Concrete
Concrete calculator
How many cubic yards the pour takes, how many bags that is, whether it is worth ordering a truck, and what the whole thing costs where you live.
Written and edited by Pablo Ruiz Quintero
Last reviewed 2026-09-16 · Nothing you type leaves your browser
What are you pouring?
Foot traffic only. 4 in over a compacted base is the standard residential section.
Dimensions
Feet and inches both work: 12' 6", 12.5, 12-6
Read as 20' × 12'
An L-shaped patio is two rectangles. Add a section rather than averaging it out.
Ready-mix is ordered in quarter-yard steps and the sub-base is never perfectly level. 5% covers the low spots and the bit left in the chute.
Leave blank to use the allowance above.
Used only to pick a regional labour rate. It stays in your browser.
You need
3.25 yd³
240 ft² · 80 ft³ · 2.96 yd³ raw · 3.11 yd³ with 5% waste · rounded up to the quarter yard
Order ready-mix
At 3.11 yd³ you would be mixing 140 bags by hand. That is a full day of heavy work and a real risk of a cold joint — order ready-mix.
What you have described
Shopping list
- Ready-mix delivered, ordered in quarter-yard steps3.25 yd³
- #3 (3/8 in) rebar at 18 in on centre, grid both ways
Or 6×6 W1.4/W1.4 welded wire mesh.320 ft - Control joints at 10 ft, max 10 ft
Cut joints no more than 2.5 × the slab thickness in feet, keep panels close to square, and cut a quarter of the depth within 6–18 hours of the pour.≈ 32 ft to cut
What it costs
National average. Material only — add labour if you are not doing it yourself.
Includes an estimated $96 short-load fee. Most plants set a four to five yard minimum and charge by the yard below it — it is the single biggest surprise on a small pour.
On this page — 10 sections
Reading the result
The short version
- The headline is what to order: raw volume, plus waste, rounded up to the quarter yard plants actually sell in.
- Under about one cubic yard, bags win. Above it, the short-load fee is cheaper than a day of mixing and the risk of a cold joint.
- The rebar and the joints are on the shopping list. They are why slabs survive, and almost no calculator computes them.
- Ordering short is the expensive mistake here — a second truck for half a yard costs more than the first one did.
The figure at the top is what you should order: raw volume plus the waste allowance, rounded up to the quarter yard that ready-mix plants actually sell in. The line under it shows the working — area, cubic feet, and the raw cubic yards before waste — so you can check it against your own arithmetic rather than trust a black box.
Concrete is the one material where ordering short is genuinely expensive. A second truck for half a yard costs more than the first one did, and a pour that stops for forty minutes while someone fetches bags leaves a cold joint — a permanent plane of weakness through the slab. Rounding up is not padding; it is the cheap side of the mistake.
order (yd³) = ceil( (L ft × W ft × T in ÷ 12 ÷ 27) × (1 + waste) × 4 ) ÷ 4
- L × W
- the plan dimensions in feet
- T
- thickness in inches
- ÷ 27
- cubic feet to cubic yards
- waste
- 5% on a prepared base, 10% on rough ground
- × 4 ÷ 4
- rounding up to the quarter yard plants sell in
Worked through — A 20 × 12 ft patio at 4 in: 240 ft² × 0.333 ft = 80 ft³ = 2.96 yd³. Add 5% → 3.11 yd³. Round up to the quarter → order 3.25 yd³. In 80 lb bags, that is 140 bags.
Bags or a truck
The crossover sits around one cubic yard, and it is not really about the price of the concrete. Below a yard, a delivery gets hit with a short-load fee that often exceeds the value of what is in the drum. Above it, you are moving and mixing forty-five or more bags — close to two tons of material — inside the window before the first batch starts to set.
Where the two curves cross
Material cost of each supply method against the size of the pour, short-load fee included. Both lines are computed with the same engine the calculator above uses.
| Bags, mixed on site | Ready-mix delivered | |
|---|---|---|
| Makes sense | Under about 1 yd³ | Above about 1 yd³ |
| Cost driver | Price per bag — no delivery | Price per yard plus the short-load fee |
| Labour | 45 bags per yard, mixed and barrowed | Placing and finishing only |
| Main risk | A cold joint if mixing falls behind | Access — a chute that cannot reach the forms |
| Timing | Yours. Stop and restart at will within reason | Theirs. The truck has other drops |
| Consistency | Varies batch to batch with how you measure water | Batched to spec at the plant |
Why we add waste, and why you can change it
A sub-base is never flat to the quarter inch. On a prepared, compacted base, five per cent covers the low spots and the concrete left in the chute. On rough ground, ten is closer to what actually gets poured. Trenches are worse again — the walls slump and over-dig is normal — which is why footings default to ten.
The percentage is shown, the reasoning is printed next to it, and you can override it. A calculator that hides its waste factor is hiding the part of the estimate most likely to be wrong, and the part you are best placed to correct: you have seen the ground, and we have not.
Rebar and control joints
Almost no concrete calculator mentions either, and both are why slabs crack. Start from the uncomfortable fact: concrete is going to crack. It shrinks as it cures, shrinkage puts it in tension, and concrete is weak in tension. Reinforcement and jointing do not prevent that. They decide where it happens and what it costs you.
Control joints want to be no more than 2.5 times the slab thickness apart in feet, cut a quarter of the depth, within six to eighteen hours of the pour. On a four-inch slab that is a joint every ten feet.
2.5×
Thickness in inches, read as feet
Maximum joint spacing
¼
Of the slab depth
How deep to cut
6–18 h
After the pour
The cutting window
≤ 1.5
Panel length to width
Keep panels near square
That spacing is a maximum, not a layout. Divide the slab into equal panels that come in under it: a twelve-foot slab with a ten-foot maximum gets two panels of six, not one of ten and one of two. A two-foot strip cracks on its own regardless of how carefully the joint next to it was cut. The calculator does that division and reports both the limit and the spacing it landed on.
Panels should also be close to square. A long thin panel cracks across its middle no matter what, because concrete shrinks along its longest dimension and a narrow panel has nowhere to relieve it.
| Slab | Joints at most | Cut depth | Reinforcement |
|---|---|---|---|
| 4 in | 10 ft | 1.00 in | #3 (3/8 in) at 18 in. Or 6×6 W1.4/W1.4 welded wire mesh. |
| 5 in | 12.5 ft | 1.25 in | #4 (1/2 in) at 18 in. Grid both ways, on chairs at mid-depth. |
| 6 in | 15 ft | 1.50 in | #4 (1/2 in) at 12 in. Grid both ways. Required for vehicle loads. |
How thick it needs to be
Thickness, not mix strength, is what carries a load. Going from 3,000 to 4,000 psi concrete buys you far less than adding an inch of depth, and costs about the same. These are the defaults the calculator applies.
| Job | Thickness | Why |
|---|---|---|
| Patio or walkway | 4 in | Foot traffic only. 4 in over a compacted base is the standard residential section. |
| Driveway (cars and light trucks) | 4 in | 4 in is code-minimum for passenger vehicles. Go to 5 in if a heavy pickup or RV parks on it. |
| Driveway (RV, heavy truck) | 6 in | 6 in with reinforcement. Anything less cracks under axle loads. |
| Garage floor | 4 in | 4 in is typical; 5–6 in where a lift or heavy equipment is planned. |
| Shed or workshop base | 4 in | 4 in with a thickened edge carries a timber-framed building. |
Where a local amendment or a drawing says otherwise, the drawing wins. These are residential norms, not a code citation, and frost-prone regions in particular have their own requirements for anything that carries a structure.
The day of the pour
Quantities are the easy part. What decides whether the slab is still flat and uncracked in five years is a handful of decisions made in a couple of hours, usually under time pressure with a truck waiting.
Do not add water. A wetter mix is easier to place and permanently weaker — water raises the slump and lowers the final strength, for the whole life of the slab. If placing is hard, the answer is more hands, a plasticiser, or a smaller pour, not a hose.
Do not stop. A pour that pauses long enough for the previous batch to start setting leaves a cold joint: two slabs touching, not one slab. That is the single strongest argument against mixing large volumes from bags — not the labour, the discontinuity.
Cure it. Concrete does not dry out to get hard; it reacts with water, and that reaction runs for weeks. A slab left to dry in sun and wind can lose a serious fraction of its potential strength in the first few days. Keep it damp, or covered, for at least a week.
Before the truck arrives
- Forms braced, square, and checked for level and fall — water has to go somewhere.
- Sub-base compacted and damp, not dusty and not puddled.
- Reinforcement on chairs at mid-depth, tied, and not resting on the base.
- A clear path for the chute, or a barrow route and enough people to use it.
- Tools and a saw ready before the pour, not fetched during it.
- Someone watching the clock for the jointing window, six to eighteen hours out.
What goes underneath
A slab is only as flat as its bed. Four inches of compacted clean stone levels the ground, gives water somewhere to go, and breaks the capillary path that would otherwise wick moisture out of the soil and into the concrete — which matters for anything that will be covered with flooring later.
That layer loses volume when it is compacted, so it has to be ordered loose rather than at the finished depth. Ordering four inches of stone and getting three and a bit is the commonest way a slab ends up sitting on a base that is thinner than it was drawn.
The gravel calculator sizes that layer — with the compaction allowance applied before you order, and the tonnage given as the range it honestly is.
Gravel calculatorFive ways this goes wrong
01Ordering exactly what the arithmetic said
The sub-base is never flat to the quarter inch, and concrete is sold in quarter-yard steps. Coming up half a yard short means a second delivery that costs more than the first.
Instead — Add the waste allowance for the ground you actually have, and let the order round up. Rounding up is the cheap side of the mistake.
02Adding water on site
A wetter mix places more easily and loses compressive strength permanently. You are trading thirty years of strength for twenty minutes of ease.
Instead — Order the slump you need, use a plasticiser if the mix is stiff, and have enough hands ready before the truck arrives.
03Laying mesh on the sub-base
Reinforcement only resists tension if it sits at mid-depth. Mesh walked into the base is at the bottom of the slab, doing close to nothing.
Instead — Chairs, tied, at mid-depth — and check it is still there after the concrete goes in.
04Skipping or delaying the joints
Past about eighteen hours the slab has already chosen where it is cracking, and it will not be where you cut.
Instead — Plan the joint layout before the pour, and cut a quarter depth within six to eighteen hours — sooner in hot weather.
05Mixing two yards from bags
That is ninety bags and most of a day. Long before you finish, the first batch has set, and you have a cold joint through the middle of the slab.
Instead — Order ready-mix above about a yard, or split the work into separate panels with a deliberate joint between them.
How the calculator works it out
- 01Measure each sectionLength and width in feet and inches. Break an irregular shape into rectangles rather than averaging it — an average is wrong at both ends.
- 02Set the thicknessFour inches for foot traffic and cars, six for heavy vehicles. Footings follow the depth on the drawing.
- 03Convert to cubic yardsLength × width × thickness in feet gives cubic feet. Divide by 27 for cubic yards.
- 04Add wasteFive per cent on a prepared base, ten on rough ground or in a trench.
- 05Round to the quarter yardPlants sell in quarter-yard steps, so an order of 2.96 yards is an order of 3.25.
- 06Choose bags or ready-mixUnder about one cubic yard, bags win. Above it, the short-load fee is cheaper than a day of mixing.
Bag yields come from the manufacturers’ own data sheets. Jointing follows the ACI guide for slabs on ground. The regional adjustment is a published federal dataset, named and dated, and it is applied to labour only — a bag of mix does not cost eleven per cent more in California, but the hour to place it does. All of it is listed on the sources page with the date each was last checked.
Nothing you type is sent anywhere. The whole calculation runs in your browser, ZIP code included — that only selects a regional labour factor, and never leaves the page.
The terms a plant will use
What the dispatcher on the phone will assume you already know.
- Ready-mix
- Concrete batched at a plant and delivered in a mixer truck. Sold by the cubic yard in quarter-yard steps, with a minimum load and a fee for falling short of it.
- Short-load fee
- The charge for ordering less than the plant's minimum, typically about $55 for every yard below 5. On a small pour it is the single biggest surprise on the invoice.
- Cold joint
- The permanent plane of weakness left when one batch starts to set before the next is placed against it. The reason a pour that stops for forty minutes is a structural problem, not a scheduling one.
- Control joint
- A cut made into the slab to create a deliberate weak line, so the inevitable shrinkage crack forms at the bottom of a straight saw cut instead of across the middle of the slab.
- Slump
- A measure of how wet the mix is. Adding water on site raises the slump and makes placing easier, and permanently lowers the strength of what you placed.
- Curing
- Keeping the slab damp while it gains strength. Concrete does not dry out to get hard, it reacts with water — a slab left to dry in the sun can lose a third of its potential strength.
- Sub-base
- The compacted layer of stone between the ground and the slab. It levels the bed, drains, and breaks the capillary path that would otherwise wick moisture up into the concrete.
- Welded wire mesh
- A prefabricated grid of light bar, commonly 6×6 W1.4/W1.4, used instead of loose rebar in slabs carrying foot traffic. Only works if it is held at mid-depth, not walked into the base.
Common questions
- How many cubic yards of concrete do I need?
- Multiply length × width × thickness in feet to get cubic feet, then divide by 27. A 20 × 12 ft patio at 4 in thick is 80 ft³, or 2.96 yd³ — call it 3.25 yd³ once you add a 5% waste allowance and round up to the quarter yard the plant sells in.
- How many bags of concrete make a cubic yard?
- Forty-five 80 lb bags, sixty 60 lb bags, or seventy-two 50 lb bags. Those are the yields printed on the bag: 0.60, 0.45 and 0.375 ft³ of set concrete respectively.
- At what point should I stop mixing bags and order a truck?
- Around one cubic yard. Below that, the short-load fee on a delivery costs more than the concrete. Above it you are mixing 45+ bags — roughly 1.8 tons of material — and you risk a cold joint if the pour takes too long.
- Why does the calculator add waste?
- Because the sub-base is never perfectly level and concrete is ordered in quarter-yard steps. Five per cent covers the low spots and what stays in the chute on a prepared base; ten is closer to reality on a rough one. The allowance is shown, explained and editable.
- What is a short-load fee?
- Most ready-mix plants set a minimum load of four to five yards and charge roughly $55 for every yard you fall short of it. On a three-yard patio that is around $100 nobody budgeted for, so it is built into the price shown here rather than hidden.
- How thick should a concrete driveway be?
- Four inches over a compacted base for cars and light trucks. Go to six inches with rebar if an RV, a boat trailer or a delivery truck will park on it. Thickness is what stops a slab cracking under axle loads, not the strength of the mix.
- Do I need rebar in a slab?
- A patio at 4 in can use 6×6 welded wire mesh. Anything carrying a vehicle wants #3 or #4 bar in a grid, on chairs at mid-depth. The calculator gives the size, the spacing and the linear feet so you can price it with the concrete.
- How far apart should control joints be?
- No more than 2.5 times the slab thickness in inches, read as feet — a joint every 10 ft in a 4 in slab. Cut a quarter of the depth, within six to eighteen hours of the pour, and keep each panel close to square.
- What goes under the slab?
- Four inches of compacted clean stone, which levels the bed, lets water drain and breaks capillary rise from the soil. The gravel calculator sizes that layer, including the compaction it will lose.
- Can I add water to the mix to make it easier to place?
- You can, and it will cost you strength permanently. Water added on site raises the slump and lowers the final compressive strength — a wetter mix is easier today and weaker for the next thirty years. If placing is hard, the answer is more hands or a plasticiser, not a hose.
- Where do the prices come from?
- Material is a national range checked against retail and supplier pricing, dated on this page. Only the labour portion is adjusted for where you are, using BEA Regional Price Parities. We do not multiply the whole cost by a regional factor — a bag of mix does not cost 11% more in California, but the hour to place it does.
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About the author
Pablo Ruiz Quintero — Editor
Editor of MaterialsEstimator. Decides which documents count as a source, what every default value is, and where the line sits between what the reader parses and what the engines compute.
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