Concrete Bag Calculator

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Concrete Bag Calculator
Rectangular Slab / WallCircular Column / Hole
80 lb bag (0.60 cu ft)60 lb bag (0.45 cu ft)40 lb bag (0.30 cu ft)
Results:
Total Bags Needed:

Total Volume: Cubic Feet
Total Volume: Cubic Yards
function toggleInputs(){var type=document.getElementById('calc_type').value;if(type==='slab'){document.getElementById('label1′).innerHTML='Length (ft):';document.getElementById('label2′).innerHTML='Width (ft):';document.getElementById('row2′).style.display='table-row';}else{document.getElementById('label1′).innerHTML='Diameter (ft):';document.getElementById('label2′).innerHTML='Depth/Height (ft):';document.getElementById('row2′).style.display='table-row';}}function calculateConcrete(){var type=document.getElementById('calc_type').value;var v1=parseFloat(document.getElementById('input1').value);var v2=parseFloat(document.getElementById('input2').value);var v3=parseFloat(document.getElementById('input3').value);var bag=parseFloat(document.getElementById('bag_size').value);var waste=parseFloat(document.getElementById('waste').value);if(isNaN(v1)||isNaN(v2)||isNaN(v3)||isNaN(waste)){alert('Please enter valid numeric values');return;}var volCuFt=0;if(type==='slab'){volCuFt=(v1*v2*(v3/12));}else{var radius=v1/2;volCuFt=(Math.PI*Math.pow(radius,2)*v2);}var totalVol=volCuFt*(1+(waste/100));var yieldFt=0.6;if(bag===60)yieldFt=0.45;if(bag===40)yieldFt=0.3;var bagsNeeded=Math.ceil(totalVol/yieldFt);document.getElementById('bagsResult').innerHTML=bagsNeeded+' ('+bag+'lb bags)';document.getElementById('cuftResult').innerHTML=totalVol.toFixed(2);document.getElementById('cuydResult').innerHTML=(totalVol/27).toFixed(3);document.getElementById('answer').style.display='block';if(document.getElementById('steps').checked){var stepTxt='Calculation: ';if(type==='slab'){stepTxt+=v1+'ft x '+v2+'ft x '+(v3/12).toFixed(3)+'ft (thickness converted to feet) = '+volCuFt.toFixed(2)+' cu ft.';}else{stepTxt+='π x '+radius+'ft² x '+v2+'ft = '+volCuFt.toFixed(2)+' cu ft.';}stepTxt+=' Including '+waste+'% waste, total volume is '+totalVol.toFixed(2)+' cu ft. Each '+bag+'lb bag yields '+yieldFt+' cu ft.';document.getElementById('stepOutput').innerHTML=stepTxt;document.getElementById('stepOutput').style.display='block';}else{document.getElementById('stepOutput').style.display='none';}}

Calculator Use

This concrete bag calculator is an essential tool for DIYers and professional contractors alike. It helps you determine exactly how many pre-mixed concrete bags you need for projects like patios, footings, fence posts, or garden walls. By entering your project dimensions, you can avoid multiple trips to the hardware store or having excess heavy bags left over.

Whether you are pouring a standard rectangular slab or a circular pier for a deck, this calculator accounts for volume and the most common bag sizes available on the market.

Project Shape
Choose between a rectangular slab (Length x Width x Thickness) or a circular column/hole (Diameter x Depth).
Bag Size
Select the weight of the bags you plan to purchase. Standard yields are 0.60 cu ft for 80lb, 0.45 cu ft for 60lb, and 0.30 cu ft for 40lb.
Waste Margin
It is standard practice to add 10% for spillages, uneven subgrades, or measurement errors.

How It Works

Concrete is measured by volume. To find the number of bags, we first calculate the total cubic feet of the space you need to fill. The formulas used by the concrete bag calculator are:

Rectangular Volume = Length (ft) × Width (ft) × [Thickness (inches) / 12]

Circular Volume = π × [Radius (ft)]² × Depth (ft)

  • Step 1: Calculate the base volume in cubic feet based on the geometry.
  • Step 2: Multiply the volume by (1 + waste percentage) to ensure you have enough material.
  • Step 3: Divide the total required volume by the yield of your chosen bag size.
  • Step 4: Round up to the nearest whole bag, as retailers do not sell partial bags.

Calculation Example

Example: You want to pour a 10-foot by 10-foot walkway that is 4 inches thick using 80lb bags, with a 10% waste margin.

Step-by-step solution:

  1. Convert thickness to feet: 4 inches / 12 = 0.333 feet.
  2. Calculate Volume: 10ft × 10ft × 0.333ft = 33.33 cubic feet.
  3. Add 10% Waste: 33.33 × 1.10 = 36.66 cubic feet.
  4. Identify Bag Yield: An 80lb bag yields 0.60 cubic feet.
  5. Calculate Bags: 36.66 / 0.60 = 61.1.
  6. Result: You will need 62 bags of 80lb concrete.

Common Questions

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

There are 27 cubic feet in one cubic yard. Since an 80lb bag yields 0.60 cubic feet, you divide 27 by 0.60, which equals 45 bags. Therefore, it takes 45 bags of 80lb concrete to make one cubic yard.

Is it cheaper to use bags or buy bulk ready-mix?

Generally, for projects requiring less than 1 cubic yard (roughly 45-50 bags of 80lb mix), bags are more cost-effective and convenient. For larger projects, ordering a ready-mix truck is usually cheaper per cubic yard, though you must pay a delivery fee and "short load" fees if the order is small.

What is the difference between 4000 psi and 5000 psi concrete bags?

PSI (pounds per square inch) measures the compressive strength. 4000 PSI is standard for walkways and slabs. 5000 PSI is high-strength concrete used for structural applications or heavy vehicle traffic where higher durability is required.

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