Concrete Mix Calculator: Cubic Yards, Bags and Cost in Seconds
Reviewed by the OnlineFree.app team · Updated
Key points
- Concrete Mix Calculator converts length, width and thickness into cubic yards, bag counts and an estimated cost on one screen.
- A 12 ft × 10 ft slab at 4 in thick needs 1.48 cu yd, or 67 bags of 80 lb mix.
- An 80 lb bag of concrete yields 0.60 cu ft, so one cubic yard takes 45 bags.
- Add a 5-10% waste allowance and always round up, because bags come in whole units.
- Bagged mix suits small pours, but compare cost per cubic yard against a ready-mix quote for larger slabs.
What does the Concrete Mix Calculator work out?
It converts a rectangular pour's length, width and thickness into cubic yards, cubic feet and cubic meters, then into bags of 60 lb or 80 lb concrete and a rough dollar total. There is no signup, no email gate and no page reload — the numbers update as you type, which is why we kept the input to three fields.
A 12 ft × 10 ft slab at 4 in thick is the classic test case. The Concrete Mix Calculator returns 40 cu ft, or 1.48 cu yd. That is 67 bags of 80 lb mix at the tool's default $6.50 per bag, roughly $435.50, and about 5,360 lb of material to move from the truck to the form.
Switch the segmented toggle in the Dimensions card to metric and the same fields accept metres and centimetres, with volume reported in cubic metres. Bag count and cost only appear once a bag size is selected, because 60 lb and 80 lb bags hold different volumes — 0.45 cu ft versus 0.60 cu ft.
How do you calculate concrete volume by hand?
Multiply length by width to get square feet, multiply by the thickness converted to feet, then divide by 27. A 12 ft × 10 ft slab at 4 in is 120 sq ft × 0.333 ft = 40 cu ft, and 40 ÷ 27 = 1.48 cu yd.
For bags, divide the cubic feet by the yield printed on the bag: 40 ÷ 0.60 = 66.7, so 67 bags of 80 lb mix. The same pour in 60 lb bags is 40 ÷ 0.45 = 88.9, rounded up to 89.
Running the arithmetic once by hand is worth the two minutes, even when you let the calculator do the work. It catches the commonest typo in the world of concrete estimating — a depth of 4 inches entered as 4 feet, which multiplies the pour by twelve and produces a bag count nobody could carry.
Bags or a ready-mix truck?
The calculator shows cost per cubic yard next to the bag total, and that is the number worth comparing with a ready-mix quote. At $6.50 a bag, 80 lb mix works out to 45 bags and about $292 per cubic yard of concrete — before you add the labour of mixing and wheeling dozens of bags.
Ready-mix suppliers bill by the cubic yard plus delivery, and most add a short-load or small-order fee when the pour is below their minimum. The tool states the truck-load equivalence as a plain sentence rather than a chart, so you can see the point where a bag run stops making economic sense.
In most markets bagged mix costs more per cubic yard than delivered concrete once you pass roughly one cubic yard, but minimums, short-load fees and regional pricing vary widely. Treat the crossover as a prompt to phone a local supplier, not as a fixed rule.
How it compares to other concrete estimators
The field is crowded: home-made spreadsheets, ready-mix suppliers' own calculators, and full takeoff suites sold to commercial estimators. The Concrete Mix Calculator sits deliberately at the simple end — one rectangular pour, three dimensions, instant output, nothing to install.
What it does not do is rebar or mesh quantities, footing schedules, stepped or curved slabs, cylindrical post holes, or mix design. A round hole has to be worked out separately as π × radius² × depth, then divided by 0.60 cu ft per 80 lb bag. For a 4 in slab with wire mesh, order the mesh on your own count.
That trade-off is the point. Estimating software earns its licence on multi-element jobs with change orders; a homeowner pouring a patio does not need a project database. If a permit packet needs image or file conversions alongside your take-off, OnlineFree.app's free online tools cover those without an account either.
Common mistakes that skew the bag count
Round up every time. Bags are sold in whole units and the tool already rounds 66.7 up to 67, but the waste toggle on top adds 5% or 10% to the volume — a 40 cu ft pour at 10% becomes 44 cu ft and 74 bags of 80 lb mix, about $481 at the default price.
Thickness is the field people get wrong. Four inches is standard for a patio or shed slab, driveways typically run 5-6 in, and footings are deeper still. Changing 4 in to 6 in raises a 12 ft × 10 ft pour from 40 to 60 cu ft, and from 67 to 100 bags of 80 lb mix — a 50% jump from one number.
Bag yields are nominal. Manufacturers label an 80 lb bag at 0.60 cu ft, but actual yield depends on how much water goes in, and over-watering lowers strength — a point the American Concrete Institute returns to in its guidance on concrete mixtures. Treat the count as a shopping list, buy one spare bag, and verify anything load-bearing with an engineer.
Frequently asked questions
How many 80 lb bags of concrete do I need for a 10 x 10 slab?
A 10 ft × 10 ft slab at 4 in thick is 33.3 cu ft, or about 1.23 cu yd. The Concrete Mix Calculator returns 56 bags of 80 lb concrete for that pour, or 75 bags of 60 lb mix, before any waste allowance. Add 5-10% for spillage and uneven subgrade and round up.
Is the Concrete Mix Calculator free, and does it store my dimensions?
Yes, it is free, and it runs entirely in your browser. There is no signup, download or account, and the dimensions you type are not saved to a server — close the tab and the numbers are gone. That makes it convenient for a phone on the job site or a quick check at the store.
How accurate is the cost estimate from the Concrete Mix Calculator?
It is only as accurate as the price you enter. The Concrete Mix Calculator pre-fills $6.50 per bag, a typical US retail figure as of 2026, and multiplies it by the rounded bag count. Edit that field to match your local store and check the receipt, because sales tax, delivery charges and short-load fees are not included.
Does the Concrete Mix Calculator work in metric units?
Yes. Switch the segmented toggle in the Dimensions card to metric and the length and width fields accept metres while thickness accepts centimetres. Volume is reported in cubic metres alongside cubic feet and cubic yards, and the bag count still works because 60 lb and 80 lb bag yields are fixed in cubic feet.
Can I use the Concrete Mix Calculator for fence post holes or footings?
It handles any rectangular volume, so square or rectangular footings and trenches are fine. Round post holes are not a shape it accepts — work out π × radius² × depth yourself, then divide that volume by 0.60 cu ft per 80 lb bag or 0.45 cu ft per 60 lb bag.