How Many 80lb Bags of Concrete for Shed Foundation? Complete Calculator Guide

Planning a shed foundation and staring at pallets of concrete bags at the store? The last thing you want is to run short mid-pour or waste money on bags you don't need. Shed foundations come in three types—piers, footings, or full slabs—and each requires different quantities. Getting the calculation right the first time saves trips to the store and ensures your shed sits on solid ground for decades.

Quick Answer: For 80lb bags, figure 0.6 cubic feet per bag. A 10x12 shed on 4 corner piers needs 12-16 bags. The same shed on a 4-inch slab needs 67-70 bags (but ready-mix is smarter at that volume). An 8x10 shed on 6 footings needs 18-24 bags. Always buy 10% extra—concrete doesn't store well long-term, but running short mid-pour is worse.

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Understanding Bag Coverage: The Math You Need

One 80-pound bag of concrete mix produces approximately 0.6 cubic feet when properly mixed. This is the industry standard, though slight variations exist between brands.

Quick conversion reference:

• 1 cubic foot = 1.67 bags (80lb)
• 1 cubic yard = 45 bags (80lb)
• 1 bag = 0.6 cubic feet

For shed foundations, you're typically dealing with cubic feet, not yards. A 12-inch diameter hole, 36 inches deep = 3.14 cubic feet = about 5.2 bags (round up to 6 bags for waste and inconsistencies).

Shed Foundation Types: Bag Requirements

Option 1: Concrete Piers (Most Common for Small Sheds)

Concrete piers work great for sheds up to 12x16 feet. They're economical, don't require frost-line-depth excavation everywhere, and use manageable bag quantities.

Typical pier specifications:

Number of piers needed:

• 6x8 shed: 4 corners = 16-20 bags
• 8x10 shed: 4 corners + 2 center = 30-36 bags
• 10x12 shed: 4 corners + 4 center = 40-48 bags
• 12x16 shed: 4 corners + 6 center = 56-64 bags

Always space piers maximum 6-8 feet apart. Heavier sheds (workshop use, heavy equipment storage) need piers every 4-6 feet.

Option 2: Continuous Footing (Perimeter Support)

Continuous footings provide more support than piers and work well for larger sheds or those on slopes.

Typical footing dimensions: 12 inches wide × 6-8 inches thick

Calculation example for 10x12 shed:

• Perimeter: (10 + 10 + 12 + 12) = 44 linear feet
• Footing volume: 44 ft × 1 ft wide × 0.5 ft thick = 22 cubic feet
• Bags needed: 22 ÷ 0.6 = 37 bags
• With 10% waste: 41 bags

Add interior footings if shed will support heavy loads. A central spine footing adds roughly 50% more bags.

Option 3: Full Concrete Slab (Maximum Stability)

Full slabs provide the best foundation but use significantly more concrete. At this volume, ready-mix becomes more economical than bags.

Cost Reality Check: At 67+ bags, you're spending $335+ on materials alone (at $5/bag). Add mixer rental ($75), labor, and time—ready-mix delivery at $300-400 becomes the smarter choice. Use our Concrete Slab Calculator to compare exact costs.

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Step-by-Step: Calculating Your Exact Needs

For Concrete Piers:

1. Determine pier diameter (typically 10-14 inches)
2. Determine depth (must be below frost line, usually 30-48 inches depending on location)
3. Calculate volume: π × radius² × height (in feet)
4. Divide by 0.6 to get bag count per pier
5. Multiply by number of piers needed
6. Add 10% for waste and inconsistencies

Example: 10x12 shed, 4 corner piers + 2 side piers (6 total)

• Pier size: 12" diameter × 36" deep
• Volume per pier: 3.14 × (0.5 ft)² × 3 ft = 2.36 cubic feet
• Bags per pier: 2.36 ÷ 0.6 = 3.9, round to 4 bags
• Total piers: 6
• Base bags needed: 6 × 4 = 24 bags
• With 10% extra: 27 bags

For Full Slabs:

1. Calculate square footage (length × width)
2. Determine thickness (4 inches for storage, 6 inches for workshop/vehicles)
3. Convert to cubic feet: sq ft × (thickness in inches ÷ 12)
4. Divide by 0.6 for bag count
5. Add 10% waste factor

Example: 10x12 shed, 4-inch slab

• Area: 10 × 12 = 120 sq ft
• Volume: 120 × (4 ÷ 12) = 40 cubic feet
• Bags needed: 40 ÷ 0.6 = 67 bags
• With 10% extra: 74 bags

Choosing the Right Foundation Type

Your shed's intended use determines the best foundation approach:

Use Piers When:

• Shed is primarily for storage (lawn equipment, bikes, seasonal items)
• Shed size is 12x16 or smaller
• Ground is relatively level (less than 6-inch slope)
• Budget is limited (piers cost 50-70% less than slabs)
• Working alone or with minimal help

Use Full Slab When:

• Shed will be a workshop with heavy equipment
• You plan to park vehicles or ATVs inside
• Floor needs to be level and smooth for workbenches
• Shed is 12x16 or larger
• Budget allows for the extra cost

Use Continuous Footings When:

• Ground has moderate slope (6-18 inches)
• Soil is poor quality or very sandy
• Shed will support heavy roof loads (snow country)
• Local codes require it (check before starting)

Mixing and Installation Tips

Mixing Equipment Options

Wheelbarrow (Hand Mixing):

• Good for: 1-10 bags total
• Cost: $0 (use what you have)
• Physical effort: High
• Best for: Small pier projects (4-6 piers)

Electric Mixer (Rental):

• Good for: 10-40 bags
• Cost: $50-75/day rental
• Physical effort: Moderate
• Best for: Medium projects (6-10 piers, small footings)

Gas-Powered Mixer (Rental):

• Good for: 40-80 bags
• Cost: $75-100/day rental
• Physical effort: Low
• Best for: Larger projects (10+ piers, continuous footings)

Pro Tip: Beyond 80 bags, seriously consider ready-mix concrete. Your time, effort, and mixer rental costs make ready-mix competitive. Use our Concrete Cost Calculator to compare exact pricing.

Mixing Best Practices

1. Use the right water amount: Follow package directions exactly—typically 3 quarts per 80lb bag. Too much water weakens concrete significantly
2. Mix thoroughly: No dry pockets, consistent color throughout. Hand-mixed concrete takes 5-7 minutes of vigorous mixing per batch
3. Work in batches: Mix only what you can place in 15-20 minutes. Concrete begins setting and loses workability
4. Consistency check: Properly mixed concrete holds its shape when formed into a ball, but flows when poured

Common Mistakes That Waste Bags

1. Underestimating Hole Volume

That 12-inch auger creates a hole larger at the bottom than top. Bell-shaped holes use 20-30% more concrete than calculated cylinder volume. Always round up and buy extra bags—they're cheap insurance.

2. Not Accounting for Soil Absorption

Dry, sandy soil absorbs moisture from concrete, requiring thicker pours. Wet the holes before pouring, or add an extra bag per pier for very porous soils.

3. Mixing Too Much at Once

Beginners mix 5-6 bags at once thinking they'll work faster. Then concrete starts setting before they finish pouring. Mix 2-3 bags max per batch unless you have multiple people placing simultaneously.

4. Using Wrong Concrete Type

"Sand mix" or "mortar mix" are NOT the same as "concrete mix." For structural foundations, you need concrete mix (contains gravel aggregate). Don't cheap out—wrong mix equals foundation failure.

Bag Storage and Shelf Life

You'll likely buy bags in advance or have leftovers. Store them properly:

• Unopened bags: 6-12 months if kept completely dry
• Opened bags: Use within 30 days, even with resealing
• Storage location: Elevated off ground (on pallets), inside garage/shed, covered with plastic
• Test old concrete: Squeeze bag—hard lumps mean it's absorbed moisture and compromised

Don't buy bags months in advance "to be ready." Concrete degrades in storage. Order 1-2 weeks before your project date, and use leftover bags for walkway repairs or small projects within a month.

Cost Comparison: Bags vs. Ready-Mix

Let's analyze a real example:

Project: 10x12 shed, full 4-inch slab

Bagged Concrete:

• Materials: 70 bags × $5.00 = $350
• Mixer rental (2 days): $150
• Labor: 2 people × 12 hours = 24 man-hours
• Total: $500 + exhausting labor

Ready-Mix Concrete:

• Concrete: 1.5 yards × $130 = $195
• Delivery: $100
• Small load fee: $75
• Labor: 2 people × 3 hours = 6 man-hours
• Total: $370 + manageable labor

Ready-mix costs $130 less AND saves 18 man-hours of backbreaking work. The break-even point is around 40-50 bags—beyond that, ready-mix wins on both cost and convenience.

Special Situations

Sloped Ground

If your shed site slopes more than 6 inches, consider stepped piers—dig holes to different depths so pier tops are level. This uses varying bag quantities per pier. Calculate each pier individually and sum the total.

Frost Line Considerations

In northern climates with deep frost lines (48+ inches), pier holes become very deep. A 14-inch diameter hole, 54 inches deep requires 7-8 bags per pier. Four corner piers = 28-32 bags just for corners. According to the International Residential Code (IRC), footings must extend below the frost line to prevent heaving.

Sandy or Unstable Soil

Poor soil requires wider pier bases (bell-bottom shape). Use a post hole digger to create a wider base— this might add 2-3 bags per pier but prevents settling that ruins sheds within a few years.

Tools and Supplies Checklist

Beyond the concrete bags, you'll need:

• Post hole digger or power auger
• Level (4-foot minimum)
• Mixing equipment (wheelbarrow or rented mixer)
• Shovel for transferring mixed concrete
• Tamping rod or stick (to eliminate air pockets)
• Gloves (concrete is caustic)
• Water source and measuring container
• Sonotube forms (if using) - reduce bag waste from irregular holes

Final Recommendations

For most DIY shed foundations, here's the smart approach:

Sheds 8x10 or smaller: Use 4-6 concrete piers with bagged concrete. Total cost: $100-150 in bags, manageable labor, completed in a weekend.

Sheds 10x12 to 12x16: Use piers if budget is tight, slab if you want maximum quality. For slabs, seriously consider ready-mix despite higher upfront cost—the convenience and consistency justify the expense.

Sheds larger than 12x16: Full slabs with ready-mix concrete, or engineered pier-and-beam foundation. At this size, you're approaching small building territory—consider consulting a structural engineer if you'll store vehicles or heavy equipment.

Start your planning with accurate calculations using our Concrete Bags Calculator or Post Hole Calculator. Buy 10% extra bags—the $25-50 in extra bags beats a $100+ emergency trip to the store when you discover you're 3 bags short with half-set concrete in your mixer.

Remember: a shed foundation done right lasts 30-50 years. The foundation done wrong fails in 3-5 years. Don't shortcut the concrete quality or quantity to save $50—it's the worst possible economy. Calculate accurately, buy the right amount, mix properly, and your shed will sit solid for decades.