To calculate how many bags of concrete you need for an 15×15 slab, you’ll first need to determine the thickness of the slab you want. Standard concrete slabs are typically 4 inches thick, but you may want a different thickness depending on your project.

Concrete is a blended mix of Portland cement, sand, aggregate, water with approve ingredient. Cement is acts as binder or adhesive materials to bind the components of concrete all together to create workable paste. Sand acts as filler material that fills the voids and gap between components of concrete and remove the air. Aggregate, stone, or gravel used to strengthen the concrete strength.

To determine how many bags of premixed concrete you will need, divide the total cubic yards needed by the yield. Use the following yields per each bag size: A 40 pound bag yields 0.011 cubic yards, 60 pound bag yields 0.017 cubic yards, 50 pound bag yields 0.0138 cubic yards and 80 pound bag yields 0.022 cubic yards.

If you measure their yields in cubic feet, then yields per each bag size are following: A 40 pound bag yields 0.30 cubic feet, 60 pound bag yields 0.45 cubic feet, 50 pound bag yields 0.375 cubic feet and 80 pound bag yields 0.60 cubic feet.

To figure the amount of yards of concrete you will need, take the all measurements Length in feet, width in feet and thickness in inches. Convert thickness in feet. Multiply its length, width and thickness to get cubic footage. Multiply the total cubic footage by 0.037 to get total yards of concrete needed.

**How many bags of concrete do i need for a 15×15 slab**

If you have a 15×15 slab, you would need approximately 2.75 yards (74.25 cubic feet, or either 124- 80lb, 165- 60lb bags) of concrete at 4″ thick, 3.5 yards (93.75 cubic feet, or either 157- 80lb, 209- 60lb bags) of concrete at 5″ thick, 2.08 yards (56.25 cubic feet, or either 94- 80lb, 125- 60lb bags) of concrete at 3″ thick, or 4.167 yards (112.5 cubic feet, or either 188- 80lb, 250- 60lb bags) of concrete at 6″ thick.

**How many bags of concrete do i need for a 15×15 slab at 4 inches thick**

At 4 inches thick, for a 15×15 slab you would need 2.75 cubic yards or 74.25 cubic feet of concrete. If you use premix concrete, either 124 80-Pound, 165 60-pound, 198 50-pound, or 248 40-pound bags of premixed concrete will be required.

**Let’s do calculation and proceeds following steps:**

● 4″ thickness in feet = 4/12 = 0.33′

● Volume of concrete in cubic feet = 15 × 15 × 0.33 = 74.25

● Volume of concrete in yards = 74.25 × 0.037 = 2.75 yards.

● No. of 80lb bags = 74.25/0.60 = 124 bags

● No. of 60lb bags = 74.25/0.45 = 165 bags

● No. of 50lb bags = 74.25/0.375 = 198 bags

● No. of 40lb bags = 74.25/0.30 = 248 bags

**How many bags of concrete do i need for a 15×15 slab at 5 inches thick**

At 5 inches thick, for a 15×15 slab you would need 3.5 cubic yards or 93.75 cubic feet of concrete. If you use premix concrete, either 157 80-Pound, 209 60-pound, 259 50-pound, or 313 40-pound bags of premixed concrete will be required.

**Let’s do calculation and proceeds following steps:**

● 5″ thickness in feet = 5/12 = 0.417′

● Volume of concrete in cubic feet = 15 × 15 × 0.417 = 93.75

● Volume of concrete in yards = 93.75 × 0.037 = 3.5 yards.

● No. of 80lb bags = 93.75/0.60 = 157 bags

● No. of 60lb bags = 93.75/0.45 = 209 bags

● No. of 50lb bags = 93.75/0.375 = 250 bags

● No. of 40lb bags = 93.75/0.30 = 313 bags.

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**How many bags of concrete do i need for a 15×15 slab at 3 inches thick**

At 3 inches thick, for a 15×15 slab you would need 2.08 cubic yards or 56.25 cubic feet of concrete. If you use premix concrete, either 94 80-Pound, 125 60-pound, 150 50-pound, or 188 40-pound bags of premixed concrete will be required.

**Let’s do calculation and proceeds following steps:**

● 3″ thickness in feet = 3/12 = 0.25′

● Volume of concrete in cubic feet = 15 × 15 × 0.25 = 56.25

● Volume of concrete in yards = 56.25 × 0.037 = 2.08 yards.

● No. of 80lb bags = 56.25/0.60 = 94 bags

● No. of 60lb bags = 56.25/0.45 = 125 bags

● No. of 50lb bags = 56.25/0.375 = 150 bags

● No. of 40lb bags = 56.25/0.30 = 188 bags.

**How many bags of concrete do i need for a 15×15 slab at 6 inches thick**

At 6 inches thick, for a 15×15 slab you would need 4.167 cubic yards or 112.5 cubic feet of concrete. If you use premix concrete, either 188 80-Pound, 250 60-pound, 300 50-pound, or 375 40-pound bags of premixed concrete will be required.

**Let’s do calculation and proceeds following steps:**

● 6″ thickness in feet = 6/12 = 0.5′

● Volume of concrete in cubic feet = 15 × 15 × 0.5 = 112.5

● Volume of concrete in yards = 112.5 × 0.037 = 4.167 yards.

● No. of 80lb bags = 112.5/0.60 = 188 bags

● No. of 60lb bags = 112.5/0.45 = 250 bags

● No. of 50lb bags = 112.5/0.375 = 300 bags

● No. of 40lb bags = 112.5/0.30 = 375 bags.

**How many bags of concrete do i need for a 15×15 slab at 2 inches thick**

At 2 inches thick, for a 15×15 slab you would need 1.38 cubic yards or 37.35 cubic feet of concrete. If you use premix concrete, either 63 80-Pound, 83 60-pound, 100 50-pound, or 125 40-pound bags of premixed concrete will be required.

**Let’s do calculation and proceeds following steps:**

● 2″ thickness in feet = 2/12 = 0.166′

● Volume of concrete in cubic feet = 15 × 15 × 0.166 = 37.35

● Volume of concrete in yards = 37.35 × 0.037 = 1.38 yards.

● No. of 80lb bags = 37.35/0.60 = 63 bags

● No. of 60lb bags = 37.35/0.45 = 83 bags

● No. of 50lb bags = 37.35/0.375 = 100 bags

● No. of 40lb bags = 37.35/0.30 = 125 bags.

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**Cost estimate for 15’×15’×4″ slab**

Pre-mixed concrete bags cost on average $3.50 per 80lb bag. There are 124 bags of 80 pound premix concrete are required for 15×15 slab at 4″ thick. Using pre-mixed bags to cover 15×15 (225 sq ft) at a depth of 4″, the estimated cost is $434 (124× $3.50 = $434).

**Cost estimate for 15’×15’×5″ slab**

Pre-mixed concrete bags cost on average $3.50 per 80lb bag. There are 157 bags of 80 pound premix concrete are required for 15×15 slab at 5″ thick. Using pre-mixed bags to cover 15×15 (225 sq ft) at a depth of 5″, the estimated cost is $549.50 (157 × $3.50 = $549.50).

**Cost estimate for 15’×15’×6″ slab**

Pre-mixed concrete bags cost on average $3.50 per 80lb bag. There are 188 bags of 80 pound premix concrete are required for 15×15 slab at 6″ thick. Using pre-mixed bags to cover 15×15 (225 sq ft) at a depth of 6″, the estimated cost is $658 (188 × $3.50 = $658).

**Conclusion:**

If you have a 15×15 slab with 4 inches thick, you would need 2.75 cubic yards or 74.25 cubic feet of concrete. If you use premix concrete, either 124 80-Pound, 165 60-pound, 198 50-pound, or 248 40-pound bags of premixed concrete will be required.