that path.
The rebar grid calculator counts steel sticks for the same slab size. To total forms, gravel, and delivery, use the project cost calculator. More tools live in the home and construction calculators hub.
Limitations and Assumptions
- The tool assumes one rectangular slab with an even thickness. It does not handle thickened edges or curves.
- A blank or invalid field shows a short message, and every result reads N/A until you fix it.
- Length and width run from 1 to 10,000 feet, and thickness from 1 to 24 inches. Waste runs from 0% to 30%, and price must be $0 or more.
- The bag count assumes 80 lb bags with a 0.60 cubic foot yield. Other bag sizes are not offered.
- The weight uses 4,050 lbs per cubic yard and includes the waste allowance.
- Local building codes and an engineer or inspector decide final slab thickness, base, and reinforcement.
FAQs About the Concrete Slab Calculator
How many cubic yards are in a 10 by 10 slab at 4 inches?
The exact volume is about 1.23 cubic yards. With the default 10% waste, the tool shows 1.36 cubic yards.
Why do I enter thickness in inches?
Slab thickness is usually quoted in inches. The tool divides that number by 12 to convert it to feet.
Should I order ready-mix or buy bags?
Bags make sense for small pads and post bases. For larger slabs, many people choose ready-mix, since mixing 100 or more bags is hard work.
Does the cost include delivery?
No. The cost is volume times your price per yard. Ask your supplier about delivery, small-load fees, and pump fees.
How much waste allowance should I use?
Many builders add 5% to 10%. Use more if the ground is uneven or the forms are not level.
Does the tool size rebar or wire mesh?
No. It covers concrete only. Reinforcement depends on how the slab will be used, so check local codes.
This calculator gives planning estimates only and is not professional engineering, electrical, or building code advice. Reviewed for general accuracy by Prof. Dr. Khalil Mudassar, PhD.