Concrete Calculator — Bags, Cubic Yards & Ready-Mix
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What this calculator does differently
Most concrete calculators handle one rectangle and stop. A real project is usually a mix: a pad plus a few post holes, a footing plus steps. Add each pour with its own shape and count, and the tool totals the volume, applies the overage you choose, and turns it into the two ways you can actually buy concrete: whole bags of a given size, or a ready-mix order rounded to the supplier's increment and minimum. Enter prices and it compares the totals, including the delivery or short-load fee that often decides small jobs. The plan stays in your browser and prints as a one-page shopping list.
The math behind each shape
- Slab or pad: length × width × thickness ÷ 12. A 10 × 10-foot pad 4 inches thick is 10 × 10 × 4 ÷ 12 = 33.3 cubic feet.
- Post hole or column: π × (diameter ÷ 2)² × depth, with inches converted to feet. Six 8-inch holes 36 inches deep come to about 6.3 cubic feet.
- Footing or strip: length in feet × width ÷ 12 × depth ÷ 12. A 20-foot footing 12 inches wide and 8 inches deep is 13.3 cubic feet.
- Steps: each step is a solid block from the ground up, so a flight of n steps is width × run × rise × (1 + 2 + … + n). Three steps 4 feet wide with a 7-inch rise and 11-inch run are 4 × 11/12 × 7/12 × 6 = 12.8 cubic feet. Steps with a hollow or gravel core use less; measure and enter the actual solid volume instead.
- Cubic yards: cubic feet ÷ 27.
Bags per cubic yard
The bag counts use the yields QUIKRETE publishes for its standard Concrete Mix: about 0.30 cubic feet for a 40 lb bag, 0.375 for 50 lb, 0.45 for 60 lb, 0.60 for 80 lb and 0.675 for the regional 90 lb bag. A cubic yard therefore takes 90, 72, 60, 45 or 40 bags. Other brands and specialty mixes can differ, so check the yield printed on the bag you plan to buy. The shopping list also shows the total bag weight, which is worth knowing before loading a car or pickup: 45 eighty-pound bags weigh 3,600 pounds.
QUIKRETE's data sheet also says to allow at least 1 cubic foot of mixer capacity for each 80 lb bag mixed at one time, which helps when planning how many batches a rented mixer will take.
Bags or ready-mix
Ready-mix is priced per cubic yard but usually sold in steps, with a minimum order and often a short-load fee for small deliveries. That means a job of 1.36 cubic yards may be billed as 1.5 yards plus delivery, while the same volume in 80 lb bags is 62 bags you mix and place yourself. Enter both sets of terms and the calculator shows the totals side by side. The cheaper option on paper also has to fit your crew, mixer and the time the concrete stays workable.
Overage is your call
Running short in the middle of a pour is worse than having a few bags left, which is why many people add an allowance for spillage, an uneven subgrade and forms that bow. How much depends on the job: a well-formed pad on a level, compacted base needs less than a hand-dug footing. The field starts at 0 so the number you see is the geometry; add the allowance yourself and watch whether it changes the bag count or pushes the ready-mix order to the next step.
Frequently asked questions
How many 80 lb bags of concrete are in a cubic yard?
QUIKRETE lists a yield of about 0.60 cubic feet for an 80 lb bag of its Concrete Mix. A cubic yard is 27 cubic feet, so it takes 27 ÷ 0.60 = 45 bags, before any allowance for spillage. The same yard takes 60 of the 60 lb bags (0.45 cu ft each) and 90 of the 40 lb bags (0.30 cu ft each).
How thick should my slab or footing be?
This calculator does not choose thickness or depth. Those are design decisions that depend on the load, soil, frost depth and local building code. Enter the dimensions from your plan, permit drawings or the product instructions, and the tool counts the material.
How much extra concrete should I order?
The overage starts at 0 so nothing is added silently. Enter an allowance you can justify for spillage, an uneven base and forms that bow. Some readers run 5 and 10 percent as two cases and compare how the bag count and ready-mix order change.
When is ready-mix cheaper than bags?
It depends on your quotes. Ready-mix is priced per cubic yard but sold in steps (often a quarter or half yard), usually with a minimum order and a delivery or short-load fee. Enter those terms and the bag price, and the tool compares the two totals for your exact volume.
How do I measure post holes or round pads?
Choose the round shape, enter the diameter and depth in inches and the number of identical holes. The volume is π × radius² × depth for each hole. Measure the hole you actually dug, since the auger size is often smaller than the finished hole.
Is my plan saved anywhere?
Only in your browser on this device. Nothing is uploaded, and Clear removes it.
Source
Bag yields and mixer guidance: QUIKRETE Concrete Mix No. 1101 product data sheet, read October 4, 2026.