Concrete volume and purchasing estimator

Concrete Slab Calculator

Calculate cubic yards or cubic meters for one or more rectangular slab sections, then compare ready-mix volume, bag quantities, truckloads and material cost.

Bag yields checked against manufacturer dataUpdated 2026-07-20

Project inputs

Measure the slab

US unitsMetric

Slab sectionsAdd rectangles when the project is not one simple shape.

Section name Length ft Width ft

Slab thickness inEnter the specified finished thickness. Ordering allowance %Covers uneven subgrade, forms and handling. Ready-mix price USD / yd³Replace with a delivered supplier quote. Truck capacity yd³Used to estimate full or partial loads. Bag sizeChoose the package you plan to buy. Yield per bag ft³Editable; use the product label when available. Price per bag USDUsed for the bagged cost comparison. Concrete density lb/ft³Normal-weight default; editable.

Concrete to order1.96 yd³

For 144 sq ft at 4 in thickness, including 10% allowance.

Calculated volume1.78 yd³

80 lb bags88 bags

Ready-mix loads0.20 loads

Ready-mix materialUSD 293

Bagged materialUSD 572

Wet concrete weight3.83 US tons

Total slab area144 sq ft

Order volume52.8 ft³

Measurement check

Slab section summary

Main slab12 ft x 12 ft144 sq ft

Transparent estimate

Calculation breakdown

1
Total area
12 ft x 12 ft = 144 sq ft
2
Slab volume
144 sq ft x 0.3333 ft = 48 ft³ = 1.78 yd³
3
Order volume
1.78 yd³ x 1.10 = 1.96 yd³
4
Bag quantity
52.8 ft³ / 0.60 ft³ per bag = 88 bags

Bag counts are rounded up to whole bags. Ready-mix suppliers may enforce minimum orders, short-load fees or full-truck pricing that this material-only estimate does not include.

Reference values

Bag yields and density assumptions

Bag yield

QUIKRETE lists approximate yields of 0.30 ft³ for 40 lb, 0.45 ft³ for 60 lb and 0.60 ft³ for 80 lb Concrete Mix bags. Product formulas differ, so the yield remains editable.

Normal-weight density

NRMCA describes normal-weight concrete as approximately 140–150 lb/ft³. The calculator starts at 145 lb/ft³ for weight planning only.

Allowance

The optional allowance increases the purchase volume for uneven subgrade and handling uncertainty. It does not replace accurate form measurements.

Primary referencesQUIKRETE Concrete Mix specification QUIKRETE concrete calculator NRMCA concrete density reference

Concrete slab calculator FAQ

How do I calculate concrete yards for a slab?

Multiply length by width by slab thickness using one unit system. Convert cubic feet to cubic yards by dividing by 27, then add the selected ordering allowance.

How many 80 lb bags make one cubic yard?

At an approximate yield of 0.60 ft³ per bag, one cubic yard requires 45 bags before any allowance. Check the specific product label because yield varies by mix.

Should I order bags or ready-mix?

Bagged mix can suit small placements, while larger pours may be more practical as ready-mix. Compare delivered pricing, minimum orders, labor, mixer capacity and placement time.

Does this calculator design the slab thickness?

No. Thickness, reinforcement, base preparation, joints and concrete strength depend on loads, soil, climate and local requirements. Enter the thickness specified for the project.

Does the price include labor and delivery?

No. The comparison covers material quantity multiplied by the entered unit price. Delivery, short-load fees, pumping, reinforcement, forms, tools, labor and taxes are separate.