One of the most common questions in construction is "how much concrete do I need?" Under-ordering means a second trip and a visible pour line. Over-ordering wastes money. This calculator gives you the exact volume for any shape โ slab, footing, column, tube, or stairs โ then converts it to the number of pre-mixed bags you'll need.
Choose your shape, enter dimensions in metric or imperial, add a waste percentage (5โ15% is typical), and get instant results: total volume in cubic yards or cubic meters, number of bags by size, estimated weight, and cost. The calculator handles all the unit conversions internally so you never have to think about feet-to-inches or cm-to-meters.
Whether you're pouring a patio, setting fence posts, building a retaining wall footer, or forming concrete steps โ this tool saves you from the math and from the hardware store return line.
How to Use
1. Select a shape: Slab, Footing/Wall, Column, Tube/Post Hole, or Steps
2. Toggle between Metric (m, cm) and Imperial (ft, in)
3. Enter your dimensions
4. Adjust waste percentage (default 10%)
5. See total volume, bag counts, weight, and estimated cost
6. Use "Copy Link" to share your calculation
Frequently Asked Questions
How much concrete do I need for a 10ร10 slab?
A 10ร10 foot slab that is 4 inches thick requires about 1.23 cubic yards of concrete, or roughly 56 bags of 80lb pre-mixed concrete. Add 10% waste for a total of about 62 bags.
How do I calculate concrete in cubic yards?
Multiply length ร width ร depth (all in feet), then divide by 27 to convert cubic feet to cubic yards. For example: 10ft ร 10ft ร 0.33ft = 33.3 cu ft รท 27 = 1.23 cubic yards.
What is the waste factor for concrete?
A waste factor of 5โ15% is standard. Use 5% for simple, flat pours on solid ground. Use 10% for most residential projects. Use 15% for irregular shapes, slopes, or when forms may shift. The extra concrete accounts for spillage, uneven subgrade, and over-fill.
How many bags of concrete do I need?
It depends on bag size. An 80lb (36kg) bag yields about 0.6 cubic feet. A 60lb (27kg) bag yields about 0.45 cu ft. A 40lb (18kg) bag yields about 0.3 cu ft. In metric, a 25kg bag yields about 0.012 mยณ and a 40kg bag yields about 0.019 mยณ. This calculator computes exact counts for all sizes.
Should I use pre-mixed bags or ready-mix truck delivery?
For projects under 1 cubic yard (about 0.76 mยณ), pre-mixed bags are practical. For 1โ3 cubic yards, it depends on your timeline and labor. For anything over 3 cubic yards, a ready-mix truck delivery is usually cheaper and faster. This calculator shows volume so you can decide.
How do I calculate concrete for round columns or post holes?
Use the Column or Tube shape option. Enter the diameter and height, and the calculator uses the formula ฯ ร (diameter/2)ยฒ ร height to compute the cylindrical volume.