Concrete Bag Calculator
Find how many bags of concrete mix you need for a slab or footing.
About Concrete Bag Calculator
Concrete is bought by volume, so the only honest way to count bags is to work out the volume first: length times width times thickness, with the thickness converted from inches to feet. A bag is then a fixed slice of that volume - printed on the sack as a yield, 0.60 cubic feet for a standard 80 lb bag - so the bag count is volume divided by yield, always rounded up, because two thirds of a bag is not something you can buy. The waste allowance matters more than people expect: a subgrade that dips half an inch across a 10 by 8 slab is most of an extra bag. Once you pass about one cubic yard, roughly 45 to 50 of the 80 lb bags, mixing by hand stops being sensible and ready-mix is normally both cheaper and better concrete, because it is batched consistently. Running out mid-pour is the one mistake you cannot correct: concrete placed against a joint that has already started to set gives you a cold joint and a permanent weak line.
Formula
Concrete Bag
Volume = length x width x thickness/12, plus waste; bags = volume / bag yield, rounded up
- Bag yield
- The finished concrete one bag makes: 0.30 cu ft for 40 lb, 0.45 for 60 lb, 0.60 for 80 lb
- thickness/12
- Thickness is entered in inches and divided by 12 to match the feet used for length and width
- cubic yard
- 27 cubic feet, the unit ready-mix is sold in
Worked example
With the default values below, Concrete Bag Calculator returns Bags Needed of 49. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Slab lengthaccepts 0–200 ft
- 10 ft
- Slab widthaccepts 0–200 ft
- 8 ft
- Thicknessaccepts 1–48 in
- 4 in
- Bag sizeone of 40 lb bag - 0.30 cu ft, 60 lb bag - 0.45 cu ft, 80 lb bag - 0.60 cu ft
- 80 lb bag - 0.60 cu ft
- Waste allowanceaccepts 0–50 %
- 10 %
Results
- Bags Needed
- 49
- Concrete Volume
- 29.33 cu ft
- Volume in Cubic Yards
- 1.09 cu yd
- Bags or Ready-Mix
- Over a cubic yard - ready-mix is usually cheaper and faster than bags
Step by step
- 1Values used
- 2Concrete Bag
- 3Bags Needed= 49
- 4Concrete Volume= 29.33 cu ft
- 5Volume in Cubic Yards= 1.09 cu yd
- 6Bags or Ready-Mix= Over a cubic yard - ready-mix is usually cheaper and faster than bags
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Concrete Bag calculated?
Volume = length x width x thickness/12, plus waste; bags = volume / bag yield, rounded up Concrete is bought by volume, so the only honest way to count bags is to work out the volume first: length times width times thickness, with the thickness converted from inches to feet. A bag is then a fixed slice of that volume - printed on the sack as a yield, 0.60 cubic feet for a standard 80 lb bag - so the bag count is volume divided by yield, always rounded up, because two thirds of a bag is not something you can buy. The waste allowance matters more than people expect: a subgrade that dips half an inch across a 10 by 8 slab is most of an extra bag. Once you pass about one cubic yard, roughly 45 to 50 of the 80 lb bags, mixing by hand stops being sensible and ready-mix is normally both cheaper and better concrete, because it is batched consistently.
Why does Concrete Bag matter?
Running out mid-pour is the one mistake you cannot correct: concrete placed against a joint that has already started to set gives you a cold joint and a permanent weak line.
What values do I need to enter?
This calculator takes 5 inputs: Slab length, Slab width, Thickness, Bag size, Waste allowance. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.