How many bags of concrete do I need?
The patio, the walkway, the pad the whole project sits on. Fill in your room, watch the answer move, then take the open math with you to the store.
How to tell if you are being overcharged
With bagged concrete the trap is comparing prices per bag when the bags do not hold the same amount.
- The QUIKRETE Concrete Mix data sheet prints the yield for every bag size: 0.30 cubic feet for 40 pounds, 0.375 for 50, 0.45 for 60, 0.60 for 80 and 0.675 for 90. Divide the shelf price by that number and compare cost per cubic foot. It is often not the biggest bag that wins.
- Twenty seven cubic feet is one cubic yard, exactly, per NIST Handbook 44. Once your slab runs into whole cubic yards, calling a ready mix supplier stops being an exotic idea. Where exactly that flips depends on your local delivery minimum, which nobody publishes, so this page will not pretend to know it.
- The same QUIKRETE sheet calls for roughly 6 pints of water per 80 pound bag and a 2 to 3 inch slump. Adding water past that makes it easier to place and weaker when it cures.
- This page gives you the volume of the slab. It says nothing about your sub-base, your forms, your rebar or your local code, and none of those are optional.
The arithmetic, in cubic feet, because that is what a bag is sold in
Length times width times thickness gives volume, and the only trap is that thickness arrives in inches while everything else is in feet. Four inches is a third of a foot. The ten by ten pad this page opens with, at four inches, is 100 square feet times 0.33 of a foot, which is 33.33 cubic feet. Add five percent for spillage and a sub-base that is never perfectly flat and you are placing about 35 cubic feet.
QUIKRETE prints the yield of every bag size on the Concrete Mix data sheet: 0.30 cubic feet from a 40 pound bag, 0.375 from 50, 0.45 from 60, 0.60 from 80 and 0.675 from 90. Thirty five cubic feet at 0.60 is 58.3, so 59 bags of eighty pounds.
Now do the multiplication nobody does. The same sheet gives a unit weight of approximately 140 pounds per cubic foot, so that pad is about 4,900 pounds of finished concrete, and 59 bags is 4,720 pounds of bags to move from a pallet to a mixer. Two and a third tons, one bag at a time. That number is not a material cost and it is the biggest number on this page.
How much extra is technical, and how much is padding
Concrete has no cut waste at all. Nothing is trimmed. What it has is spillage, a sub-base that dips where you tamped it less, and forms that bow slightly outward under the weight of wet concrete. That is why the extra field on this page starts at five percent rather than ten, and why it is marked as your judgment: no manufacturer publishes a figure for it, and this site will not pretend one exists.
What genuinely moves the number is depth accuracy. On a hundred square feet, half an inch of extra depth is 4.2 cubic feet, which is seven more bags. If your excavation is a half inch deeper in the middle than at the edges, that is where your extra bags go, and it is a reason to check the sub-base rather than a reason to buy blindly.
The point where the number you were given stops adding up
Multiply the bags quoted by the yield of that bag size, and compare against your slab volume. Fifty nine eighty pound bags is 35.4 cubic feet, against a 33.3 cubic foot slab, which is a six percent allowance and entirely reasonable. Seventy bags of the same size is 42 cubic feet against the same slab, a twenty six percent allowance, and that is a question.
The second test is the one that catches more money. Ask what the price per cubic foot is, not the price per bag. If the answer is a bag price, the person quoting is either not comparing or is hoping you are not.
Five bag sizes are five different products at the register
This is the whole trap on this page in one sentence: the bags do not hold the same amount, so the bag price tells you almost nothing. Divide the shelf price by the yield printed on the sheet and you get cost per cubic foot, which is the only number that lets you compare a forty pound bag with a ninety pound bag honestly.
| Bag | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 50 lb | 0.375 cu ft | 72 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
| 90 lb | 0.675 cu ft | 40 |
The bags per cubic yard column comes from one line in NIST Handbook 44: 27 cubic feet is one cubic yard, exactly. Divide 27 by the yield and you have it. It is often not the biggest bag that wins, and it changes store to store and week to week, which is why the comparison has to be done in front of the shelf rather than remembered.
That column is also the moment the conversation changes. Forty five eighty pound bags is one cubic yard. The pad above is 1.23 cubic yards. QUIKRETE says to allow at least one cubic foot of mixer capacity for every eighty pound bag mixed at a time, which means that pad is dozens of separate batches, each one needing about six pints of water, roughly forty four gallons of water in total. Nobody publishes the delivery minimum or the short load fee at your local ready mix plant, so this page will not invent a crossover point. It will tell you what to ask: what is your minimum load, and what is the short load charge under it. Once your slab is past a cubic yard, that phone call costs you nothing and can save you a weekend.
The lines under the slab and over it
The data sheet describes a whole procedure, and almost every step in it is a purchase that is not concrete:
- Forms and stakes. The instructions begin with staking out the area, removing sod or soil to depth, and nailing and staking forms securely in place.
- A compacted sub-base. Tamp and compact until firm, and dampen the sub-grade before the concrete goes down without leaving standing puddles. A slab on soft ground cracks regardless of how correctly you counted bags.
- A grooving tool. The sheet calls for control joints one inch deep, cut every six to eight feet. Those joints are how you decide where the slab cracks instead of finding out.
- Curing. Five days in warm weather at 70 degrees or above, seven days in colder weather between 50 and 70. The sheet offers a curing compound, or wet burlap, plastic sheeting or waterproof paper, or a sprinkler kept running continuously. Skipping this is the cheapest way to ruin a slab you poured correctly.
- A screed board, a bull float or darby, and an edging tool. All named in the finishing instructions, none of them included with the bags.
Reinforcement, local code and footing requirements are not on this page and are not optional. The sheet itself points load bearing work back to local building codes.
Questions that come up in the aisle
How many bags make a cubic yard?
Forty five, if they are eighty pound bags. Ninety, if they are forty pound bags. The table above has the rest. This is the number that tells you whether you are doing a project or starting a career.
How much water goes in?
The sheet gives a starting content and a maximum for each bag size. For eighty pounds it is six pints to start and nine at the outside, aiming for a two to three inch slump. Water past that maximum makes the mix easier to place and weaker once it cures, which is a trade nobody makes on purpose.
How thin can I pour this?
This product is designed for pouring two inches thick or more. Below that you are using the wrong material, not saving money.
When can I walk on it?
Strength is published by age, not by day of the week: 2,500 psi at seven days and 4,000 psi at twenty eight days by ASTM C39. Curing runs five to seven days depending on temperature, and the sheet says to protect fresh concrete from freezing for the first forty eight hours.
Does the bag price include the sand and gravel?
Yes. This is a pre-blended mixture of cement and aggregates that needs only water, which is exactly why its yield in cubic feet is printed and why cost per cubic foot is a fair comparison. Buying cement alone and adding your own aggregate is a different calculation entirely.
Where the bag price above comes from
Yields, water, slump, strength, curing times and the cubic yard conversion are all published and all dated by the manufacturer or by NIST. The dollar range is read a different way, off one product page at Sutherlands, a home improvement and lumber chain out of Kansas City, Missouri whose pages publish a price for every one of its stores instead of one national number:
- QUIKRETE Concrete Mix, 80 pound bag, item 471474: 5.69 to 7.29 dollars across the 24 stores with it in stock, read 2026-08-23. Open the product page.
It is the same 80 pound QUIKRETE bag whose yield is cited above, which makes this the cleanest reading on the site: one product, one page, twenty four stores, one day. It is still not a national average, and concrete is heavy, so freight and your distance from a yard show up in the tag more than they do for anything else here. Sixty of these bags weigh most of two and a half tons, which is also worth knowing before you plan a trip in a car.
The arithmetic that matters is cost per cubic foot, and it is the one to run on the shelf in front of you. Divide the tag by the yield printed on the bag and compare that against a ready mix truck by the cubic yard. It takes about a minute and it is the single highest paid minute of this whole project. Type your number into the price field and the page reruns on yours.
Sources. QUIKRETE, Concrete Mix, Product No. 1101, data sheet, revised October 2022, quikrete.com, opened 2026-08-23, for yields, water, slump, unit weight, strength, joints and curing. NIST Handbook 44 (2026), Appendix C, nvlpubs.nist.gov, opened 2026-08-23, for 27 cubic feet to the cubic yard, exactly. Sutherlands, the product page linked above, read 2026-08-23, for the dollar range.
The other calculators
Or start from the room instead
Nobody wakes up needing concrete. Each room below opens with its own drawing, its own starting size and only the calculators that room needs.
What this page does not count. This page gives you the volume of a rectangular slab and turns it into bags. It says nothing about your sub-base, your forms, your reinforcement, your control joints, your curing or your local code, and a slab poured on soft ground will crack no matter how many bags you counted correctly.
An estimate, not a quote. Everything above is arithmetic on the measurements you typed and on figures printed in the documents cited on this page. The label on the product in your hand outranks all of it, which is why every one of those fields stays editable.