How Many Bags of Concrete Do I Need? 80 lb, 60 lb and 50 lb Bags

An 80 lb bag yields 0.60 cubic feet, so one cubic yard (27 cubic feet) takes 45 bags; 60 lb bags (0.45 cu ft) take 60 and 50 lb bags (0.375 cu ft) take 72.

To size your own job, divide the concrete volume you need, in cubic feet, by the yield printed on the bag, then round up. A 10 ft × 12 ft slab, 4 in thick, needs about 74 bags of 80 lb mix with a 10% allowance. Yields vary by product, so check your bag.

How do you calculate how many bags of concrete you need?

volume (cu ft) = length (ft) × width (ft) × thickness (in) ÷ 12
bags = ceil( volume × (1 + allowance) ÷ yield per bag )

1. Find the volume of the slab, footing or hole in cubic feet. Convert thickness from inches to feet by dividing by 12.
2. Add an allowance. Most estimators use 10% for a simple slab and 12% to 15% for stepped, curved or congested pours. This is a convention, not a standard. Multiply by 1.10, do not add 10 to the bag count.
3. Divide by the yield on the bag, in cubic feet per bag.
4. Round up to whole bags. You cannot buy 0.3 of a bag, and a pour that runs short is harder to fix than a spare bag.

How many bags of concrete are in a yard?

Yield depends on the product, not just the weight. These are printed on the manufacturers’ data sheets for two common mixes.

The table follows the QUIKRETE Concrete Mix No. 1101 data sheet. The SAKRETE High Strength sheet lists the same yields for 40, 60 and 80 lb (and 0.68 for 90 lb), and has no 50 lb size. Fast-setting, high-early, countertop and grout mixes can differ. If your bag prints a different yield, use it. Cubic yards come from dividing cubic feet by 27, as explained in how many cubic feet are in a cubic yard.

Worked example: a 10 ft × 12 ft slab, 4 in thick

• Volume: 10 × 12 × (4 ÷ 12) = 40.000 cubic feet, which is 1.4815 cubic yards.
• With a 10% allowance: 40 × 1.10 = 44.000 cubic feet, or 1.6296 cubic yards.
• 80 lb bags (0.60): 44 ÷ 0.60 = 73.33, so 74 bags.
• 60 lb bags (0.45): 44 ÷ 0.45 = 97.78, so 98 bags.
• 50 lb bags (0.375): 44 ÷ 0.375 = 117.33, so 118 bags.

Without the allowance the counts are 67, 89 and 107 bags. Concrete weighs about 150 lb per cubic foot, so the slab itself is 40 × 150 = 6,000 lb (3 short tons). Plan the haul too: 74 bags of 80 lb is 5,920 lb of bagged mix, close to 3 short tons to load, move and stack.

Slab quick table

These counts include a 10% allowance and assume the yields above (0.60 cu ft for 80 lb, 0.45 cu ft for 60 lb). Use them for planning, not as a substitute for your own measurements.

How many bags of concrete does a fence post or tube take?

For a round hole or tube, the volume is π × (diameter ÷ 24)² × depth, with the diameter in inches and the depth in feet. For a post set in concrete, subtract the volume of the post itself.

Tubes. A 4 ft tube holds this much, which agrees with the QUIK-TUBE table for 80 lb bags: 8 in diameter, 1.3963 cu ft, 3 bags; 10 in, 2.1817 cu ft, 4 bags; 12 in, 3.1416 cu ft, 6 bags. Use the inside diameter of the tube.

A 4×4 fence post. The QUIKRETE Fast-Setting data sheet sizes the hole at three times the post width, with a depth of half the above-ground post height. For a 6 ft fence, that is a 3 ft deep hole. A 4×4 is really 3.5 in square, so the hole is 10.5 in across.

• Hole: π × (10.5 ÷ 24)² × 3 = 1.8040 cubic feet
• Post: (3.5 ÷ 12)² × 3 = 0.2552 cubic feet
• Concrete per post: 1.8040 − 0.2552 = 1.5488 cubic feet
• Standard mix, 80 lb (0.60): 2.58, so 3 bags per post
• Fast-Setting, 60 lb (0.45): 3.44, so 4 bags per post

For ten posts, that is 15.49 cubic feet, or 25.8 bags of 80 lb mix. Round up to 26, or 29 with a 10% allowance. Frost depth and local code can require deeper holes than the half-height rule, and gate posts are often set deeper. Call 811 before you dig. For 6×6 posts and a table by fence height, see how much concrete per fence post.

Bags or ready-mix?

Bags make sense for small pours. As a rough guide, once you pass about 200 bags (120 cubic feet, a little over 4 cubic yards of 80 lb mix), get a ready-mix quote too. Ready-mix is sold by the cubic yard, usually in quarter-yard steps, so the 10 × 12 ft slab above (1.6296 yd³ with allowance) rounds to 1.75 yd³. Ask the supplier about minimum loads and short-load fees, which many apply to small orders. If you are buying bags by the pallet, how many bags of concrete are on a pallet gives the count and weight.

Assumptions and limits

• Forms are rectangular and the thickness is uniform. Uneven subgrade, form flex and spillage are what the allowance is for.
• The numbers are quantities, not design. Slab thickness, reinforcement, base and footing size depend on local code, soil, loads and frost depth. Ask your building department or a licensed engineer.
• Water, strength and curing are product-specific and are on the bag.
• Wet concrete is caustic. Wear gloves and eye protection.

Questions people ask