Concrete Calculator
Concrete is the one material you cannot take back to the store. Once the truck tips its drum, the meter is running — and running short mid-pour is the kind of afternoon people talk about for years. You need the volume right the first time, in the unit your supplier quotes: cubic yards in the US, cubic metres in most of the world, and cubic feet when you are sizing up bagged mix from Home Depot. This concrete calculator turns your slab's length, width, and thickness into all three, with the depth entered in inches, feet, centimetres, or metres — because nobody measures a 4-inch slab in feet by choice. Short loads are expensive, and most plants charge a fee under 3 to 4 yards, so knowing your real number before you pick up the phone saves both money and the look the dispatcher gives you.
How to use this calculator
- Set your forms or string lines first, then measure inside them. The slab can only be as true as the forms — a 2x4 laid flat gives about 3.5 inches of depth, standing on edge about 5.5.
- Measure the length inside the forms in feet (or metres). Hook your tape on the inside face of the form board and pull to the far inside face.
- Measure the width the same way, at the widest point. Bowed forms make a bowed slab — and extra concrete you never planned for.
- Enter the depth (thickness) in inches — 4 inches is the standard for patios, walkways, and shed slabs. Use the dropdown to switch units if you measured in centimetres.
- Check the depth in several spots with a tape, not just one corner. Excavations are never perfectly flat; the shallowest and deepest spots can differ by an inch.
- Press Calculate. You get cubic yards (what the ready-mix plant quotes), cubic feet (for bag math), and cubic metres.
- Add your margin: 5% if the subgrade is flat and compacted, 10% if it is hand-dug and lumpy. Ordering exactly the calculated number is how pours run short.
- Call the plant the day before with the number plus margin. Tell them the slab size, the depth, and when you want the truck — plants schedule in windows, not exact minutes.
Formula
Worked example
Let us price a driveway pad: 24 feet long, 10 feet wide, 5 inches thick (driveways carry cars, so 5 inches instead of the 4 a patio gets). Convert the depth: 5 / 12 = 0.4167 ft. Multiply: 24 x 10 x 0.4167 = 100 ft3. To cubic yards: 100 / 27 = 3.70 yd3. To cubic metres: 100 x 0.0283 = 2.83 m3. Add the pro margin — 10%, because hand-dug ground is never flat: 3.70 x 1.10 = 4.07 yd3. Round up to 4.1 yd3 when you call the plant (they sell fractional yards, so do not round to 5). At roughly $140 to $160 per yard for standard 3000-psi mix in most US markets, that is about $574 to $656 for the load — and at 4.1 yards you are safely over the short-load fee most plants charge under 3 yards, a fee that alone runs $50 to $100. Bag-math check, in case you are tempted: 100 ft3 / 0.45 ft3 per 60-lb bag = 223 bags. At about $4.50 a bag, that is $1,000 in bags plus a full day of mixing — versus one truck and a morning. The truck wins by a mile.
Measurement tips
- Order for the morning. Concrete sets on its own schedule — a 9 a.m. delivery gives you the whole day to place, screed, and finish before it decides it is done.
- Line up the crew before the truck. Two wheelbarrows and three people minimum for a 4-yard pour — the driver waits about 30 minutes free, then charges by the minute.
- Wet the subgrade before the pour. Dry dirt sucks water out of the mix and weakens the surface — one pass with a garden hose an hour before is cheap insurance.
- Rent a bull float and a magnesium hand float from Home Depot ($15 to $25 a day each). Your knees and the slab's surface will both thank you.
- Keep a straight 2x4 or aluminum screed board as long as the slab is wide. Screeding off the forms is how you get a flat slab; eyeballing is how you get a birdbath.
- Buy rebar or wire mesh for anything vehicles touch. Number 3 rebar on 18-inch centers runs about $5 to $7 a stick at the lumberyard; wire mesh sheets are cheaper and fine for patios.
- Tell the plant your slump. A 4-inch slump is standard and strong; asking for it wetter makes placing easier but weakens the concrete. Do not add water on site unless the driver okays it.
Common mistakes
- Measuring the hole instead of the concrete. If you dig 6 inches deep for a 4-inch slab over a gravel base, the concrete volume is 4 inches — the gravel is not the plant's problem, and ordering 6 inches of concrete is 50% too much.
- Forgetting the slope. A patio needs about 1/4 inch of fall per foot away from the house — on a 10-foot run that is 2.5 inches of extra depth at the far end nobody calculated.
- Ordering to the exact decimal. Excavations are lumpy, wheelbarrows spill, and the chute keeps a little. The 5 to 10% margin is not caution, it is physics.
- Pouring on uncompacted soil. Soft ground settles, the slab cracks, and no volume calculator can fix a bad base — tamp it with a plate compactor ($60 to $80 a day to rent).
- Starting the pour with no help lined up. Concrete does not wait for your brother-in-law. A 4-yard pour needs to be placed and screeded inside about two hours.
- Finishing too early. Working the surface while bleed water is still sitting on top traps it and weakens the finish — wait until the sheen disappears, then float.
Factors that affect accuracy
- Slab thickness. Volume scales directly with depth — going from 4 to 6 inches for a driveway adds 50% more concrete. Measure twice, pour once.
- Subgrade flatness. A hand-dug base that sits an inch deeper in the middle adds concrete across the whole slab. Screed the base level and your volume lands within 5%.
- Form spread. Wet concrete pushes forms outward — a 2x4 held with one stake every 4 feet will bow. Stake every 2 to 3 feet and brace the corners, or the slab ends up wider (and thirstier) than planned.
- Reinforcement. Rebar and mesh displace almost no concrete, so ignore them in the math. But the chairs and spacers holding the steel up can float if you do not tie the steel down.
- Weather. Hot, dry wind speeds up setting and eats your finishing time; cold slows everything down. Neither changes the volume, but both change the day — schedule around them.
- Supplier minimums and fees. Under about 3 cubic yards most plants add a short-load fee; under 1 yard, bagged mix from the home center usually wins on price and sanity.
Frequently asked questions
How much concrete do I need for a 12x12 patio at 4 inches?
12 x 12 x 0.333 ft = 48 ft3, which is 1.78 yd3. Order about 2 yards with margin — and check the short-load fee, because 2 yards sits under most plants' 3-yard threshold.
Should I buy 60-lb or 80-lb bags?
Same concrete, different workout. An 80-lb bag yields 0.6 ft3, a 60-lb bag 0.45 ft3. For a small job (under about 15 ft3) bags are fine; above that, price ready-mix — your back will vote for the truck.
How thick should a shed slab be?
4 inches on compacted gravel handles any normal shed. Go 5 to 6 inches if you are parking a riding mower or a car on it — the extra inch is cheap compared to a cracked slab.
Can I pour concrete in sections?
Yes — that is what control joints and cold joints are for. But each section needs its own volume calculation, and a cold joint (poured on a different day) is weaker than one continuous pour.
How long before I can walk or drive on new concrete?
Foot traffic in 24 to 48 hours, vehicles after about 7 days — and full strength takes 28 days. The volume you order today becomes a driveway next week, not tomorrow.
What does '3000 psi' mean on the delivery ticket?
It is the compressive strength — 3,000 pounds per square inch after 28 days of curing. Standard for patios and driveways; step up to 4000 psi in freeze-thaw climates where road salt is used.
Why did my slab crack if I ordered the right volume?
Volume was never the risk — the base, the joints, and the curing were. Uncompacted soil, missing control joints (cut every 8 to 10 feet), or drying too fast in sun and wind will crack a slab that had exactly the right yards.