Sizing Calculators
Work out how much to buy before you buy it.
Measure the space, convert the measurement to the volume or capacity the product is sold in, add a stated allowance, and round up to whole units. The section holds one calculator today, the Concrete Calculator, and it follows that method.
At DIY Repair Guide, we compiled the bag yields in the calculator from the Quikrete Concrete Mix No. 1101 data sheet, which gives them as approximate figures. We also ran the calculator's formulas on seven example pours and saved the output. No pour was made.
The sections below cover the method, the calculator's inputs and outputs, how to check measurements, why results are estimates, and where the calculator's answer stops.
How Sizing Calculators Work
Sizing takes four steps. Measure the space, convert the measurement into the unit the product is sold in, add an allowance, and round the result up to whole units.
Measuring comes first. The Concrete Calculator takes length and width in feet and thickness in inches, so one dimension entered in the wrong unit changes the result.
Converting turns the measurements into a volume. For a rectangular slab, footing or wall, the calculator multiplies length in feet by width in feet by thickness in inches, then divides by 12 to get cubic feet. Dividing cubic feet by 27 gives cubic yards, because one cubic yard is 27 cubic feet.
The allowance is an extra percentage for waste. The calculator's default is 10 percent, and the field can be changed. That default is our own planning assumption, with no published standard behind it.
Rounding up applies to whole bags. The calculator divides cubic feet by the yield of one bag and rounds the bag count up. A result of 0.60 cubic feet is one 80 lb bag, and 0.61 cubic feet is two.
The Concrete Calculator: Inputs and Outputs
The Concrete Calculator accepts two shapes. For a rectangular slab, footing or wall, it takes length in feet, width in feet and thickness in inches. For a round column or post hole, it takes diameter and depth, both in inches.
Two more inputs apply to either shape. A count of identical pieces multiplies the volume of one piece. The bag size is set to 40, 50, 60, 80 or 90 lb, and it defaults to 80 lb.
The outputs are cubic feet and cubic yards, each shown with and without the extra percentage. The calculator also returns a bag count rounded up and the dry mix weight of those bags.
Take a 10 ft x 10 ft x 4 in slab as a worked case. Multiplying 10 by 10 by 4 and dividing by 12 gives 33.33 cubic feet, or 1.23 cubic yards. Adding 10 percent extra brings the volume to 36.67 cubic feet. Dividing 36.67 by the 0.60 cubic foot yield of an 80 lb bag gives 61.1, which rounds up to 62 bags.
One cubic yard is 27 cubic feet, so 27 divided by 0.60 is 45 bags of 80 lb with no extra, which weigh 3,600 lb.
For a round column or post hole, the volume is pi times the square of the diameter in inches divided by 24, times the height in inches divided by 12. That gives cubic feet, and the calculator then applies the same bag and extra steps as for a slab.
The calculator uses these bag yields from the Quikrete data sheet, and each bag count is cubic feet to order divided by the yield, rounded up:
- 40 lb: 0.30 cubic feet per bag
- 50 lb: 0.375 cubic feet per bag
- 60 lb: 0.45 cubic feet per bag
- 80 lb: 0.60 cubic feet per bag
- 90 lb: 0.675 cubic feet per bag
Check Your Measurements First
A dimension entered in the wrong unit changes the answer.
Measure each side the formula uses. For a slab, that means length, width and thickness. For a round form, it means diameter and depth. To read the tape itself, see how to read a tape measure.
If a dimension is a fraction of an inch, convert it before you enter it. The input fields take decimal numbers. See the fraction to decimal chart for converting fractions.
Count identical pieces only when they share the same dimensions. The count field multiplies one piece's volume, so a count used for pieces of different sizes gives a figure that matches none of them.
Three inputs need a check before you read the result:
- Feet or inches: length and width are in feet, while thickness, diameter and depth are in inches.
- Thickness: a 4 inch thickness is entered as 4, and the formula divides it by 12.
- Rounding: 0.61 cubic feet of mix needs two 80 lb bags, because the count rounds up.
Why Results Are Estimates
Every bag count from the calculator is an estimate. Yield is approximate and depends on added water, so the bag count is a planning figure.
The 10 percent waste default is our planning assumption, and no published standard sets it. The calculator lets the percentage be changed.
An uneven subgrade, an over-dug hole or leaking forms call for more than 10 percent extra.
The bag count would change if the yield figures changed, because each count is cubic feet divided by one bag yield. The yields used here are the approximate values on the Quikrete data sheet.
Where the Calculator Stops
The calculator returns volume, bag counts and dry mix weight. It does not return a price, and this section does not publish prices, delivery minimums or truck sizes.
The calculator takes thickness as an input. It does not say what thickness a given load needs, and this section does not state the strength a pour must reach.
For deck structure, see the deck framing page.
This section does not cover your local code.
Next step: measure the slab, footing, wall or post hole in the units the calculator asks for, then enter those figures into the Concrete Calculator.