How to measure a slab
- Measure the inside of the forms, not the outside of the boards. A 2×4 form board is 1½ in thick, so measuring outside-to-outside on a 10 ft slab overstates each dimension by 3 in.
- Take the depth in at least five places. Check the corners and the middle with a tape measure from a string line pulled tight across the form tops down to the compacted base. Use the average. Low spots in the base are the most common reason people run out of concrete.
- Split odd shapes into rectangles. An L-shaped patio is two rectangles. Run the calculator for each (or use "Number of slabs" if they are identical) and add the results.
- Note any thickened edges. Many slabs have a deeper perimeter or a thickened area under posts. Set Thickened edge to Yes and enter the edge depth and width from your plans. For a thickened area under posts, use the footing calculator and add it.
Thickened edges and contractor quotes
A monolithic slab with a thickened edge has a deeper strip around the perimeter. The calculator adds a ring of concrete: perimeter strip area × (edge depth − slab thickness), where the strip area is 2 × (length + width) × edge width − 4 × edge width² so the corners are not counted twice. The default edge, 12 in deep in total and 12 in wide, is an editable assumption and not a code requirement. Use the size on your plans or the one your building department requires. With the default 10 × 10 ft slab, 4 in thick: 0.98 yd³ (26.4 ft³) of extra concrete, for 2.34 yd³ (63.1 ft³) in total.
For a quote, open “Product sizes, prices and costs”. Enter the concrete price, then optional delivery, markup and tax. Markup is applied to materials and delivery, and tax is applied last. Costs are rounded to whole currency units.
The formula behind the numbers
A flat slab is a box, so its volume is length × width × thickness. Lengths are in feet and thickness is usually in inches, so divide the thickness by 12 first. One cubic yard is 27 cubic feet (3 ft × 3 ft × 3 ft), so divide cubic feet by 27 for the number ready-mix suppliers quote.
Example: 10 ft × 10 ft × (4 ÷ 12) ft = 33.33 ft³, and 33.33 ÷ 27 = 1.23 yd³. Add 10% for waste and you get 1.36 yd³ (36.7 ft³).
For bagged mix, divide the cubic feet by the bag's yield. Quikrete's data sheet lists approximate yields of 0.30 ft³ for a 40 lb bag, 0.45 ft³ for 60 lb and 0.60 ft³ for 80 lb. The calculator always rounds bags up, because you can't buy part of a bag. For metric users, an 80 lb bag yields about 17 litres (0.017 m³), per the same sheet.
How many bags for common slab sizes
These figures come straight from the calculator with waste set to 0%. Add your own allowance (5–10% is typical) before buying.
| Slab size | Area | Volume | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 4 × 4 ft | 16 ft² | 0.2 yd³ (5.3 ft³) | 9 | 12 |
| 8 × 8 ft | 64 ft² | 0.79 yd³ (21.3 ft³) | 36 | 48 |
| 10 × 10 ft | 100 ft² | 1.23 yd³ (33.3 ft³) | 56 | 75 |
| 10 × 12 ft | 120 ft² | 1.48 yd³ (40 ft³) | 67 | 89 |
| 12 × 12 ft | 144 ft² | 1.78 yd³ (48 ft³) | 80 | 107 |
| 12 × 16 ft | 192 ft² | 2.37 yd³ (64 ft³) | 107 | 143 |
| 16 × 20 ft | 320 ft² | 3.95 yd³ (106.7 ft³) | 178 | 238 |
| 20 × 20 ft | 400 ft² | 4.94 yd³ (133.3 ft³) | 223 | 297 |
| 24 × 24 ft | 576 ft² | 7.11 yd³ (192 ft³) | 320 | 427 |
| Slab size | Area | Volume | 40 kg bags | 25 kg bags |
|---|---|---|---|---|
| 1.2 × 1.2 m | 1.44 m² | 0.144 m³ (144 L) | 8 | 13 |
| 2.4 × 2.4 m | 5.76 m² | 0.576 m³ (576 L) | 31 | 50 |
| 3 × 3 m | 9 m² | 0.9 m³ (900 L) | 49 | 77 |
| 3 × 3.6 m | 10.8 m² | 1.08 m³ | 58 | 93 |
| 3.6 × 3.6 m | 12.96 m² | 1.3 m³ | 70 | 111 |
| 3.6 × 4.8 m | 17.28 m² | 1.73 m³ | 93 | 148 |
| 4.8 × 6 m | 28.8 m² | 2.88 m³ | 154 | 247 |
| 6 × 6 m | 36 m² | 3.6 m³ | 193 | 308 |
| 7.2 × 7.2 m | 51.84 m² | 5.18 m³ | 277 | 443 |
Past about 1.5–2 yd³, the bag count gets large. A 12 × 12 ft slab at 4 in already needs 80 bags of 80 lb mix, which is more than 3 tons of bags to lift and mix by hand. That's usually the point where ordering ready-mix makes sense.
How thick should the slab be?
Thickness drives the volume more than anything else: going from 4 in to 6 in adds 50% more concrete. Pick it for the load the slab will carry and what your local code requires, not by habit.
| Use | Typical thickness | Notes |
|---|---|---|
| Walkway, patio, shed floor (light) | 4 in | Common minimum for flatwork on a compacted base |
| Driveway, passenger cars | 4 in minimum | The Michigan Concrete Association guideline minimum |
| Driveway with delivery vehicles | 5 in minimum | Same guideline; many cities require a thicker apron at the street |
| Garage floor, RV or truck parking | 5–6 in | Check local code and the loads involved |
| Thin topping or repair | Under 2 in | Use a topping or resurfacer, not regular concrete mix. Quikrete rates its Concrete Mix for pours of 2 in or more |
| Use | Typical thickness | Notes |
|---|---|---|
| Walkway, patio, shed floor (light) | 100 mm | Common minimum for flatwork on a compacted base |
| Driveway, passenger cars | 100 mm minimum | The Michigan Concrete Association guideline minimum |
| Driveway with delivery vehicles | 125 mm minimum | Same guideline; many councils require a thicker crossover at the street |
| Garage floor, RV or truck parking | 125–150 mm | Check local code and the loads involved |
| Thin topping or repair | Under 50 mm | Use a topping or resurfacer, not regular concrete mix. Quikrete rates its Concrete Mix for pours of about 50 mm or more |
Here's what a 10 × 10 ft slab needs at different thicknesses, before waste:Here's what a 3 × 3 m slab needs at different thicknesses, before waste:
| Thickness | Volume | 80 lb bags |
|---|---|---|
| 3.5 in | 1.08 yd³ (29.2 ft³) | 49 |
| 4 in | 1.23 yd³ (33.3 ft³) | 56 |
| 5 in | 1.54 yd³ (41.7 ft³) | 70 |
| 6 in | 1.85 yd³ (50 ft³) | 84 |
| Thickness | Volume | 40 kg bags | 25 kg bags |
|---|---|---|---|
| 7.5 cm | 0.675 m³ (675 L) | 37 | 58 |
| 10 cm | 0.9 m³ (900 L) | 49 | 77 |
| 12.5 cm | 1.13 m³ | 61 | 97 |
| 15 cm | 1.35 m³ | 73 | 116 |
Structural slabs, such as house foundations or slabs carrying point loads, are designed by an engineer or set by local code. Use their numbers.
Worked examples
Backyard patio, 12 × 16 ft, 4 in thickBackyard patio, 3.66 × 4.88 m, 10.2 cm thick
Length 12 ft · Width 16 ft · Thickness 4 in · Number of slabs 1 · Waste / overage 8 %Length 3.66 m · Width 4.88 m · Thickness 10.2 cm · Number of slabs 1 · Waste / overage 8 %
- Concrete volume: 2.56 yd³ (69.1 ft³)Concrete volume: 1.96 m³
- 80 lb bags: 11640 kg bags: 105
At over 2 cubic yards this is a ready-mix job for most people. Ask the plant for 2.5 yd³ to cover base variations.At over 1.53 m³ this is a ready-mix job for most people. Ask the plant for 1.91 m³ to cover base variations.
Shed pad, 8 × 10 ft, 4 in thickShed pad, 2.44 × 3.05 m, 10.2 cm thick
Length 8 ft · Width 10 ft · Thickness 4 in · Number of slabs 1 · Waste / overage 10 %Length 2.44 m · Width 3.05 m · Thickness 10.2 cm · Number of slabs 1 · Waste / overage 10 %
- Concrete volume: 1.09 yd³ (29.3 ft³)Concrete volume: 0.831 m³ (831 L)
- 80 lb bags: 4940 kg bags: 45
- 60 lb bags: 6625 kg bags: 71
This is manageable with bags and a rented mixer over a long morning, with two people.
Two identical AC pads, 3 × 4 ft, 4 in thickTwo identical AC pads, 0.91 × 1.22 m, 10.2 cm thick
Length 3 ft · Width 4 ft · Thickness 4 in · Number of slabs 2 · Waste / overage 10 %Length 0.91 m · Width 1.22 m · Thickness 10.2 cm · Number of slabs 2 · Waste / overage 10 %
- Concrete volume: 0.33 yd³ (8.8 ft³)Concrete volume: 0.249 m³ (249 L)
- 80 lb bags: 1540 kg bags: 14
Bags or a ready-mix truck?
Both give you the same material. What differs is labour, timing and the minimum order.
- Bags make sense for small pads, post footings, steps and repairs, or anywhere a truck can't reach. Bags you don't open can often go back to the store, so it costs little to over-buy by a few.
- Ready-mix arrives as one consistent batch, which matters for a large slab that has to be finished in one go. Trucks commonly carry around 8–10 yd³ (approximate), and many plants charge a short-load fee below a minimum order. Call two or three local suppliers, give them the cubic yards from this calculator, and ask for the delivered total including any fees.
- Mixing trailers or "U-cart" concrete from some yards sit in between. They're good for 1–2 yd³ when you can tow.
Whichever you choose, have helpers, tools and forms ready before the concrete arrives. With bags, set up a mixing station close to the forms.
Rounding the order: bags or a truck
Under the results you'll see a ready-mix order rounded up to the next ¼ yd³ and a short bags-or-truck note. Many plants quote in steps like this, and some use ½ yd³, so check with yours. The note uses approximate thresholds from published cost guides. Below about ½ yd³, bags usually win. Between about ½ and 1 yd³, it depends on the plant's short-load fee. Above about 1 yd³ (45 bags of 80 lb mix), ready-mix is usually cheaper and much less work. Many plants charge a short-load fee on orders below about 3–5 yd³, and some set the line higher, so ask for their minimum before you compare. For the default 10 × 10 ft slab at 4 in with 10% waste, that's 1.36 yd³ (36.7 ft³), ordered as 1.5 yd³ (40.5 ft³), or 62 bags of 80 lb mix.
Under the results you'll see a ready-mix order rounded up to the next 0.25 m³ and a short bags-or-truck note. Many plants quote in steps like this, so check with yours. The note uses approximate thresholds from published cost guides. Below about 0.4 m³, bags usually win. Between about 0.4 and 0.75 m³, it depends on the plant's short-load fee. Above about 0.75 m³, ready-mix is usually cheaper and much less work. Bunnings estimates about 110 bags of 20 kg concrete per cubic metre (the calculator's yield gives about 107). Many plants charge a short-load fee on orders below roughly 2–4 m³, so ask for their minimum before you compare. For the metric default 3 × 3 m slab at 10 cm with 10% waste, that's 0.99 m³ (990 L), ordered as 1 m³, or 106 bags of 20 kg.
What affects the cost
Concrete prices vary by region, season and mix, so the calculator only multiplies by prices you enter. The main factors are:
- Volume and minimum-load fees. Small orders cost more per yard.
- Mix design. Higher strength, fiber, air entrainment for freeze-thaw areas, colour, and set accelerators or retarders all change the price.
- Delivery distance and time. Long waits at the site can be billed by the minute.
- Pump or buggy rental if the truck can't back up to the forms.
- Base and reinforcement. Gravel, rebar or mesh, forms, expansion joint material and curing compound. Use the gravel and rebar calculators for those.
Common mistakes
- Forgetting the base is uneven. Half an inch low across a 10 × 10 slab is an extra 4.2 ft³, which is seven 80 lb bags.
- Adding water to make bagged mix easier to place. Extra water weakens the concrete. Follow the bag's water range.
- Skipping control joints. The Michigan Concrete Association guideline spaces joints no more than about two times the slab thickness in feet (8 ft for a 4 in slab), cut to at least ¼ of the depth, with panels as square as possible.
- Ordering exactly the calculated volume. Order 5–10% extra. Running out mid-pour leaves a cold joint.
Safety
Wet concrete is caustic. OSHA warns that cement can cause severe chemical burns that may not hurt until hours later. Wear alkali-resistant gloves, long sleeves, waterproof boots and eye protection, and wash concrete off your skin promptly with clean water. Lift bags with your legs, and get help with 80 lb bags.
Checked against
Each case below is re-run against the calculator by our automated test suite.
| Case | Our result | Checked against | Result |
|---|---|---|---|
| 10 × 10 ft slab, 4 in thick, no waste: volume | 1.23 yd³ (33.3 ft³) | calculator.net concrete calculator | Match |
| Same slab with 10% waste: 80 lb bags | 62 | calcs24 concrete calculator (worked example) | Match |
| 20 × 20 ft slab, 4 in thick, 12 × 12 in thickened edge 12 in deep, no waste: extra concrete | 1.88 yd³ (50.7 ft³) | Hand calculation | Match |
| Case | Our result | Checked against | Result |
|---|---|---|---|
| 3.05 × 3.05 m slab, 10.2 cm thick, no waste: volume | 0.944 m³ (944 L) | calculator.net concrete calculator | Match |
| 6.1 × 6.1 m slab, 10.2 cm thick, 30.5 × 30.5 cm thickened edge 30.5 cm deep, no waste: extra concrete | 1.43 m³ | Hand calculation | Match |
Frequently asked questions
How many 80 lb bags make a cubic yard?
At the published 0.60 ft³ per bag, 27 ÷ 0.60 = 45 bags. With 60 lb bags it is 60, and with 40 lb bags 90, before waste.
How much concrete do I need for a 10 × 10 slab?
At 4 in thick, 33.3 ft³ (1.23 yd³). That is 56 bags of 80 lb mix with no waste, or 62 with 10% added.
How much concrete for a 20 × 20 slab?
At 4 in, about 4.94 yd³ before waste. At that size a ready-mix delivery is the practical option.
Can I pour a slab thinner than 4 inches?
For decorative or very light use it is sometimes done, but thin slabs crack more easily. Quikrete rates its standard Concrete Mix for pours of 2 in or more. Use a topping or resurfacer for thinner layers.
Does the calculator include the gravel base or rebar?
No. It counts concrete only. Use the gravel and rebar calculators for those, and add the results to your project list.
How do I calculate concrete for a circular slab?
Use the column calculator with the circle's diameter as the "diameter" and the slab thickness (in feet) as the height, or find the area with the square footage calculator and multiply by the thickness.
Sources & assumptions
The results are planning estimates, and the cost figures only use prices you enter. Product yields, densities and coverage rates vary, so check the label or ask your supplier, and follow local codes. Figures marked "approximate" or "rule of thumb" are general practice, not exact standards.
- Quikrete Concrete Mix data sheet (bag yields, minimum thickness)
- Michigan Concrete Association, Driveway & Sidewalk Guidelines (2025)
- ShortLoad: What is a short-load fee? (typical plant minimums of 3–5 yd³)
- CementCalcs: ready-mix vs bagged concrete breakeven (about ½–1 yd³)
- Bunnings Workshop: about 110 bags of 20 kg concrete per cubic metre
- OSHA: Preventing Skin Problems from Working with Portland Cement
See how we calculate and test our formulas and the changelog. Last updated 2026-10-08.
Reviewed by the MaterialWhiz Editorial Team. This estimate uses the assumptions shown in the calculator. Review the result against your supplier's product coverage and site conditions. Last reviewed: October 10, 2026. See how we calculate · Report a correction.