Concrete Bag Calculator
Concrete Bag Calculator
Calculate how many bags of concrete you need for your project. Enter dimensions, get instant bag counts for all sizes with cost comparison.
Enter total number of identical areas
Per Area Volume
0 cu yd
Total Volume Needed
0 cu yd
3D Shape Preview
Why Use Our Concrete Bag Calculator?
Stop guessing how many bags of concrete you need. Our bagged concrete calculator gives you accurate bag counts, cost comparisons, and the most economical option for your project.
Concrete Bags Calculator
Instantly calculate bags of concrete needed for slabs, footings, posts, and any rectangular pour. Supports 40lb, 60lb, and 80lb bag sizes.
Bag Size Comparison
Compare all available bag sizes side by side. See bags needed, total cost, and savings for each option to find the most economical choice.
Sack Concrete Calculator
Whether you call them bags, sacks, or packs — this calculator handles all bagged concrete products. Works for columns, sonotubes, and any shape.
Waste Factor Built In
Every calculation includes a configurable waste factor (default 10%) so you always order enough concrete bags. Adjust in Settings to match your project.
Imperial & Metric Support
Calculate concrete bags in pounds (40lb, 60lb, 80lb) or kilograms (20kg, 25kg, 30kg). Switch between feet/inches and meters/centimeters instantly.
Ready Mix Concrete Bags
Calculator works with all ready mix concrete bags from Quikrete, Sakrete, and other brands. Market-specific pricing for US, Canada, UK, and Australia.
How to Calculate Bags of Concrete
Follow these steps to calculate concrete bags for any project. Works for slabs, footings, posts, and any rectangular concrete pour.
Measure Your Project Area
Measure the length, width, and thickness of the area you need to fill with concrete. Use a tape measure for accuracy.
Tip: For irregular shapes, break them into rectangles and calculate each section separately.
Enter Dimensions in Calculator
Input your measurements into the concrete bag calculator above. Choose between imperial (feet/inches) or metric (meters/cm) units.
Note: Common slab thickness is 4 inches. Footings are typically 8-12 inches deep.
Review Bags Needed Per Size
The calculator instantly shows how many bags you need for each size — 80lb, 60lb, and 40lb. Each result includes the waste factor so you order enough.
Important: An 80lb bag covers 0.6 cu ft, 60lb covers 0.45 cu ft, and 40lb covers 0.3 cu ft.
Pick the Best Bag Size for Your Budget
Compare total costs across bag sizes. Larger bags (80lb) usually cost less per cubic foot, but smaller bags (40lb) are easier to handle. Our bag concrete calculator shows the savings for each option.
Pro Tip: For projects over 1 cubic yard (45+ bags of 80lb), consider ready-mix delivery instead of bagged concrete.
Concrete Bag Calculation Formula
Step 1: Calculate Volume
Volume (cu ft) = Length × Width × (Thickness ÷ 12)
Step 2: Add Waste Factor
Adjusted Volume = Volume × 1.10
Step 3: Calculate Bags Needed
Bags = Adjusted Volume ÷ Coverage per Bag
Bag Coverage Reference:
- • 80 lb bag: 0.6 cubic feet
- • 60 lb bag: 0.45 cubic feet
- • 40 lb bag: 0.3 cubic feet
- • 30 kg bag: 17 liters (0.60 cu ft)
- • 25 kg bag: 14 liters (0.49 cu ft)
- • 20 kg bag: 11 liters (0.39 cu ft)
Frequently Asked Questions About Concrete Bags
Common questions about calculating bags of concrete, coverage per bag, cost comparison, and how to use our concrete bag calculator.
How many bags of concrete do I need?
The number depends on your project size and bag weight. Calculate the total cubic feet (Length × Width × Depth in feet), add 10% for waste, then divide by the bag coverage. For 80lb bags, divide by 0.6. For 60lb, divide by 0.45. For 40lb, divide by 0.3. Our concrete bag calculator does this math instantly.
How many 80lb bags of concrete in a cubic yard?
One cubic yard equals 27 cubic feet. Since each 80lb bag covers 0.6 cubic feet, you need 45 bags per cubic yard (27 ÷ 0.6 = 45). With a 10% waste factor, order about 50 bags. For 60lb bags, you need 60 per yard. For 40lb bags, 90 per yard.
How to calculate bags of concrete for a slab?
Multiply slab length × width × thickness (all in feet). For a 10ft × 10ft × 4" slab: 10 × 10 × 0.33 = 33.3 cu ft. With waste: 36.7 cu ft. For 80lb bags: 36.7 ÷ 0.6 = 62 bags. Use our slab calculator for quick volume calculations, then come back here for detailed bag analysis.
What is the cheapest bag size to buy?
The 80lb bag is almost always the cheapest per cubic foot of concrete. At typical pricing ($6.50 per bag), you pay about $10.83 per cubic foot. A 60lb bag at $5.25 costs $11.67 per cubic foot. A 40lb bag at $4.00 costs $13.33 per cubic foot. Bigger bags = better value.
How much does a bag of concrete cover?
An 80lb bag of concrete mix covers 0.6 cubic feet when mixed. That fills a space roughly 12" × 12" × 7.2" deep. A 60lb bag covers 0.45 cu ft (about 12" × 12" × 5.4"). A 40lb bag covers 0.3 cu ft (about 12" × 12" × 3.6").
How to calculate concrete bags for a post hole?
For round post holes, use the formula: Volume = π × radius² × depth. A 10" diameter × 36" deep hole = 3.14 × 0.417² × 3 = 1.64 cu ft. That's about 3 bags of 80lb concrete per post. For fence posts, most people use 1-3 bags per post depending on hole size.
Should I use 40lb, 60lb, or 80lb bags?
Choose based on your project and physical ability. 80lb bags offer the best value but are heavy — ideal for large projects when you have help. 60lb bags balance cost and weight. 40lb bags are easiest to handle, good for small repairs and when working alone. Our calculator for bags of concrete shows the cost difference.
How many bags of concrete for a 10×10 slab?
For a 10×10 foot slab at 4" thick: Volume = 10 × 10 × 0.33 = 33.3 cu ft (1.23 cu yd). With 10% waste: 36.7 cu ft. That equals 62 bags of 80lb, 82 bags of 60lb, or 123 bags of 40lb concrete. At this quantity, ready-mix delivery may be more practical. For footing projects, bag counts can add up quickly.
What is a concrete mix calculator for bags?
A concrete mix calculator for bags converts your project dimensions into the number of pre-mixed concrete bags required. Unlike ordering ready-mix by the yard, bagged concrete is sold by weight (40lb, 60lb, 80lb). This concrete calculator bags tool bridges the gap between volume and bag count.
How many bags of concrete do I need for a sidewalk?
A typical 3ft wide × 4" thick sidewalk needs about 3.3 cu ft per linear foot. For a 20ft sidewalk: 20 × 3 × 0.33 = 20 cu ft. With waste: 22 cu ft = 37 bags of 80lb concrete. Use the calculator above to adjust for your exact sidewalk dimensions.
When should I use ready-mix instead of bags?
Consider ready-mix concrete delivery when you need more than 1 cubic yard (about 45 bags of 80lb). Ready-mix costs $100-150 per cubic yard delivered, while 45 bags at $6.50 each = $292. Ready-mix saves money, time, and labor on bigger pours. Our ready mix concrete bags calculator helps you compare.
How do I calculate concrete bags for metric measurements?
Switch to metric mode in Settings. Enter length and width in meters, thickness in centimeters. The calculator converts to metric bag sizes (20kg, 25kg, 30kg) with local pricing. A 30kg bag covers 17 liters. Multiply L(m) × W(m) × D(cm) × 10 to get liters needed.
What waste factor should I use?
Use 10% waste factor for most projects (the default). Increase to 15% for irregular shapes or rough ground. For very precise forms with smooth subgrade, 5% may suffice. Adjust the waste factor in Settings. It's always better to have extra bags than to run short mid-pour.
How many bags of concrete for a 4×4 post?
A 4×4 fence post in a standard 8" diameter × 24" deep hole needs about 0.7 cu ft of concrete — roughly 2 bags of 40lb or 1 bag of 80lb. For a 6×6 post in a 12" diameter × 36" deep hole, you need about 2 bags of 80lb concrete.
Concrete Bag Guide
Everything you need to know about buying, calculating, and using bagged concrete for your project.
Bag Size Selection
- • 80 lb bags: Best value, heaviest to carry
- • 60 lb bags: Good balance of cost and weight
- • 40 lb bags: Easiest to handle, highest per-unit cost
- • 80lb bags need water: ~3 quarts per bag
- • Working time: 15-20 minutes after mixing
- • Full cure: 28 days for maximum strength
When to Use Bags vs Ready-Mix
Use Bags When:
- • Project under 1 cubic yard
- • No truck access to pour site
- • Pouring in stages over days
- • Small repairs and patching
Use Ready-Mix When:
- • Project over 1 cubic yard
- • Need consistent mix quality
- • Large continuous pours
Mixing & Handling Tips
- • Mix one bag at a time for best results
- • Add water gradually — don't oversaturate
- • Use a wheelbarrow or mixing tub
- • Wear gloves — wet concrete burns skin
- • Store unused bags in dry location
- • Opened bags lose effectiveness after 3 months
Quick Reference: Common Projects & Bags Needed
| Project | Size | Volume (cu ft) | 80lb Bags | 60lb Bags | 40lb Bags |
|---|---|---|---|---|---|
| Fence Post | 8" dia × 24" deep | 0.7 | 2 | 2 | 3 |
| Small Slab | 4' × 4' × 4" | 5.3 | 10 | 13 | 20 |
| Sidewalk Section | 3' × 10' × 4" | 10 | 19 | 25 | 37 |
| Patio Slab | 10' × 10' × 4" | 33.3 | 62 | 82 | 123 |
| Footing | 16" × 8" × 20' | 17.8 | 33 | 44 | 66 |
| 1 Cubic Yard | 27 cu ft | 27 | 50 | 66 | 99 |
* Bag counts include 10% waste factor. Actual amounts may vary by brand and mix.