Dry Pour Concrete Calculator: How Much Mix Do You Need?

This calculator works out the volume of dry concrete mix (in cubic feet or cubic meters) you need for a slab, footing, or column, based on the dimensions you enter. You input the length, width, and depth (or diameter and height for a cylinder), and it returns the required dry volume, accounting for the typical 10–15% overage for waste and compaction. The result is the amount of bagged mix (like 60-lb or 80-lb bags) or bulk cubic yards you should order.

Formula

The calculator uses the basic volume formula for a rectangular prism:

Volume (cubic feet) = Length (ft) × Width (ft) × Depth (ft)

Since you enter depth in inches, the calculator converts it to feet by dividing by 12. Then it multiplies the raw volume by (1 + waste percentage/100) to account for spillage, uneven subgrade, and compaction. The result is the dry concrete volume you need to order.

• L = length of the slab or footing (feet)
• W = width (feet)
• D = depth or thickness (inches, converted to feet)
• Waste % = extra material added for safety (default 10%)

For a cylinder (like a deck post footing), the formula is π × radius² × height. The same waste factor applies.

Worked Example

Let’s use the default values: a slab that is 10 ft long, 4 ft wide, and 4 inches deep, with 10% waste.

1. Convert depth to feet: 4 in ÷ 12 = 0.333 ft
2. Raw volume = 10 × 4 × 0.333 = 13.32 cu ft
3. Add 10% waste: 13.32 × 1.10 = 14.65 cu ft
4. 60-lb bags: 14.65 ÷ 0.45 = 32.56 → round up to 33 bags
5. 80-lb bags: 14.65 ÷ 0.60 = 24.42 → round up to 25 bags
6. Cubic yards: 14.65 ÷ 27 = 0.54 yd³

The calculator shows 14.65 cu ft, 33 bags of 60-lb mix, 25 bags of 80-lb mix, and 0.54 cubic yards. That matches the computed result above.

How to Use the Dry Pour Concrete Calculator

Enter the dimensions of your project in feet and inches. For a slab, measure the length and width from the outside edges of the form. For a footing, measure the diameter (or radius) and the depth. The calculator works for any shape if you use the appropriate volume formula – for a cylinder, multiply π × radius² × height.

After you get the volume, decide how you will buy concrete:

• Bagged mix – 60-lb bag yields about 0.45 cu ft; 80-lb bag yields about 0.60 cu ft. These are typical yields for standard concrete mixes, but check the bag label because some blends (e.g., high-strength or fiber-reinforced) may differ.
• Bulk delivery – Order in cubic yards (1 yd³ = 27 cu ft). Most ready-mix trucks deliver a minimum of 1 yard, and you pay for the full yard even if you use less.

Always round up to the nearest whole bag. It’s better to have a little extra than to run short mid-pour.

Common Mistakes and Misconceptions

One frequent error is mixing up units. If you measure length in feet and width in inches, the volume will be wrong. Always convert everything to the same unit before calculating.

Another mistake is ignoring the waste factor. For a simple slab on flat ground, 5–10% waste is enough. For footings with irregular shapes or rebar obstructions, use 15–20%. The calculator defaults to 10%.

A common misconception is that the dry volume equals the wet volume. Actually, dry concrete mix takes up more space than the same weight of wet concrete because water fills the voids. But when you buy bagged mix, the bag volume is the dry volume, and that’s what you need. The calculator gives the dry volume, not the wet volume.

When to Use This Calculator

Use it for any project where you need to order concrete:

• Patio slabs and walkways
• Footings for decks, posts, or fences
• Concrete pads for AC units, generators, or sheds
• Steps or small retaining walls

It’s also handy for estimating the cost of bagged concrete – multiply the number of bags by the local price per bag. For large projects (more than 1 cubic yard), ready-mix delivery is usually more economical than bags.

Bagged vs. Ready-Mix: Which Should You Choose?

Bagged concrete is convenient for small projects (under 1 cubic yard) because you can carry it, mix it in a wheelbarrow or mixer, and avoid delivery minimums. However, mixing many bags is labor-intensive and can be inconsistent if you don’t measure water carefully.

Ready-mix concrete is delivered in a truck and poured directly into the forms. It’s faster and more consistent, but you must be prepared to receive the whole load at once. Most companies charge a short-load fee if you order less than the minimum (often 1 cubic yard). For a 4-inch-thick slab, 1 cubic yard covers 81 square feet – that’s about 9×9 ft.

For most DIY projects under 1 yard, bags are simpler. For anything larger, consider ready-mix.

Cost Considerations

Bagged concrete costs more per cubic foot than ready-mix, but you avoid delivery fees. As a rough guide, a 60-lb bag costs about $4–$6, so 33 bags for the example slab would cost roughly $130–$200. Ready-mix might cost $150–$200 per cubic yard plus delivery, so the 0.54-yard slab would be about $100–$150 but with a minimum order fee, it could be $200–$300. Prices vary widely by region and time, so check local suppliers.

Don’t forget the cost of rebar or wire mesh, form lumber, and concrete tools. These are separate from the concrete itself.

Safety and Preparation Tips

Concrete is heavy – a 60-lb bag is a lot to lift repeatedly. Use proper lifting technique and consider a wheelbarrow to move bags. Wear gloves and eye protection when mixing, as cement dust can irritate skin and eyes. If you get wet concrete on your skin, rinse immediately with clean water.

Prepare the subgrade properly: compact the soil, add a gravel base for drainage, and set forms level. For a slab that will bear heavy loads, add rebar or wire mesh. The calculator assumes a uniform depth – if your subgrade is uneven, you’ll need more concrete.

Mix concrete in small batches, adding water gradually until it reaches a workable consistency. Too much water weakens the concrete. Follow the instructions on the bag.

Frequently Asked Questions

How many 80-lb bags do I need for a 10×10 slab that is 4 inches thick?
First, volume = 10×10×(4/12) = 33.33 cu ft. Add 10% waste = 36.67 cu ft. Divide by 0.60 cu ft per 80-lb bag = 61.1, so you need 62 bags. Round up to be safe.

What is the difference between dry volume and wet volume?
Dry volume is the volume of the dry mix ingredients before water is added. Wet volume is the volume after mixing, which is slightly less because water fills the voids between particles. When ordering bags, you order the dry volume. The calculator gives dry volume.

Can I use this calculator for a round footing?
Yes, but you need to use the cylinder formula: π × radius² × height. For example, a 12-inch diameter (6-inch radius) footing that is 24 inches deep: π × (0.5 ft)² × 2 ft = 1.57 cu ft. Add 10% waste = 1.73 cu ft, so about 4 bags of 60-lb mix.

How much does a 60-lb bag of concrete cover?
A 60-lb bag yields about 0.45 cubic feet of concrete. At a 4-inch depth, that covers 1.35 square feet (0.45 / (4/12) = 1.35). So for a 10×10 slab (100 sq ft), you need about 74 bags (100 / 1.35 = 74.07).

Should I always add waste?
Yes, it’s wise. Even with careful measuring, you may spill some, or the subgrade may be slightly deeper in spots. A 10% waste factor is a good minimum. For complex forms or if you’re new to concrete, use 15–20%.

Is it cheaper to mix my own concrete from cement, sand, and gravel?
For very large projects, buying bulk materials can be cheaper. For small DIY jobs, bagged mix is usually more convenient and cost-effective because you avoid buying large quantities of sand and gravel that you may not use up. Also, bagged mix has the right proportions already.

Related

• Drywall Framing Calculator: Studs & Sheets
• Drywall Grid Calculator: How to Estimate Framing
• Drywall Grid Calculator Square Feet

in l | Length | ft | 10 | 0
in w | Width | ft | 4 | 0
in d | Depth | in | 4 | 0
in waste | Waste % | % | 10 | 0
out dry_vol | Dry volume | cu ft | (l * w * (d / 12)) * (1 + waste / 100) | 2
out bags_60 | 60-lb bags | bags | ceil(((l * w * (d / 12)) * (1 + waste / 100)) / 0.45) | 0
out bags_80 | 80-lb bags | bags | ceil(((l * w * (d / 12)) * (1 + waste / 100)) / 0.6) | 0
out cubic_yds | Cubic yards | yd³ | ((l * w * (d / 12)) * (1 + waste / 100)) / 27 | 2