How many bags of concrete do I need for a 10x10 slab?

A 10×10 ft slab at 4 inches thick requires 1.23 cubic yards. That equals 56 bags of 80 lb concrete, 74 bags of 60 lb, or 111 bags of 40 lb. With a 10% waste factor, buy 62 bags of 80 lb concrete. At 6 inches thick, you need 84 bags of 80 lb concrete.

How many 80 lb bags of concrete are in a cubic yard?

One cubic yard of concrete requires 45 bags of 80 lb concrete. Each 80 lb bag yields 0.60 cubic feet, and 1 cubic yard = 27 cubic feet. So 27 ÷ 0.60 = 45 bags per cubic yard.

How many bags of concrete do I need for a 12x12 slab?

A 12×12 ft slab at 4 inches thick needs 1.78 cubic yards, or 80 bags of 80 lb concrete (88 with 10% waste). At 5 inches thick: 2.22 yd³, or 100 bags. At 6 inches thick: 2.67 yd³, or 120 bags of 80 lb concrete with waste.

Is it cheaper to buy bags of concrete or order ready-mix?

For projects under 1 cubic yard, bagged concrete is usually cheaper. For anything over 1 cubic yard, ready-mix is more economical when you factor in the labor cost of mixing. Ready-mix costs $125–$175 per cubic yard delivered, while bagged concrete costs $180–$250 per yard in materials alone — plus hours of mixing labor.

How many bags of concrete do I need for fence posts?

For a standard 10-inch diameter fence post hole at 3 feet deep, you need about 2.7 bags of 80 lb concrete per hole. For an 8-inch hole at 2 feet deep: about 1.2 bags. Multiply by the number of posts to get your total. Always round up and keep one or two spare bags on hand.

How many bags of concrete do I need for a 20x20 slab?

A 20×20 ft slab at 4 inches thick needs 4.94 yd³, which is 222 bags of 80 lb concrete (245 with waste). At 5 inches thick: 6.17 yd³, or 278 bags. At this volume, ordering ready-mix concrete is strongly recommended over mixing bags manually.

What is the yield of an 80 lb bag of concrete?

An 80 lb bag of Quikrete or Sakrete concrete mix yields approximately 0.60 cubic feet (0.022 cubic yards) of mixed concrete. A 60 lb bag yields 0.45 ft³. A 40 lb bag yields 0.30 ft³.

How many bags of Quikrete do I need for a slab?

Calculate your cubic footage (L × W × thickness in feet), then divide by 0.60 for 80 lb Quikrete bags, or 0.45 for 60 lb bags. For a 10×10 slab at 4 inches: 33.3 ft³ ÷ 0.60 = 56 bags of 80 lb Quikrete. Add 10% — buy 62 bags.

How much water do I add to an 80 lb bag of concrete?

An 80 lb bag of concrete typically requires 3 quarts (0.75 gallons) of water. Add water gradually and mix thoroughly. Do not add extra water to make the mix more fluid — excess water weakens the concrete significantly. The mix should have a peanut butter consistency.

How many bags of concrete do I need for a 4x4 post?

A 4×4 post set in a 10-inch diameter hole at 2 feet deep needs about 1.2 bags of 80 lb concrete. At 3 feet deep (below frost line in most of the US), use 1.8 bags. Round up to 2 bags per hole to be safe.

How Many Bags of Concrete Do I Need? (Calculator + Charts)

How Many Bags of Concrete Do I Need?

Enter your project dimensions and instantly get the exact bag count for 40, 60, and 80 lb bags — with waste factor included.

✓ 40 / 60 / 80 lb Bags ✓ Cubic Yards Included ✓ 10% Waste Built In ✓ 100% Free

Quick Answer

For a standard 10×10 ft slab at 4 inches thick, you need 62 bags of 80 lb concrete (including 10% waste). One cubic yard requires 45 bags of 80 lb, 60 bags of 60 lb, or 90 bags of 40 lb concrete. Use the calculator below for your exact dimensions.

🧱 Concrete Bag Calculator

Length (ft)

Width (ft)

Thickness (in)

Qty of Sections

Volume (cubic yards)

Waste Factor (%)

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Cubic Yards

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Cubic Feet

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Cubic Meters

Bags needed (includes 10% waste):

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40 lb Bags

0.30 ft³ yield

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60 lb Bags

0.45 ft³ yield

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80 lb Bags

0.60 ft³ yield

The Formula: How to Calculate Bags of Concrete

Every concrete bag calculation comes down to three steps. Here’s the full formula with a worked example:

Step 1: Volume (ft³) = Length × Width × (Thickness ÷ 12)
Step 2: Cubic Yards = ft³ ÷ 27
Step 3: 80 lb Bags = Cubic Yards × 45 (+10% waste)
60 lb Bags = Cubic Yards × 60 (+10% waste)
40 lb Bags = Cubic Yards × 90 (+10% waste)

Example: 10×10 slab, 4″ thick → 10×10×(4÷12)=33.3 ft³ → ÷27=1.23 yd³ → ×45×1.10 = 62 bags (80 lb)

⚠️ Always add a 10% waste factor. Running short on pour day forces you to stop mid-slab — this creates a “cold joint” where the first batch hardens before you add the second. Cold joints are a structural weak point and are permanent. It is always better to have extra bags than to run short.

How Many Bags of Concrete Per Cubic Yard?

This is the single most important reference table for concrete bag calculations. One cubic yard = 27 cubic feet. Each bag size has a fixed yield:

Concrete bag yield and bags required per cubic yard
Bag Size	Yield (ft³)	Yield (yd³)	Bags Per Cubic Yard	Coverage @ 4″ Thick
40 lb bag	0.30 ft³	0.011 yd³	90 bags	~1.1 ft²
50 lb bag	0.375 ft³	0.014 yd³	72 bags	~1.4 ft²
60 lb bag	0.45 ft³	0.017 yd³	60 bags	~1.7 ft²
80 lb bag	0.60 ft³	0.022 yd³	45 bags	~2.2 ft²

The 80 lb bag is the most economical choice for most DIY projects — it has the highest yield per bag and costs less per cubic foot than smaller bags. Use 60 lb bags if you’re mixing by hand and the weight is an issue. Avoid 40 lb bags except for very small repairs.

40 lb bag

0.30 ft³

90 bags/yd³

60 lb bag

0.45 ft³

60 bags/yd³

80 lb bag

0.60 ft³

45 bags/yd³

How Many Bags for Every Common Slab Size

Use this quick-reference chart for common residential concrete slab sizes. All figures include a 10% waste factor and use 80 lb bags. For 60 lb bags, multiply cubic yards by 66 (60 × 1.10). For 40 lb bags, multiply by 99 (90 × 1.10).

Bags of 80 lb concrete needed by slab size (with 10% waste)
Slab Size	Sq Ft	At 3.5″ thick	At 4″ thick	At 5″ thick	At 6″ thick
4 × 4 ft	16	7	8	10	12
4 × 8 ft	32	14	16	20	24
6 × 6 ft	36	16	18	22	27
8 × 8 ft	64	28	32	40	48
10 × 10 ft	100	43	62	77	93
10 × 20 ft	200	86	99	123	148
12 × 12 ft	144	61	88	110	132
12 × 20 ft	240	104	119	148	178
16 × 16 ft	256	110	126	157	189
20 × 20 ft	400	173	222	278	333
24 × 24 ft	576	249	285	356	427
30 × 30 ft	900	389	444	556	667

Highlighted rows = most commonly searched slab sizes. For projects over 1 yd³, consider ordering ready-mix concrete.

How Many Bags for Specific Projects

How Many Bags for a 10×10 Slab?

A 10×10 ft slab is one of the most common DIY concrete projects. Here’s the exact count at every thickness:

Thickness	Cubic Yards	40 lb Bags	60 lb Bags	80 lb Bags
3.5 in (walkway)	1.07 yd³	118	71	53
4 in (patio / slab)	1.23 yd³	122	82	62
5 in (driveway)	1.54 yd³	153	102	77
6 in (garage floor)	1.85 yd³	184	122	93

How Many Bags for a 12×12 Slab?

A 12×12 ft slab at 4 inches thick needs 1.78 yd³, or 88 bags of 80 lb concrete with waste. At 6 inches thick (garage floor standard): 2.67 yd³ → 132 bags. At this volume, you are right at the break-even point between bags and ready-mix — either is viable.

How Many Bags for Fence Posts?

Post holes use the round column formula: π × radius² × depth. Here are pre-calculated counts:

Hole Diameter	Depth	Volume	80 lb Bags (per hole)
6 in	2 ft	0.03 yd³	1.5 bags
8 in	2 ft	0.05 yd³	2.5 bags
10 in	3 ft	0.11 yd³	3 bags (buy 3)
12 in	3.5 ft	0.17 yd³	5 bags (buy 5)
12 in	4 ft	0.19 yd³	5–6 bags

For 8 fence posts at 10″ × 3″ depth: 8 × 3 = 24 bags of 80 lb concrete. Always buy 2–3 extra bags to account for loose or oversized holes.

How Many Bags for a Sidewalk?

Standard residential sidewalks are 4 ft wide and 3.5–4 inches thick. A 4 ft × 20 ft sidewalk at 4 inches thick needs 0.99 yd³ — approximately 49 bags of 80 lb concrete with waste. A 4 ft × 50 ft sidewalk: 2.47 yd³ → 122 bags (at this volume, order ready-mix).

How Many Bags for Steps / Stairs?

A standard 4-step staircase (3 ft wide, 7″ rise, 10″ run) uses approximately 0.5–0.7 yd³ of concrete — roughly 25–32 bags of 80 lb concrete. Use our stairs calculator for exact figures based on your step count and dimensions.

Step-by-Step: How to Calculate Bags for a Patio

Real worked example for a 14 × 12 ft backyard patio at 4 inches thick:

Convert thickness to feet

4 inches ÷ 12 = 0.333 ft

Calculate cubic feet

14 × 12 × 0.333 = 55.9 ft³

Convert to cubic yards

55.9 ÷ 27 = 2.07 yd³

Add 10% waste

2.07 × 1.10 = 2.28 yd³ — your order quantity

Calculate 80 lb bags

2.28 × 45 = 103 bags of 80 lb concrete

Decide: bags or ready-mix?

At 2.28 yd³, ordering ready-mix is recommended. If using bags, you’ll need to mix 103 bags — plan for a full day with a rented mixer.

When to Use Bags vs. Ready-Mix Concrete

Volume	Best Option	Reason
Under 0.5 yd³	Bags	Small repairs, post holes — bags are fast and convenient
0.5 – 1.0 yd³	Bags	Still manageable — rent a small mixer to save mixing time
1.0 – 2.0 yd³	Either	Compare ready-mix delivery fee vs. bag cost + labor
Over 2.0 yd³	Ready-mix	Cheaper per yard and avoids cold joints from batch-to-batch mixing

Cost comparison per cubic yard (2025):
Bagged concrete (80 lb bags): $180–$250/yd³ in materials alone — plus 1–2 hours of mixing labor per yard.
Ready-mix delivered: $125–$175/yd³ including delivery. Minimum order is usually 1 yd³; short-load fee applies under 5 yd³.

Pro Tips Before You Buy Concrete Bags

📦

Round up, never down

If your calculation gives 57.3 bags, buy 60. An extra bag costs $6–$8. Running short mid-pour costs far more.

💧

Don’t over-water

An 80 lb bag needs ~3 quarts of water. Adding extra water weakens the mix. Each gallon added per yd³ reduces strength by ~200 PSI.

📅

Check the batch date

Old bags partially cure inside the bag, reducing yield and strength. Look for bags made within the last 6 months.

🔧

Rent a mixer

Hand-mixing is practical up to 15 bags. For larger pours, rent a drum mixer — it pays for itself in time and quality.

🌡️

Temperature matters

Don’t pour below 40°F or above 90°F without special precautions. Hot weather dries the concrete too fast; cold weather stops curing.

🏗️

Prep your subbase first

Compact the soil base and add 4″ of gravel. A bad subbase causes settling and cracking — no amount of concrete fixes a soft base.

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Frequently Asked Questions

The 20 most-asked questions about concrete bag calculations, answered with exact numbers.