Concrete calculator
Enter the shape and size of your pour. You get cubic feet, cubic yards and the number of bags, waste included, with the manufacturer's yield per bag.
Concrete calculator
33.33 cubic feet of concrete (36.67 with 10% waste), or 1.04 cubic metres.
62 bags of 80 lb pre-mixed concrete at 0.6 cubic feet per bag. Above about 1 cubic yard, ready-mix delivered by truck is usually cheaper than bags.
Formula: volume = length x width x thickness (all in feet), cubic yards = cubic feet / 27, bags = volume / yield per bag, rounded up. Yields are the manufacturer's published figures for its standard concrete mix; check the bag you buy.
How to measure a slab, footing or post hole
Concrete is sold by volume, so every job comes down to length times width times depth. Measure the inside of the forms, not the outside: the forms take up space the concrete will not. For a slab, take the length and width in feet and the thickness in inches; if the ground is not level, measure the thickness at the thickest point, since the concrete fills the low spots first. For a footing or a wall, the width and depth are usually in inches and the run in feet. For a post hole or a round pier, the diameter and depth in inches and the number of holes.
The calculator converts everything to cubic feet, adds your waste allowance, divides by 27 for cubic yards, and divides by the yield of one bag for the bag count, rounded up. The yields (0.30, 0.375, 0.45 and 0.60 cubic feet for 40, 50, 60 and 80 lb bags, and 0.675 for the 90 lb bag sold in some regions) are the figures on the manufacturer's data sheet for its standard concrete mix, given as approximate; fast-setting and high-strength mixes yield slightly differently, so read the bag.
Bags or ready-mix: where the line is
Bags are convenient up to a point, and the point is weight. One cubic yard is 45 bags of 80 lb, or 3,600 pounds to carry, open, mix and place before the first batch sets. Most people who have done it once draw the line around half a cubic yard for hand mixing and one cubic yard with a rented mixer. Past that, a ready-mix truck delivers the whole pour wet at once, which also gives a stronger, more uniform slab with no cold joints between batches.
Ready-mix is priced per cubic yard, with a short-load fee when you order less than the truck's minimum. Ask the plant for its minimum, its short-load fee and its waiting time per hour, and compare the total with the bag count from the calculator times the bag price at your store.
How much concrete common jobs take
Volumes below are computed with the calculator's formula, at the thickness shown and with no waste allowance. Add 10 percent before you buy.
| Job | Dimensions | Cubic feet | Cubic yards | 80 lb bags |
|---|---|---|---|---|
| Mailbox or fence post | 12 in hole, 30 in deep | 1.96 | 0.07 | 4 |
| Air conditioner pad | 3 x 3 ft, 4 in thick | 3.0 | 0.11 | 5 |
| Hot tub or shed pad | 8 x 8 ft, 4 in thick | 21.3 | 0.79 | 36 |
| Patio | 12 x 12 ft, 4 in thick | 48.0 | 1.78 | 80 |
| Single-car driveway | 10 x 20 ft, 5 in thick | 83.3 | 3.09 | 139 |
| Footing for a small wall | 20 ft run, 16 in wide, 8 in deep | 17.8 | 0.66 | 30 |
Computed from length x width x thickness; bags at 0.60 cubic feet each, rounded up.
Mistakes that leave you short
The thickness is where most estimates go wrong. A slab marked 4 inches on the plan is often 4.5 inches once the gravel base is raked out, and that half inch is 12 percent more concrete. Measuring the outside of the forms adds the thickness of the lumber to the length and width, which over-orders slightly; measuring a hole at the top, where it is widest, under-orders if the auger tapered. For a sloping site, measure the depth at several points and use the average, then raise the waste allowance.
The second mistake is forgetting what else goes in the hole. A footing with a keyway or a thickened edge, a slab with a turned-down edge at the perimeter, a pier with a bell at the bottom: each is a second shape to add. Run the calculator once per shape and add the results.
What to settle before you pour
Volume is the easy part. Check with your building department whether the slab or footing needs a permit and an inspection, what minimum thickness and reinforcement it requires, and how deep footings must go to sit below frost. Compact the base, set the forms level and braced, and have help lined up: an 80 lb bag mixes in about three minutes and a yard of bags is forty-five of them. Order the ready-mix for early morning in summer, so the pour sets before the heat of the day.
Questions people ask
How many bags of concrete do I need for a 10 by 10 slab?
A 10 by 10 foot slab 4 inches thick is 33.3 cubic feet, or 1.23 cubic yards. At 0.60 cubic feet per 80 lb bag of standard concrete mix that is 56 bags, 62 with a 10 percent waste allowance. At that size a short load of ready-mix is worth pricing against the bags.
How thick should a concrete slab be?
Patios, walkways and shed floors are commonly poured at 4 inches; driveways that carry cars at 4 to 6 inches; slabs that carry trucks or heavy equipment thicker still. Your local building department sets the minimum for anything structural, and frost depth sets the footing depth: ask before you pour.
How much does a yard of concrete weigh?
Standard concrete weighs about 145 to 150 pounds per cubic foot, so a cubic yard (27 cubic feet) weighs roughly 3,900 to 4,050 pounds, or about two tons. That is why bags stop making sense past a cubic yard: you would be lifting two tons through a mixer.
Do I round up the number of bags?
Yes. The calculator rounds up to the next whole bag after adding your waste allowance, because an unopened spare bag is cheap and a pour that runs short leaves a cold joint you cannot undo.
What does the waste allowance cover?
Uneven ground under the slab, forms that bow a little, spillage, and the concrete left in the mixer and wheelbarrow. Ten percent is a common allowance for a small pour on compacted gravel; on rough or sloping ground, use more.
Can I use this for a post hole?
Choose "round column or post hole", enter the diameter and depth of the hole in inches and the number of posts. The result is the concrete for the hole without subtracting the post itself; for a 4x4 post in a 12 inch hole the difference is small and goes into the waste allowance.