Concrete Bag Calculator: Free Online Bagged Concrete Estimator
Use our free concrete bag calculator to determine exactly how many bags of concrete mix you need for your slab, footing, or small project. Designed for DIY homeowners, handymen, and small-job contractors, this tool calculates the number of bags required based on your slab dimensions and bag weight. Enter your length, width, thickness, and bag size to know exactly how many bags to buy before your next trip to the hardware store.
How to use this calculator.
- Measure your project area. Measure the length and width of the slab in feet, and the desired thickness in inches.
- Choose your bag weight. Select 80 lb, 60 lb, or 40 lb bags. An 80 lb bag yields about 0.6 cubic feet, a 60 lb bag yields about 0.45 cubic feet, and a 40 lb bag yields about 0.3 cubic feet of cured concrete.
- Review your results. The calculator shows the volume in cubic feet and the total number of bags needed. Always buy 1-2 extra bags for waste and unexpected needs.
How do you calculate number of concrete bags needed?
The number of bags is calculated by dividing the total concrete volume by the yield per bag based on concrete density.
Volume (cu ft) = Length (ft) × Width (ft) × Thickness (in) ÷ 12
Bags Needed = Volume (cu ft) × 133 lb/ft³ ÷ Bag Weight (lb)
Where:
- 133 lb/ft³ is the approximate density of cured concrete.
- An 80 lb bag yields approximately 0.6 cu ft (80 lb ÷ 133 lb/ft³).
- A 60 lb bag yields approximately 0.45 cu ft.
- A 40 lb bag yields approximately 0.3 cu ft.
Example calculation.
Let's calculate the bags needed for a garden shed slab.
Scenario: You are pouring a shed slab that is 8 ft long and 6 ft wide at 4 inches thick, using 80 lb bags of concrete mix.
- Calculate the volume: 8 ft × 6 ft × 4 in ÷ 12 = 16 cu ft
- Calculate bags needed: 16 cu ft × 133 lb/ft³ ÷ 80 lb = 26.6 bags
Result: You need approximately 27 bagsof 80 lb concrete mix (rounding up). Buy 28-29 bags to have a buffer. If you used 60 lb bags instead, you would need about 36 bags.
Tips from the field.
- Buy all bags from the same lot. Concrete mix from different manufacturing batches can have slight color variations. Buy everything at once and check the lot numbers on the bags.
- Rent a mixer for medium jobs. If you need more than 10-15 bags, rent a small electric concrete mixer. Mixing 20+ bags by hand in a wheelbarrow is exhausting and leads to inconsistent mix quality.
- Mix exactly to the water spec. Use the amount of water specified on the bag: no more, no less. Too much water weakens the concrete significantly. A mix that is 1 gallon too wet per bag can reduce compressive strength by 20-30%.
- Store bags dry. Keep unopened bags off the ground on a pallet and covered with a tarp. Moisture-hardened bags are unusable. Do not buy bags that feel hard or lumpy.
- Consider ready-mix for larger jobs. If you need more than 40-50 bags, call for ready-mix delivery. It costs less per cubic yard and produces better, more consistent concrete with far less labor.
Disclaimer: This calculator provides estimates for planning purposes only. Actual bag yield may vary based on mix design, water content, and placement method. Always check the manufacturer's stated yield on the bag. Buy extra bags to account for spillage and waste. PavementCalculator.com assumes no liability for material overages or shortages.
Cost, price, and density figures on this page reflect approximate U.S. averages as of August 2026 and vary by region, supplier, and material availability.
Frequently asked questions.
How many bags of concrete do I need for a 10×10 slab?
For a 10 ft × 10 ft slab at 4 inches thick, you need about 56 bags of 80 lb concrete mix. The volume is 33.33 cubic feet, and each 80 lb bag yields approximately 0.6 cubic feet. For 60 lb bags, you would need about 74 bags.
How many cubic feet does an 80 lb bag of concrete make?
An 80 lb bag of standard concrete mix yields approximately 0.6 cubic feet of cured concrete. A 60 lb bag yields about 0.45 cubic feet, and a 40 lb bag yields about 0.3 cubic feet. Always check the manufacturer's yield on the bag for the most accurate estimate.
Should I use bagged concrete or ready-mix for my project?
For small projects under 1 cubic yard, bagged concrete is convenient and cost-effective. For projects over 1 cubic yard, ready-mix concrete delivered by a concrete truck is typically cheaper per cubic yard and ensures consistent quality. The break-even point is usually around 40-50 bags of 80 lb mix.
How long does bagged concrete take to cure?
Bagged concrete sets in about 24 to 48 hours but continues to cure and gain strength for 28 days. Wait at least 24 hours before light foot traffic, 7 days before vehicle traffic on driveways, and keep the surface moist during the first few days for optimal strength development.
How do I use this calculator?
Enter your project dimensions in the input fields above and the results update automatically. For the most accurate results, measure carefully and use consistent units (feet for length/width, inches for depth). Add 5-10% extra for waste and variations.