How to do it
- 1
Measure the project area
Measure to the inside of the forms, in more than one place along each dimension, and use the larger figure where they differ. Square footage is length times width for a rectangle, pi times the radius squared for a circle, and half the base times the height for a triangle.
- 2
Take the thickness from your specification
Thickness comes from your plan, your designer, or local code. It is the most sensitive number in the calculation, so it should never be guessed or borrowed from another project.
- 3
Calculate cubic feet, then cubic yards
Multiply the area in square feet by the thickness in feet to get cubic feet, then divide by 27 for cubic yards. The one-step version is square feet times thickness in inches divided by 324.
- 4
Convert to bags if you are buying bags
Divide the cubic feet needed by the yield printed on the bag, then round up. Never use a remembered yield: it differs by product and bag weight.
- 5
Handle footings, post holes, and form tubes separately
Rectangular footings use length times width times depth. Round elements use pi times the radius squared times the depth. Calculate each type on its own and add the totals.
- 6
Add every section before converting
For a project with several parts, total all the areas or volumes first and convert once at the end. Converting each part separately and adding rounded figures introduces avoidable error.
- 7
Choose your own allowance
Add a percentage for subgrade variation, over-dig, and spillage, based on how the site actually looks and how costly a shortfall would be. There is no universal figure.
- 8
Compare ready-mix and bags on your own numbers
Price both routes with your quoted rate per cubic yard and your bag price, remembering that fixed delivery fees weigh most heavily on small orders.
- 9
Confirm the order with the supplier
Take your volume in cubic yards, your mix, and your schedule to the supplier, and confirm increments, access, and timing before booking.
Formula
Cubic yards = (square feet x thickness in inches) / 324
For round elements such as post holes and form tubes, use pi times the radius squared times the depth to get cubic feet, then divide by 27.
The formula for each part of a project
Most projects are a combination of these. Calculate each part in cubic feet, add them, then convert once.
- Slab, patio, driveway
- Length(ft) x width(ft) x thickness(ft)
- Circular pad
- pi x (diameter(ft) / 2)^2 x thickness(ft)
- Strip or pad footing
- Length(ft) x width(ft) x depth(ft)
- Post hole or form tube
- pi x (diameter(ft) / 2)^2 x depth(ft)
- Post hole with the post subtracted
- Hole volume - post volume
- Cubic feet to cubic yards
- Cubic feet / 27
- Bags
- Cubic feet / bag yield, rounded up
Worked example
A project with a slab, a footing, and six post holes, worked through end to end
- 1Slab: 16 ft x 20 ft at 4 in. Area 320 sq ft, volume 320 x 0.3333 = 106.67 cubic feet.
- 2Footing: 30 ft long, 1.5 ft wide, 8 in deep. Volume 30 x 1.5 x 0.6667 = 30 cubic feet.
- 3Post holes: 12 in across, 36 in deep, six of them. Each is pi x 0.5^2 x 3 = 2.36 cubic feet, so 14.14 cubic feet total.
- 4Add everything: 106.67 + 30 + 14.14 = 150.81 cubic feet.
- 5Convert once: 150.81 / 27 = 5.59 cubic yards.
The whole project needs about 5.59 cubic yards. Adding the three parts in cubic feet before converting keeps the arithmetic clean and avoids compounding rounding.
Worked examples
The same project in bags
Converting the 150.81 cubic feet above at a 0.6 cubic foot bag yield
- 1Divide by the yield entered: 150.81 / 0.6 = 251.35 bags.
- 2Round up: 252 bags.
- 3At a 0.45 yield the same project would take 336 bags instead.
252 bags at that example yield, or 336 at a lower one. The difference of 84 bags for the same project is why the yield must come from the bag you are buying.
Checking a supplier's figure
A supplier quotes 6 cubic yards for the project above
- 1Your own calculation: 5.59 cubic yards.
- 2Difference: 6 - 5.59 = 0.41 cubic yards, about 7 percent more.
- 3That is a plausible allowance rather than an error, so it is worth asking what it covers.
Doing your own take-off does not replace the supplier's, but it tells you whether a quoted quantity is in the right range and gives you a specific question to ask.
Start with the area, not the volume
Almost every concrete question begins as an area question. How big is the slab, the patio, the driveway, the pad. Get that right and the rest is arithmetic; get it wrong and no amount of careful conversion will save the estimate.
Measure to the inside of the forms, because that is the space concrete will occupy. Measure each dimension in at least two places, since forms and marked outlines are rarely as parallel as they look, and use the larger figure or the average when they differ.
For circles, measure the diameter through the center rather than doubling a radius taken from one point. Because area depends on the square of the radius, a small diameter error has an outsized effect on the result.
Thickness is a specification, not a choice you make here
Volume scales directly with thickness, which makes it the most sensitive input in the whole calculation. Moving a 500 square foot slab from 4 inches to 5 inches adds 1.54 cubic yards, a 25 percent increase for one inch.
That sensitivity is exactly why YardCalc does not suggest thicknesses. What a slab, footing, driveway, or pad needs depends on the loads it will carry, the ground beneath it, the reinforcement in it, the climate, and the applicable code. Those are engineering and regulatory questions, not arithmetic ones.
Take the thickness from your project drawings, your designer, or your building department, and enter that. If a result looks wrong by a large factor, the thickness field is the first place to look, since entering inches where feet were expected changes the answer twelvefold.
Measuring the parts that are not slabs
Footings are calculated like slabs, just with different proportions: length along the trench, width across it, and depth as the concrete thickness rather than the excavation depth. For a footing that turns corners, measure along the centerline, because the outside face double-counts corners and the inside face undercounts them. The concrete footing calculator handles these.
Post holes and round form tubes are cylinders, so they use pi times the radius squared times the depth. The critical detail is that volume grows with the square of the diameter: a 12 inch hole holds four times what a 6 inch hole holds at the same depth. Use the post hole calculator or the form tube calculator for these.
A post sitting in a hole displaces concrete, and subtracting it gives the more precise figure. Whether it is worth doing depends on the ratio: a 3.5 inch post in a 12 inch hole takes up about 11 percent of the volume, while the same post in an 8 inch hole takes up around 25 percent.
Adding several sections
Most real projects have more than one part. A patio and its walkway, a driveway and its apron, a slab and its thickened edge, a deck with footings and piers. Each is simple alone and awkward if you try to average them into one shape.
The reliable method is to calculate each part in cubic feet, add them all, and convert to cubic yards once at the end. Converting each part separately, rounding each, and summing the rounded figures compounds error unnecessarily.
Keep a written list of the parts as you measure. The most common omission is not a mismeasurement but a forgotten section: the walkway, the landing, the step, the pad by the gate.
Adding an allowance carefully
Calculated volume assumes exact dimensions and a perfectly flat base. Neither holds in practice. Subgrades dip, forms bow, trenches over-dig, and some material is always lost to spillage and to what stays in the chute or mixer.
How much to add depends on the job. A slab on a well-prepared, laser-graded base needs less than a footing poured against rough-cut earth. A pour that must be continuous justifies more caution than a set of fence posts you can top up next weekend.
YardCalc does not publish an allowance percentage, because a single figure would be wrong for most projects and would be treated as authoritative. Look at the actual site, weigh what a shortfall would cost you, choose a figure, and note it so you can tell the supplier what your number already includes.
Ready-mix or bags
Both deliver the same calculated volume. This compares how they differ in practice so you can weigh them against your own site, not so the page can pick one.
| Question | Ready-mix | Bags |
|---|---|---|
| Ordered in | Cubic yards | Whole bags, via the printed yield |
| Best suited to | Larger continuous pours | Small pours, posts, piers, repairs |
| Access needed | Truck access and chute reach, or a pump | Anywhere you can carry bags and mix |
| Placement pace | Set by delivery, usually fast | Set by your mixing capacity |
| Topping up | Difficult once the truck leaves | Easy, buy another bag |
| Main cost driver | Rate per yard plus fixed fees | Price per bag times the count |
Ordering checklist
Before placing a concrete order, settle each of these:
- Your calculated volume in cubic yards, with the allowance you chose noted separately.
- The thickness and dimensions you calculated from, so you can answer questions about them.
- The mix your project specifies, confirmed with whoever specified it.
- Whether the pour has to be continuous, and how many loads that means.
- The supplier's order increments and how partial loads are handled.
- Site access: truck route, overhead clearance, turning space, chute reach, and whether a pump is needed.
- Timing, and whether the forms, reinforcement, and crew will be ready when the concrete arrives.
- What happens to any surplus, and what happens if you run short.
Common mistakes
The errors that account for most concrete shortfalls:
- Multiplying by thickness in inches without converting to feet, overstating the volume twelvefold.
- Measuring outside the forms rather than inside.
- Averaging an irregular outline into a single rectangle.
- Forgetting a section entirely, most often a walkway, landing, or thickened edge.
- Using a bag yield from memory rather than from the bag being bought.
- Confusing hole diameter with post width on post holes.
- Converting each section separately and adding rounded results.
- Ordering the bare geometric figure with no allowance on a pour that cannot be topped up.
- Treating any calculated figure as a final order quantity without talking to the supplier.
Square feet to concrete cubic yards by thickness
Cubic yards required for a given area at a given thickness. Thickness columns are reference values, not recommendations.
| Area | 2 in | 3 in | 4 in | 5 in | 6 in | 8 in | 12 in |
|---|---|---|---|---|---|---|---|
| 25 sq ft | 0.154 | 0.231 | 0.309 | 0.386 | 0.463 | 0.617 | 0.926 |
| 50 sq ft | 0.309 | 0.463 | 0.617 | 0.772 | 0.926 | 1.23 | 1.85 |
| 100 sq ft | 0.617 | 0.926 | 1.23 | 1.54 | 1.85 | 2.47 | 3.7 |
| 150 sq ft | 0.926 | 1.39 | 1.85 | 2.31 | 2.78 | 3.7 | 5.56 |
| 200 sq ft | 1.23 | 1.85 | 2.47 | 3.09 | 3.7 | 4.94 | 7.41 |
| 300 sq ft | 1.85 | 2.78 | 3.7 | 4.63 | 5.56 | 7.41 | 11.11 |
| 500 sq ft | 3.09 | 4.63 | 6.17 | 7.72 | 9.26 | 12.35 | 18.52 |
| 750 sq ft | 4.63 | 6.94 | 9.26 | 11.57 | 13.89 | 18.52 | 27.78 |
| 1,000 sq ft | 6.17 | 9.26 | 12.35 | 15.43 | 18.52 | 24.69 | 37.04 |
Bag-yield math: bags per cubic yard
Mathematical bag-yield examples showing 27 cubic feet divided by each yield. Use the yield printed on the actual concrete bag or supplier documentation.
| Bag volume | Bags per cubic yard | Whole bags for 1 cubic yard |
|---|---|---|
| 0.33 cu ft | 81.82 | 82 |
| 0.45 cu ft | 60 | 60 |
| 0.6 cu ft | 45 | 45 |
| 0.75 cu ft | 36 | 36 |
| 1 cu ft | 27 | 27 |
Round element volume examples
Full cylinder volume for a range of hole and tube sizes, before any post is subtracted. These sizes demonstrate the arithmetic and are not recommended dimensions.
| Diameter x depth | Cubic feet each | Cubic yards each | Number that fit in 1 cubic yard |
|---|---|---|---|
| 8 in x 24 in | 0.698 | 0.0259 | 38 |
| 10 in x 36 in | 1.64 | 0.0606 | 16 |
| 12 in x 36 in | 2.36 | 0.0873 | 11 |
| 12 in x 48 in | 3.14 | 0.116 | 8 |
Frequently asked questions
- How much concrete do I need?
- Measure the area in square feet, multiply by the thickness in inches, and divide by 324 to get cubic yards. For round elements such as post holes, use pi times the radius squared times the depth to get cubic feet, then divide by 27. Add every part of the project before converting.
- How many yards of concrete do I need for a 20 x 20 slab?
- A 20 by 20 foot slab is 400 square feet. At 4 inches that is 4.94 cubic yards, at 5 inches 6.17 cubic yards, and at 6 inches 7.41 cubic yards. Use the thickness from your project specification.
- How many bags of concrete do I need?
- Divide the volume you need in cubic feet by the yield printed on the bag, then round up. One cubic yard is 27 cubic feet, which is 45 bags at a 0.6 cubic foot yield and 60 bags at a 0.45 yield.
- How do I measure for concrete?
- Measure to the inside of the forms, take each dimension in more than one place, and split irregular outlines into shapes you can measure exactly. For circles, measure the diameter through the center rather than doubling a radius.
- How much extra concrete should I order?
- There is no universal figure, and YardCalc does not publish one. The right allowance depends on how level the subgrade is, whether the pour is against forms or earth, how carefully it was measured, and what a shortfall would cost on your job. Choose a percentage yourself and tell the supplier what your number includes.
- Should I order ready-mix or buy bags?
- It depends on the volume, site access, how fast the concrete must be placed, and how much mixing labor you have. Large continuous pours generally favor delivery; small pours, posts, and repairs often favor bags. Price both from your own figures with the concrete cost calculator.
- Do I need to subtract the post from a post hole?
- Only if you want the tighter figure. A 3.5 inch post in a 12 inch hole displaces about 11 percent of the volume, while the same post in an 8 inch hole displaces around 25 percent. Leaving the post in gives a conservative estimate.
- Can I trust a supplier's quantity without checking?
- Doing your own take-off is worthwhile even when a supplier or contractor provides one. It tells you whether a quoted quantity is in the right range and gives you a specific question to ask if it is not, which is far easier to resolve before the pour than during it.
Every formula on this page is exact geometry, with one cubic yard treated as exactly 27 cubic feet. No thickness, footing size, hole size, bag yield, allowance percentage, or price is prescribed: each of those is a project decision or a supplier figure. Concrete calculations estimate material quantity from geometry and user-entered values. Confirm mix design, thickness, reinforcement, code requirements, bag yield, delivery, and final order quantity with the appropriate supplier or professional.
Results are planning estimates. Last reviewed 2026-07-28.