Concrete Mix Calculator: How to Use It Step by Step
Reviewed by the OnlineFree.app team · Updated
Key points
- Concrete volume is length × width × thickness divided by 27 to convert cubic feet to cubic yards.
- An 80 lb bag of concrete yields about 0.60 cubic feet; a 60 lb bag yields about 0.45 cubic feet.
- The Concrete Mix Calculator returns cubic yards, cubic feet, cubic meters, bag counts and estimated cost in one screen.
- Add 5% waste for formed slabs on flat ground and 10% for hand mixing, footings and fence-post holes.
- Bagged concrete suits small pours; a full driveway of 8.89 cubic yards is roughly 400 bags and better delivered.
How do you calculate concrete volume?
Concrete volume is length × width × thickness, all in the same unit, then divided by 27 to convert cubic feet into cubic yards. A 12 ft × 10 ft slab at 4 in thick is 40 cubic feet, which is about 1.48 cubic yards.
Bag counts come from bag yield, not from the volume alone. An 80 lb bag of concrete yields roughly 0.60 cubic feet, so 40 cu ft ÷ 0.60 = 66.7, rounded up to 67 bags. Switch to 60 lb bags at 0.45 cu ft and the same slab needs 89 bags — a 22-bag swing for one pour.
The Concrete Mix Calculator does that arithmetic in one screen: enter the three dimensions and it returns cubic yards as the headline number, with cubic feet and cubic meters alongside, plus bags and an estimated cost.
Step by step: using the Concrete Mix Calculator
Start with the Imperial/Metric toggle in the Dimensions card header, because it changes the units you type. Imperial means feet for length and width and inches for thickness. Metric means meters for length and width and centimeters for depth — not millimeters, not meters.
Fill in the three numeric fields: Length (the longest side), Width (the shortest side) and Thickness/Depth. Values are converted to feet internally, so a 4 in slab is entered as 4, never as 0.33.
Next pick the bag size — 80 lb (0.60 cu ft) is the default — and, if you want a cost figure, edit Price per bag. The field is pre-filled with 6.50 USD as a placeholder; type what your local store actually charges for a single bag, not a pallet price divided by bag count unless that is genuinely your cost.
Read the output in order: cubic yards first, then bags rounded up to a whole bag, then the estimated total, cost per cubic yard and approximate weight. It is one screen, single column, no signup and no page reload. If you want a second tool for the decorative side of a project, OnlineFree.app hosts the whole free collection.
Bags vs. ready-mix: which should you order?
Bagged concrete makes sense for small pours: slabs under roughly 2 cubic yards, footing pads, fence-post holes. A 20 ft × 24 ft driveway at 6 in thick is 240 cubic feet — 8.89 cubic yards — which works out to 400 bags of 80 lb concrete. That is about 32,000 lb of bag material to carry, open, mix and place by hand, and it is not a realistic single pour.
At that scale a ready-mix truck is the standard answer, and the calculator's cost-per-cubic-yard line is the fairest way to compare. Bagged concrete almost always costs more per cubic yard than delivered ready-mix; the trade-off is that trucks carry minimum loads and charge short-load fees for partial loads. The National Ready Mixed Concrete Association explains how ready-mix ordering and load sizes work.
The tool states this comparison as a plain sentence rather than a chart, so read it as a nudge: if your volume is large, call a supplier for a quote before you load 400 bags into a cart.
Waste allowance: how much extra concrete to order
Real pours lose material to uneven subgrade, spillage, over-excavated edges and the inch or two left in the mixer. That is why the calculator offers an optional 5% or 10% waste allowance toggled on top of the raw volume.
Take the 12 ft × 10 ft × 4 in slab. Raw volume is 40 cu ft (1.48 cu yd). Add 10% and you are ordering 44 cu ft, or 1.63 cu yd, which is 74 bags of 80 lb instead of 67. Add 5% and you are at 42 cu ft and 70 bags.
Rule of thumb we use: 5% for a formed slab on a flat, compacted base, 10% for hand-mixed pours, footings, fence posts and any job where the ground is rough or the forms are homemade. Running short mid-pour is far more expensive than one extra bag.
How thick should your slab be?
Thickness is the input people get wrong most often, because it is the one dimension measured in inches (or centimeters) rather than feet or meters. Four inches is the common residential slab thickness for patios, shed floors and walkways. Driveways that carry vehicles are usually 5 to 6 inches. Footings and post holes are deeper and are usually calculated per hole rather than as one rectangle.
A useful sanity check: doubling the thickness doubles the volume. Going from 4 in to 8 in on the same 12 ft × 10 ft footprint takes you from 1.48 cubic yards to 2.96 cubic yards, and the bag count doubles with it.
The Concrete Mix Calculator converts the depth to feet for you, so enter 4, 5 or 6 as inches rather than converting by hand — decimal conversions like 0.33 are where most manual take-offs go wrong.
Common mistakes this calculator won't catch
Mixing up cubic feet and cubic yards is the classic error. There are 27 cubic feet in a cubic yard, so 40 cu ft is 1.48 cu yd, not 40 cu yd — a mistake that would multiply your order by 27.
The tool sizes concrete only. It does not include gravel or crushed-stone subbase, sand bedding, rebar or mesh, form lumber, expansion joints, curing compound or delivery and short-load fees. Add those separately when you build a real budget.
Bag yields are nominal figures published by manufacturers; actual yield shifts with moisture, how you mix, and how much you lose to the wheelbarrow and the hose. Cost estimates are only as good as the price per bag you typed, and prices vary by region, retailer and pallet quantity. For structural work — foundations, retaining walls, anything inspected — treat the output as a planning estimate and confirm volumes, mix design and reinforcement with your supplier or a licensed engineer; the American Concrete Institute publishes the standards that govern that side of the job.
Frequently asked questions
How many 80 lb bags of concrete do I need for a 10x10 slab?
A 10 ft × 10 ft slab at 4 in thick needs 56 bags of 80 lb concrete. The volume is 33.3 cubic feet, or 1.23 cubic yards, and an 80 lb bag yields about 0.60 cubic feet. Add a 10% waste allowance in the Concrete Mix Calculator and the count rises to 62 bags.
Does the Concrete Mix Calculator work in metric units?
Yes. Switch the Imperial/Metric toggle and the tool accepts meters for length and width and centimeters for thickness, returning cubic meters as well as cubic yards and cubic feet. For example, 3 m × 3 m × 10 cm equals 0.9 cubic meters, roughly 1.18 cubic yards. Enter 10 for the depth, not 0.1.
Is bagged concrete or a ready-mix truck cheaper?
Delivered ready-mix is usually cheaper per cubic yard than bagged concrete, but trucks carry minimum loads and add short-load fees for partial deliveries. The Concrete Mix Calculator shows cost per cubic yard so you can compare it against a supplier quote. Get the quote before committing to a large bagged order.
How accurate is the estimated cost from the Concrete Mix Calculator?
It is as accurate as the price you enter. The 6.50 USD default is only a placeholder, so replace it with your store's actual per-bag price. The estimate covers bags only and excludes tax, delivery, subbase gravel, rebar, forms and any tool rental.
Can I use the Concrete Mix Calculator for footings and fence posts?
You can, by entering the length, width and depth of each footing or hole and multiplying the bag count by the number of holes. Use the 10% waste allowance for post holes, since irregular excavation usually takes more concrete than the measured rectangle suggests.