How Much Concrete Do I Need?

The one formula behind every estimate

Concrete is sold by volume, so every estimate begins with the volume of the space you want to fill. For a flat rectangular slab the volume is length times width times thickness. If all three are in feet, the answer is in cubic feet. Divide by 27 to get cubic yards, because a cubic yard is 3 ft by 3 ft by 3 ft.

The only real trap is thickness. Slabs are measured in inches but length and width are in feet, so thickness must be divided by 12 before multiplying. A 4 in slab is 0.3333 ft thick. Forgetting that step gives an answer that is wrong by a factor of twelve.

Formula: yd³ = (L ft × W ft × T in ÷ 12) ÷ 27. Add waste, then round up.

Step by step: measure, convert, multiply, add waste

1. Measure the length and width of the area in feet. Use the formwork dimensions, not the excavation.
2. Decide the thickness in inches and divide by 12 to convert it to feet.
3. Multiply length × width × thickness to get cubic feet.
4. Divide cubic feet by 27 for cubic yards. This is the net volume.
5. Add 5 to 10% for waste and round up to the next quarter or half yard.

That order never changes, whatever the shape. For a circle you replace length × width with π × radius squared. For a wall you replace width by height and thickness stays thickness. The slab calculator and the other calculators on this site run the same steps for you and show them in a Show the math panel, so you can check each line.

Worked example: a 12 × 16 ft patio, 4 in thick

1. Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Length 12 ft; Width 16 ft; Thickness 4 in = 0.3333 ft
2. Volume = 12 × 16 × 0.3333 × 1 = 64 ft³
3. Net volume = 64 ft³ ÷ 27 = 2.3704 yd³ (× 0.0283168 = 1.8123 m³)
4. Order quantity = 2.3704 yd³ × (1 + 10%) = 2.6074 yd³ = 70.4 ft³
5. 80 lb bags = 70.4 ft³ ÷ 0.6 ft³ per bag = 117.33 → 118 bags (always round up)
6. Weight = 70.4 ft³ × 150 lb/ft³ = 10,560 lb

The net volume is 2.370 yd³ (64.00 ft³). With 10% waste the order is 2.61 yd³. In bags that is 118 bags of 80 lb mix or 157 bags of 60 lb mix. Either way this is beyond what most people want to mix by hand, which is why a truck order makes sense for a job this size.

How thickness changes the answer

Thickness has the largest effect on the total, and it is also the most variable measurement on site. Each extra inch on a 12 × 16 ft slab adds about 0.59 yd³. The table shows the same footprint at four thicknesses.

See the slab thickness guide for what each thickness is normally used for.

Formulas for other shapes

For an irregular shape, split it into rectangles and circles, calculate each and add them. For a sloped base, measure the thickness at several points and use the average.

How much waste to add

Waste covers spills, uneven sub-grade, form movement and the little that stays in the truck chute. Five to ten percent is the common advice. Use 5% on a flat, well-compacted base with tight forms. Use 10% on a typical residential job. Go to 15% or more for trenches, excavated footings and anything poured over rough ground, because the depth is rarely what you drew.

Running short is worse than having extra. A second delivery means another short-load fee, and a cold joint where new concrete meets partially hardened concrete. If you are close to a round number, round up. For the 12 × 16 ft slab above, 2.61 yd³ rounds to 2.75 or 3 yards depending on what your plant sells.

Bags or a truck?

The break between the two depends on volume and effort. One cubic yard needs 45 bags of 80 lb mix. Mixing and placing that many bags by hand is a long day. As a rule of thumb, bags suit up to about one yard, ready-mix suits above two, and between the two you should price both. The thresholds are an estimate, not a rule: the delivery fee and short-load fee often decide it. Use the cost calculator with your own quotes, and read bagged vs ready-mix concrete.

Mistakes that cause shortages

• Forgetting to divide thickness by 12.
• Measuring the excavation instead of the forms.
• Mixing units, such as feet for length and meters for width.
• Using the nominal lumber size instead of the actual size for forms.
• Ignoring footings, thickened edges and steps, which add to the slab volume.
• Subtracting reinforcement. Rebar takes up almost no volume, so do not subtract it.
• Not rounding up when the plant sells in quarter-yard steps.

A checklist before you order

1. Measure again, on site, with a tape.
2. Write down length, width and thickness for each part.
3. Calculate each part and add the totals.
4. Add waste and round up.
5. Check the minimum load and short-load fee with the plant.
6. Plan the base, reinforcement and curing before the truck arrives.

Coverage numbers worth remembering

• One cubic yard covers 81 ft² at 4 in, 64.8 ft² at 5 in and 54 ft² at 6 in.
• A 10 ft × 10 ft slab at 4 in is about 1.24 yd³ net.
• One 80 lb bag makes 0.60 ft³, so 45 bags make a yard.
• A cubic yard of concrete weighs about 4,050 lb.

Frequently asked questions