Concrete Calculator
Calculate the amount of cement, sand, and water needed for your concrete project. Get accurate material calculations and cost estimates based on your mix ratios.
Accurate Concrete Material Calculation
Our concrete calculator helps you determine the exact amount of cement, sand, and water needed for your project based on your mix ratios. Simply enter your dimensions and the ratio of cement, sand, and water. The calculator will provide you with detailed material requirements and cost estimates including material costs and labor fees.
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VOLUME
MATERIALS
Cost Breakdown
How to Estimate Concrete for a Slab
Concrete is volume, and the arithmetic that catches people is the unit conversion: multiply length by width by thickness in feet, then divide by 27 to get cubic yards. A 4 inch slab is 0.333 feet thick, not 4. Ready-mix is sold by the cubic yard and bags are sold by volume yield, so the same number answers both. Everything else on this page — the base, the rebar, the second pour you did not plan for — is downstream of getting that one number right.
1. Convert the thickness to feet
4 inches is 4 ÷ 12 = 0.333 ft. 6 inches is 0.5 ft. Doing this first prevents the mistake that ruins the rest.
2. Multiply the three dimensions
Length x width x thickness, all in feet. A 20 x 12 patio at 4 inches is 20 x 12 x 0.333 = 80 cubic feet.
3. Convert to cubic yards
Divide by 27. 80 ÷ 27 = 2.96 cubic yards.
4. Break an odd shape into rectangles
An L-shaped patio or a drive with a turning bay is two or three rectangles, each worked out separately and added. For a circle, use radius squared x 3.14 x thickness. Estimating an irregular slab as one big rectangle overshoots badly, and estimating it as its smallest dimension leaves you mixing bags to finish a truck pour.
5. Add for waste and subgrade
Add 5 to 10%. Ground is never perfectly flat and the low spots take real concrete. That takes 2.96 to about 3.25 cubic yards. On a hand-dug subgrade, or clay that has been rained on, use the top of that range.
6. Decide bags or ready-mix
An 80 lb bag yields about 0.6 cubic feet, so a cubic yard takes 45 bags. Past roughly half a yard, mixing by hand stops making sense — the pour takes longer than the concrete stays workable, and you get a cold joint where one batch met the next.
7. Order in the units your supplier sells
Ready-mix is ordered in cubic yards, usually to the quarter yard, and most plants charge a short-load fee under a full truck. It is worth calling with your yardage before you finalise a quote: the fee for a 3 yard load and for a 9 yard load is the same paperwork, and it changes which jobs are worth combining into one morning.
Worked Example: a 20 x 12 Patio Slab
20 by 12 feet, 4 inches thick, on prepared gravel.
| Item | Working | Result |
|---|---|---|
| Thickness in feet | 4 ÷ 12 | 0.333 ft |
| Volume in cubic feet | 20 x 12 x 0.333 | 80 cu ft |
| Cubic yards | 80 ÷ 27 | 2.96 cu yd |
| With 10% allowance | 2.96 x 1.10 | 3.25 cu yd |
| If bagged instead | 80 cu ft ÷ 0.6 | ~134 x 80 lb bags |
Worked Example: a 24 x 20 Driveway at 5 Inches
A two-car driveway carrying vehicle weight, so 5 inches rather than 4, with wire mesh and a compacted gravel base. This is the same arithmetic, but the thickness change is where the yardage jumps — and it is the job people underestimate, because they price it off a patio they did last month.
| Item | Working | Result |
|---|---|---|
| Thickness in feet | 5 ÷ 12 | 0.417 ft |
| Volume in cubic feet | 24 x 20 x 0.417 | 200 cu ft |
| Cubic yards | 200 ÷ 27 | 7.41 cu yd |
| With 10% allowance | 7.41 x 1.10 | 8.15 → order 8.25 cu yd |
| Same slab at 4 inches | 24 x 20 x 0.333 ÷ 27 | 5.93 cu yd |
| What the extra inch costs | 7.41 − 5.93 | 1.48 cu yd, a quarter more concrete |
| Gravel base, 4 inches | 24 x 20 x 0.333 ÷ 27 | 5.93 cu yd of aggregate |
Bag Yields and Slab Thicknesses
| Item | Figure | Note |
|---|---|---|
| 80 lb bag | ~0.60 cu ft | 45 bags to the cubic yard. |
| 60 lb bag | ~0.45 cu ft | 60 bags to the cubic yard. |
| 40 lb bag | ~0.30 cu ft | 90 bags to the cubic yard. Repairs only. |
| 4 inch slab | 0.333 ft | Patios, walkways, shed bases. |
| 5–6 inch slab | 0.417–0.5 ft | Driveways and anything carrying vehicles. |
| 1 cubic yard | 27 cu ft | Covers 81 sq ft at 4 inches thick. |
| 1 cubic yard at 5 in | — | Covers 65 sq ft. |
| 1 cubic yard at 6 in | — | Covers 54 sq ft. |
| Ready-mix ordering unit | Cubic yards | Usually to the quarter yard, with a short-load fee under a full truck. |
Concrete for Common Slab Sizes
The sizes that come up over and over, worked out both ways. Yardage includes no waste allowance — add 5 to 10% to what you order. Bag counts are 80 lb bags at 0.6 cubic feet each, rounded up.
| Slab | At 4 inches | At 6 inches | As 80 lb bags (4 in) |
|---|---|---|---|
| 4 x 4 pad (16 sq ft) | 0.20 cu yd | 0.30 cu yd | 9 bags |
| 8 x 8 (64 sq ft) | 0.79 cu yd | 1.19 cu yd | 36 bags |
| 10 x 10 (100 sq ft) | 1.23 cu yd | 1.85 cu yd | 56 bags |
| 10 x 20 (200 sq ft) | 2.47 cu yd | 3.70 cu yd | 112 bags |
| 12 x 12 (144 sq ft) | 1.78 cu yd | 2.67 cu yd | 80 bags |
| 16 x 20 (320 sq ft) | 3.95 cu yd | 5.93 cu yd | 178 bags |
| 20 x 20 (400 sq ft) | 4.94 cu yd | 7.41 cu yd | 223 bags |
| 24 x 24 (576 sq ft) | 7.11 cu yd | 10.67 cu yd | 320 bags |
| 30 x 40 shop floor (1,200 sq ft) | 14.81 cu yd | 22.22 cu yd | Truck territory |
What a Concrete Slab Costs to Pour
The ready-mix is the line everyone prices first, and on a residential slab it is rarely the majority of the invoice. Concrete is sold by the cubic yard delivered, so the calculator above gives you the biggest single number in one step. Then comes everything that has to happen before and after the truck arrives: the excavation, the gravel, the forms, the steel, the placing crew and the finishing. On a small pour with a pump and a stamped finish the concrete itself can be under half the total, which is why a cost per square foot copied off a national average tells you almost nothing about the job in front of you.
Yardage, and the short-load fee underneath it
Ready-mix is priced per cubic yard delivered, with a short-load surcharge below the supplier's minimum. That fee is why two 3-yard pours on separate days cost noticeably more than one 6-yard pour, and why it is worth combining a slab with the footing or the walkway when the schedule allows. The threshold and the surcharge are printed on your supplier's price sheet. Ask for both before you quote.
What is under the slab
Excavation depth, the compacted gravel base, the vapour barrier and hauling the spoil away are a full line of their own, and on a sloping or clay site they can rival the concrete. This is also the line that gets cut to win a job and cracks the slab two winters later.
Thickness, which is a volume decision and not a surface one
Going from 4 to 5 inches on a 400 square foot slab is not a small percentage of a small number, it is over a cubic yard and a half of extra concrete. A driveway carrying vehicles needs the thickness. A shed base does not.
Reinforcement
Welded mesh, a rebar grid at a stated spacing, or fibre in the mix are three different prices and three different labour times. A rebar grid also needs chairs and tying time, which is hours rather than materials.
How the concrete gets from the truck to the form
A chute reaches roughly the length of the truck. Past that it is wheelbarrows, a buggy or a pump, and a pump is a fixed cost with a minimum charge that lands on a small job as hard as on a big one. Back-garden access with no truck route is the most commonly underquoted item on a residential pour.
The finish
A broom finish is one pass. Power-trowelled, exposed aggregate, coloured or stamped work is finishing labour that scales with the surface area rather than the volume, and stamped work brings release agent, sealer and a second visit.
Weather
Hot-weather retarders, cold-weather accelerators, blankets and a cancelled pour are all real costs. On a driveway the curing window also decides when the customer can park on it, which is a scheduling promise worth making carefully.
Where the Money Goes on a Concrete Job
Price each of these separately with your own suppliers and rates. The point of the list is that only the first line comes out of a yardage calculation.
| Line item | What drives it | What gets missed |
|---|---|---|
| Ready-mix delivered | Cubic yards x delivered price per yard, plus any short-load fee | Ordering the exact calculated yards. A slab that runs 3% over means a second truck or a cold joint. |
| Excavation and base | Depth, soil type, gravel thickness, compaction passes, spoil removal | Haulage of the spoil, and the day the digger cannot get in. |
| Forms | Linear feet of edge, plus stakes, ties and release agent | Steps, curves and thickened edges add edge without adding square footage. |
| Reinforcement | Mesh, bar size and grid spacing, chairs, tying hours | Chairs and tie wire are cheap. The hours to place a grid are not. |
| Placing | Crew size x hours, plus a pump or buggy where the chute does not reach | The pump minimum charge on a small pour. |
| Finishing | Surface square footage and the finish specified | Stamped and coloured work needs a return visit for sealer. |
| Joints and curing | Saw cuts per linear foot, curing compound or blankets per square foot | Cutting control joints late is what produces the random crack the customer calls about. |
| Cleanup | Washout, waste concrete disposal, protecting what is next to the pour | There is no legal free place to wash out a truck. |
What Goes Under and Inside the Slab
The concrete is usually the smaller part of the estimate. These are the items that sit either side of it, and the ones a homeowner never thinks to ask about until the slab has already cracked.
The gravel base is its own volume
Four inches of compacted crushed stone under a slab is the same arithmetic as the concrete — length x width x 0.333 ÷ 27 — and it has to be quoted and hauled. On a 24 x 20 driveway that is another 5.93 cubic yards of aggregate. Aggregate is sold by the ton in most yards rather than the yard, so ask your supplier what a yard of their base material weighs before you convert.
Compaction is what stops the cracking
A base that has not been compacted in lifts settles unevenly, and an unsupported slab cracks wherever the void ended up. This is invisible in the finished job and it is the single most common cause of a callback two winters later.
Rebar or mesh, and how much
Wire mesh in a residential slab, rebar on a grid where there is vehicle load or a footing. Rebar quantity is linear feet: divide the slab length by the spacing for the number of bars in each direction, then add the laps. At 18 inch centres a 20 x 12 slab takes roughly 9 bars one way and 14 the other.
Control joints are planned, not hoped for
Concrete cracks; joints decide where. The usual guide is joints no further apart than 24 to 30 times the slab thickness — on a 4 inch slab that is 8 to 10 feet — and panels kept roughly square rather than long and thin.
Curing time versus load time
Foot traffic in a day or so, furniture in a few days, vehicles at 7 days at the earliest and comfortably at 28, which is when the design strength is measured. Telling a customer this up front prevents the phone call where a truck has been parked on a four-day-old driveway.
Weather changes the order
Heat shortens working time, so a big pour on a hot day wants either an earlier start or an admixture. Cold slows the set and freezing ruins it. Neither changes the yardage — both change whether the yardage arrives in one load or two.
Where Estimates Go Wrong
Using inches where the formula wants feet
Putting 4 in instead of 0.333 overstates the pour twelvefold. It is the single most common concrete estimating error.
Ordering the exact calculated amount
Subgrade is never level. Running short mid-pour means a cold joint through the middle of a slab that should have been monolithic.
Bagging a job that wanted a truck
Past about half a yard the mixing time, the water and the consistency all work against you, and the slab shows it.
Quoting the concrete and forgetting the base
Aggregate, hauling, forms, mesh and the labour to compact are frequently more than the ready-mix itself. An estimate built only from yardage looks competitive right up to the point where it is unprofitable.
Adding water at the truck to make it easier to place
Every gallon added past the mix design costs strength and adds shrinkage cracking. If it is going to be a hard placement, that is a decision for the mix and the timing, not for the chute.
Pricing a driveway off a patio
The extra inch or two of thickness is not a rounding difference — the same footprint at 5 inches instead of 4 takes a quarter more concrete, and it usually adds rebar and a deeper base as well.
Concrete Calculator — Common Questions
What does a concrete slab cost?
Take the yardage from the calculator above, multiply it by your supplier's delivered price per cubic yard, and you have the largest single line. Then price the excavation and gravel base, the forms, the reinforcement, the placing crew and the finishing separately, because on a small residential slab those together often exceed the concrete. Watch two items in particular: the short-load fee below your supplier's minimum, and a pump where the truck chute cannot reach the forms.
How many cubic yards of concrete do I need for a 20x12 patio?
At 4 inches thick, 2.96 cubic yards before waste — call it 3.25 ordered. At 6 inches it is 4.44 before waste.
How much concrete do I need for a 10x10 slab?
1.23 cubic yards at 4 inches thick, or about 56 x 80 lb bags. At 6 inches it is 1.85 cubic yards. Add 5 to 10% to whatever you order.
How many 80 lb bags of concrete are in a cubic yard?
About 45. Each 80 lb bag yields roughly 0.6 cubic feet and there are 27 cubic feet in a yard.
How thick should a concrete slab be?
4 inches for patios, walkways and shed bases. 5 to 6 inches for driveways or anything taking vehicle weight.
How much does a cubic yard of concrete cover?
81 square feet at 4 inches thick, 65 at 5 inches, 54 at 6 inches.
Should I use bags or order a ready-mix truck?
Bags up to about half a cubic yard — a small pad, a set of post bases, a repair. Past that the mixing cannot keep pace with the placing, and you end up with cold joints between batches. A truck also gives you a consistent mix design, which bags mixed by eye on site do not.
Do I need rebar in a concrete slab?
Wire mesh is normal in a residential slab and rebar goes in where there is vehicle load, a footing, or a slab thicker than about 5 inches. Neither prevents cracking — the base does that. What steel does is hold the two sides of a crack together so it stays a hairline instead of becoming a trip hazard.
How long before you can drive on a new concrete driveway?
A week at the very earliest for a car, and 28 days is when concrete is measured at its design strength. Heavy vehicles and dumpsters should wait for the full cure. This is worth putting in writing on the estimate, because the damage from an early load is not repairable.
How do I turn cubic yards into a price for the customer?
Take your yardage, add the waste allowance, and multiply by your supplier's current per-yard price — then add the short-load fee if you are under a full truck, the base material, the forms, the steel and your labour hours. The calculator above asks for your own rates rather than assuming a national average, because the yard price you are quoted this month is the only one that matters to your margin.
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