Concrete Bag Calculator
Count the bags of premix for a slab, some post holes, or a tube column. Enter the dimensions and the yield printed on your bag, and get the bag count plus the volume in cubic feet and yards — so you know when it's time to call for ready-mix instead.
Slabs and steps are usually 4 in; a bag-friendly pad rarely needs more.
Square post width — leaves out the space the post takes up. Blank fills the whole hole.
For round form tubes and columns.
Prefilled from the bag size — confirm the yield printed on your bag, since it varies by mix.
Advanced: price
Formula & how it works
Slab: volume = length × width × (thickness ÷ 12) ft³.
Holes / tubes: volume = π × (diameter ÷ 24)² × (depth ÷ 12) × count ft³. A square post, if entered, is subtracted from each hole.
Bags = volume × (1 + waste) ÷ yield per bag, rounded up. One cubic yard is 27 ft³.
Worked example
A 10 × 10 ft pad at 4 in thick is 33.3 ft³. With 10% waste that's about 36.7 ft³ — roughly 1.4 cubic yards. At 0.6 ft³ per 80 lb bag, that's 62 bags. At that point the cubic-yard figure is a hint that ready-mix would be easier than sixty-two bags in a wheelbarrow.
Bagging concrete well
Yield is a bag number, not a universal one
Every mix packs differently, so the cubic feet a bag yields is printed right on it — and fast-setting, crack-resistant, and high-strength blends don't match the standard number. Enter the yield from your actual product rather than trusting a rule of thumb, especially on a bigger pour where a small per-bag difference multiplies into several bags.
Know when to switch to ready-mix
Bags shine for post holes, steps, small pads, and repairs. Once the volume climbs past about a cubic yard, the bag count and the mixing labor both get painful, and a ready-mix truck is usually cheaper and far faster. Watch the cubic-yard readout as much as the bag count.
Buy a spare and mix to the line
Running short mid-pour means a cold joint where fresh concrete meets set concrete, so round up and keep a spare bag on hand. And follow the water amount on the bag — too much water weakens the finished concrete more than almost anything else you can do.