Material estimator

Concrete Calculator

Estimate concrete volume, cubic yards, bag counts, total bag weight, and material cost for slabs, pads, and small pours.

Reviewed for formula logic and buying assumptions on May 21, 2026.

How This Calculator Works

Use this concrete calculator when you need a fast buying number before going to the store or calling for ready-mix. It converts feet and inches into cubic yards, compares common bag sizes, and flags when a pour is large enough to consider delivery.

Formula

Volume = length x width x thickness. Convert thickness from inches to feet, divide cubic feet by 27 for cubic yards, then divide cubic feet by bag yield for bag count.

Assumptions

Default bag yields are 40 lb = 0.30 ft^3, 50 lb = 0.375 ft^3, 60 lb = 0.45 ft^3, and 80 lb = 0.60 ft^3. Add 5-10% overage for uneven base, spillage, and form variation.

Measurement Checklist

Measure the formed length and width after forms are set, not just the planned drawing size.
Measure slab thickness in inches and use the average thickness when the base is uneven.
Calculate thickened edges, steps, or footings separately and add them to the slab volume.
Confirm whether your supplier or bag label uses a different yield than the planning defaults.

Practical Examples

A 12 ft by 10 ft slab at 4 inches thick with 10% overage needs about 1.63 cubic yards, or about 98 bags of 60 lb mix / 74 bags of 80 lb mix.
A small 8 ft by 8 ft pad at 3.5 inches thick with 10% overage needs about 0.76 yd^3, or about 35 bags of 80 lb mix.
A 30 ft by 3 ft sidewalk at 4 inches thick with 10% overage needs about 1.22 yd^3, or about 55 bags of 80 lb mix.
Two 6 ft by 4 ft shed pads at 4 inches thick with 10% overage need about 0.65 yd^3, or about 30 bags of 80 lb mix.

Before You Buy

Buying Guidance

Common concrete mix bags are 40, 50, 60, and 80 lb. The heavier bags usually reduce bag count but are harder to move and lift into a mixer.
Use bagged concrete for small repairs, steps, pads, and isolated pours. Compare ready-mix when the estimate is near one cubic yard or higher.
Ask about short-load, delivery, fuel, and standby fees before deciding that ready-mix is more expensive than bags.

Waste Rules

Use 5% overage for tight forms and a compacted, even base. Use 10% or more for hand-dug forms, uneven subgrade, or first-time DIY pours.
Measure thickness in several spots. One extra inch across a slab changes the cubic yards much more than most people expect.

Common Mistakes

Do not include gravel base, reinforcement, forms, expansion material, or finishing tools in the concrete quantity.
Do not plan a large pour around hand mixing unless you have enough labor and time to place the concrete before it starts setting.

Plan the Rest of the Job

Common Questions

How many bags are in one cubic yard?

One cubic yard is 27 cubic feet. Using common yields, that is about 90 bags of 40 lb, 72 bags of 50 lb, 60 bags of 60 lb, or 45 bags of 80 lb concrete mix.

When should I consider ready-mix delivery?

Once the estimate is around one cubic yard or more, compare bagged cost, hauling weight, and mixing time against local ready-mix and short-load fees.

Should I add waste?

Yes. For most DIY pours, 5-10% is a practical starting point. Irregular forms and rough subgrade may need more.