Planning a concrete pour
Concrete is bought by volume: cubic feet in a bag, cubic yards or cubic metres from a ready-mix truck. Every estimate in this aisle starts the same way, with length × width × depth, and ends with a decision about how to buy it. Small jobs such as fence posts, landing pads and repairs come in bags you mix yourself. Slabs and footings of more than about a cubic yard are usually cheaper and far easier from a truck.
Unlike paint or flooring, you cannot pick up a missing quart halfway through. Once concrete starts to set, a cold joint where a second batch meets the first is a weak spot. That is why the calculators here round up generously and add a waste allowance, and why it is worth measuring twice before you order.
Measure depth carefully, it matters most
Length and width are easy to get right with a tape. Depth is where estimates go wrong. A slab specified at 4 inches that averages 4½ inches because the base dips in the middle uses 12% more concrete than planned. Check the depth at several points across the forms with a string line pulled tight between the form tops, and use the average.
For footings and post holes, measure the hole you actually dug, not the one on the plan. Trench sides wander and holes come out wider at the top. If the hole is noticeably larger than planned, enter the larger size; the waste allowance is for spillage and small errors, not for a hole that is two inches wider all round.
- Patios, walks and shed floors: 4″ (10 cm)
- Driveways and slabs for vehicles: 5–6″ (12–15 cm)
- Wall footings: as specified on your plans, commonly 8″ deep or more
- Fence posts: about one third of the post height above ground, below the frost line
Bags or ready-mix
Bagged concrete mix is sold in 80, 60 and 40 lb bags in the US and usually 25 kg bags elsewhere. The standard published yields are about 0.60 cubic feet for an 80 lb bag, 0.45 for a 60 lb bag and 0.30 for a 40 lb bag, which works out to 45, 60 and 90 bags per cubic yard. A 25 kg bag makes roughly 12 litres. Always check the yield printed on the bag, because high-strength, fast-setting and fiber-reinforced mixes differ.
Mixing 45 bags by hand is a long, heavy day, and the first batches start to stiffen before the last ones are placed. For anything over about one cubic yard, compare the price of a ready-mix delivery. Plants sell by the cubic yard or cubic metre, often with a minimum order and a fee for small loads, so ask when you call. Have the truck’s route to the forms planned and clear before it arrives.
Waste allowance and rounding up
The calculators add 10% by default. That covers a base that is not perfectly flat, forms that bow slightly under the weight, concrete left in the wheelbarrow or chute, and small measuring errors. A carefully graded and compacted base can get by with 5%; a rough excavation or an irregular hole may need 15%.
After waste, every list rounds up to whole purchasable units: whole bags, or whole cubic yards and cubic metres of ready-mix. The detail line shows the exact amount, so if your supplier sells part loads in quarter or half yards you can order closer to it. Running short is far more expensive than a few leftover bags, which keep for months if stored dry.
Forms, base and reinforcement
Slabs need forms at least as deep as the slab: a 4-inch slab takes 2×6 boards, because a 2×4 is only 3½ inches deep. Stake the forms on the outside about every three feet and at each corner so they hold their line when the concrete pushes against them. Under the slab, a compacted layer of gravel gives an even base and helps drainage.
Reinforcement controls cracking and adds strength, but only when it sits inside the concrete. Rebar is sold in 20 ft lengths in the US and 6 m lengths in most metric markets, in sizes such as #3 and #4 or 10 and 12 mm. Where bars meet end to end they overlap, commonly by 40 bar diameters, and every crossing is tied with wire. Chairs or bricks hold the grid near the middle of the slab during the pour.
Bar size, spacing and footing dimensions for structural work are set by local building codes and by your plans. Use the calculators to turn those numbers into a shopping list, not to choose them.
Common mistakes
The most frequent problems are easy to avoid once you know them. Most come from ordering too little, pouring too fast, or not preparing the ground under the concrete.
- Ordering exactly the calculated volume with no allowance, then running short on the last corner
- Pouring on loose fill or frozen ground instead of compacted, undisturbed soil
- Adding extra water to make the mix easier to work, which weakens the concrete
- Letting rebar or mesh lie on the ground where it does nothing
- Forgetting to call the utility locating service before digging post holes and footings
- Not having enough help, tools or daylight for the size of the pour
Calculators in this aisle
- Concrete slab calculator: Slab size in, shopping list out: ready-mix by the yard or bags for small pours, plus the forms to hold it.
- Concrete bag calculator: Pour size in, bag count out, for whichever bag size the store has on the pallet.
- Fence post concrete calculator: Posts and holes in, bags out: concrete for every hole with the post itself taken off, plus gravel for drainage.
- Footing calculator: Continuous footings, square pads or round piers: concrete by the yard or the bag, and the rebar for a wall footing.
- Rebar calculator: Slab size and spacing in, a rebar grid out: stock bars to buy, ties for every crossing and chairs to hold it up.