How much concrete do I need?
A slab is an area and a thickness, and concrete is sold by the cubic yard. The conversion is one line of arithmetic. The decisions around it, how thick, bags or truck, and what goes underneath, are what actually change the number.
How much concrete do you need?
Working out a whole yard rather than one bed? Sketch it to scale in the planner and every area is measured, costed and totalled together.
The formula
cubic yards = (square feet × thickness in inches) ÷ 324
A 10 by 12 foot patio at 4 inches is (120 × 4) ÷ 324 = 1.48 cubic yards. Order 1.6, because concrete is the one material where running short is genuinely unrecoverable: the first pour has started curing by the time you get back with more, and the join between them is a crack waiting to happen.
Ten per cent over is the standard allowance, and it covers the two things that always happen. The sub-base is never perfectly flat, so the middle is deeper than you measured, and the forms bow outward slightly under the weight.
How thick
| What it carries | Slab | Compacted base under it |
|---|---|---|
| Patio, walkway, shed floor | 4 in | 4 in |
| Car driveway | 4 – 5 in | 4 – 6 in |
| Trucks, RV or heavy loads | 6 in | 6 – 8 in |
| Footing for a wall or post | varies | below the frost line |
Four inches is the number for almost every yard project, and going to five adds a quarter to your order for very little. What actually decides whether a slab cracks is the base under it and the joints in it, not another inch of concrete.
The base is compacted gravel, and it is not optional. It gives the water somewhere to go and the slab something that does not move to sit on. Concrete poured straight onto soil moves with the soil, and soil moves every winter.
Bags or a truck
An 80 lb bag of ready-mix yields about 0.6 cubic feet, so it takes 45 bags to make a cubic yard. A 60 lb bag yields 0.45, which is 60 to the yard. That is the arithmetic; here is the practical version.
- Under half a yard, so a post, a small pad or a set of steps: bags, mixed in a wheelbarrow or a rented mixer.
- Half a yard to about a yard is the awkward middle. Forty-five bags is 3,600 pounds to carry, and it all has to be mixed and placed before any of it sets. Doable with help; miserable alone.
- Over a yard, call for ready-mix. Most companies have a one-yard minimum and a short-load fee, and it is still the right answer: the truck delivers a consistent mix, all at once, and you spend the day placing rather than mixing.
The reason this matters more than the price is that concrete has a clock on it. You have somewhere between forty-five minutes and a couple of hours from water to unworkable, depending on the temperature. Anything you cannot place and finish inside that window should have arrived in a truck.
What else the order needs
- Forms: straight lumber, staked every three or four feet. The forms hold the shape and the level, and a bowed form is a bowed slab for good.
- Reinforcement: wire mesh or half-inch rebar on a grid, held up in the middle of the slab rather than lying on the ground where it does nothing.
- Control joints: cut or tooled every 8 to 10 feet, and at least a quarter of the slab's depth. Concrete cracks; joints decide where.
- Expansion material where the new slab meets the house, a path or an existing slab, so the two can move independently.
Weather and curing
Pour between about 50°F and 85°F if you can choose. Hot sun takes the water out of the surface before the slab has set, which leaves it dusty and weak; frost overnight on a fresh pour ruins it outright.
Then keep it damp. Concrete gains strength by a chemical reaction with water, not by drying, and a slab kept wet for the first week ends up substantially stronger than one left in the sun. Cover it with plastic or spray it down morning and evening. It is free, and it is the single biggest thing an amateur pour gets wrong.
Walkable in a day, furniture after three, and a car after a week. Full strength takes about 28 days.
Ordering
- Work out the yardage, add ten per cent, and round up to the increment your supplier sells.
- Have more hands than you think you need. Placing a yard is fast and finishing it is not.
- Have everything ready before the truck arrives: forms staked, base compacted, mesh laid, tools out.
- Ask about the short-load fee and the waiting-time charge. Both are normal and both surprise people.
Lay out your slab in the planner
Draw the patio, set the depth, and the planner gives you the square footage and the cubic yards together, and counts the gravel base under it as its own order, which is the half of the job that usually gets forgotten.