Bags
Concrete Bag Calculator
How many bags of concrete mix you need, with typical yields for 40 to 80 lb bags.
Open full page →Concrete estimating
Concrete Calculator
Estimate cubic yards of concrete, bags or cost for slabs, footings, columns and post holes. Everything runs in your browser.
Financial estimator
Concrete Cost Calculator
Compare the cost of bags against a ready-mix truck with your own prices.
Open full page →Step by step
How to use this concrete calculator
Pick the shape that matches your pour — slab, footing or column — and type the dimensions you measured. Each shape shows only the fields it needs, so the form stays short and nothing asks for numbers you do not have. The drawing updates as you type, so a mistyped number is easy to spot, and the results card shows the volume in cubic yards and cubic feet (or cubic meters with the metric toggle), the estimated bag count and the total bag weight. Use the wastage slider to add 0–20% extra material; 10% is a common starting point for spillage and uneven ground. Working on more than one pour? Add another shape to build up a whole project — a patio plus six piers, for example — and read a single grand total for everything together. When the numbers look right, copy a plain-text summary for your notes, print a one-page sheet to take to the supplier, or clear the project and start a new one. Nothing you enter ever leaves your browser.
Formulas and coverage
How many cubic yards of concrete do you need?
All three shapes come down to the same idea: multiply the three dimensions together to get cubic feet, then divide by 27 to get cubic yards, because one cubic yard is 27 cubic feet. Thickness and depth are usually measured in inches, so divide by 12 first to convert them to feet.
- Slab: length (ft) × width (ft) × (thickness in ÷ 12) ÷ 27
- Footing: length (ft) × (width in ÷ 12) × (depth in ÷ 12) ÷ 27
- Column: 3.14 × (diameter in ÷ 24)² × height (ft) × count ÷ 27
With metric units the same formulas apply to meters and centimeters and the result is cubic meters; there are about 1.31 cubic yards in a cubic meter. A quick mental check helps: one cubic yard covers roughly the floor of a home garage at a walking-pace thickness. The calculator handles every conversion for you and shows both unit systems side by side in the results.
How much area does one cubic yard cover?
For slabs, divide 324 by the thickness in inches: that is the square footage one cubic yard covers at that depth. (The 324 is 27 cubic feet per yard times 12 inches per foot.) The most common thicknesses work out like this:
| Slab thickness | One cubic yard covers |
|---|---|
| 4 in (10 cm) | 81 ft² (7.5 m²) |
| 5 in (13 cm) | 64.8 ft² (6.0 m²) |
| 6 in (15 cm) | 54 ft² (5.0 m²) |
As a scale reference, a two-car driveway of around 400 ft² poured at 4 in needs roughly five yards before any allowance — a size where a ready-mix truck usually starts to make sense.
Bags or ready-mix?
Bagged mix suits smaller pours: no minimum order, no delivery window to coordinate, and the work can be spread across a weekend. With a typical yield of 0.60 ft³, an 80 lb bag works out to about 45 bags per cubic yard (example) — manageable for a garden pad or a few post holes, and the bag calculator does the counting for you. Larger volumes tip the other way: mixing a hundred bags by hand is a long day, and that is where a ready-mix truck earns its per-yard price. Trucks bring a delivery fee and often a minimum order, which matters on small pours. Bag prices also climb faster than people expect once the count passes a few dozen — twenty bags is an afternoon, two hundred is a different conversation. Put your own quotes into the concrete cost calculator and it shows the break-even volume where the cheaper option changes hands.
Wastage allowance
Why add a wastage allowance?
Real pours rarely match the drawing. Ground is uneven, holes cave in a little at the sides, spillage happens, and some mix always stays behind in the mixer or the wheelbarrow. Over-excavation and a thick spot in the middle of a slab quietly swallow material too. An allowance of 5–10% (example) covers most of this; the slider goes up to 20% for rough ground or a first pour. Coming up one bag short in the middle of a finish is a far bigger problem than returning one unopened bag, so the bag count always rounds up, never down.
Worked examples
A 10 × 10 ft patio slab at 4 in. 10 × 10 × (4 ÷ 12) = 33.3 ft³, which is 33.3 ÷ 27 = 1.23 yd³. With a 10% allowance the order becomes about 1.36 yd³, or 62 of the 80 lb bags at a typical yield.
Six deck piers, 12 in diameter × 4 ft. One pier holds 3.14 × (12 ÷ 24)² × 4 = 3.14 ft³, so six piers take 18.85 ft³, or 0.70 yd³. Add 10% and the order is about 0.77 yd³ — comfortably a bag job. Post holes for a fence use the same math with a shorter depth.
Metric: a 3 × 3 m slab at 10 cm is 0.9 m³, about 1.18 yd³. If your supplier quotes in yards, order a little above the conversion rather than below it.
Common mistakes this calculator helps you avoid
- Typing inches into a feet field or the other way around — the drawing changes shape when the numbers do not fit together.
- Skipping the wastage allowance and coming up one bag short halfway through the pour.
- Rounding down: 0.64 yd³ is not "about half a yard" on ordering day.
- Forgetting that the gravel base under a slab is a separate material with its own volume and its own delivery — the slab calculator estimates it alongside the concrete.
- Comparing a bag price against a truck price without the delivery fee or the supplier's minimum order in the picture.
What these estimates are — and are not
Every number here is a planning estimate built from the dimensions you enter and typical material yields. Site conditions, subgrade, formwork and supplier practices all move real quantities, and pour thickness, reinforcement and mix strength come from your plans and local building practice — not from a calculator. Use the results to budget, size the order and compare quotes; confirm the details with your supplier before the truck is booked.
Knowledge base
Frequently Asked Questions
How many 80 lb bags of concrete are in a cubic yard?
About 45 bags with the typical yield of 0.60 ft³ per 80 lb bag (example — check the yield printed on the bag you buy). The count shifts with bag size: roughly 60 bags of 60 lb or 90 bags of 40 lb per cubic yard. The calculator always rounds the count up to whole bags.
How much area does one cubic yard of concrete cover?
At 4 in thick, one cubic yard covers about 81 ft². At 5 in it covers about 64.8 ft², and at 6 in about 54 ft². Divide 324 by the thickness in inches to get the coverage for any other depth.
Should I add extra material for wastage?
In most cases yes. Spillage, uneven ground, over-excavation and mix left in the equipment usually consume 5–10% more than the raw geometry suggests (example). The wastage slider adds up to 20%, and the bag count always rounds up so a whole-bag order never falls short.
Do I need a gravel base under my slab?
It depends on your soil, climate and local building practice — many slabs are poured over a compacted gravel base rather than raw soil. The slab calculator has an optional section that estimates the volume and weight of that base from the slab area and the depth you enter, so you can plan it as a separate material.
How thick should my slab be?
That decision belongs to your plans and your local building authority, because it depends on loads, soil and use. As scale, 4 in is a thickness commonly seen for garden pads and walkways and 5–6 in for driveways, but treat those as orientation examples, not recommendations. The coverage table shows how thickness changes the volume you need.
Bags or ready-mix for a project my size?
Bags win on flexibility for small pours — under about one cubic yard — because there is no minimum order or delivery window. Larger pours usually cost less per yard from a truck once the delivery fee is spread out. Enter your own quotes in the cost calculator and the break-even volume shows where the answer changes for your prices.
Can I work in meters and centimeters?
Yes. Every calculator has an imperial/metric toggle. Dimension labels switch with it, you enter measurements in the unit shown, and the results appear in cubic meters as well as cubic yards and cubic feet so you can match whichever unit your supplier quotes.
Are these results a quote?
No. Everything on this site is a planning estimate computed in your browser from the values you enter. Real quantities and prices depend on site conditions and on your suppliers. Nothing you type is stored or sent anywhere, and the site never quotes prices — you supply those yourself.
More Calculators
Concrete Slab Calculator
Concrete volume for slabs and footings, plus an optional gravel base estimate.
Tube Form Calculator
Concrete for round columns, deck piers and post holes, per column and in total.
Concrete Bag Calculator
How many bags of concrete mix you need, with typical yields for 40 to 80 lb bags.
Concrete Cost Calculator
Compare the cost of bags against a ready-mix truck with your own prices.