Working out how much concrete you need comes down to two steps: find the volume, then convert it to the units concrete is actually sold in — bags or cubic yards of ready-mix. Everything after that is deciding how much extra to buy and whether you can physically place it before it sets.
Step 1 — find the volume
Multiply length × width × thickness, with the thickness converted from inches to feet:
volume (ft³) = length (ft) × width (ft) × thickness (in) ÷ 12
volume (yd³) = volume (ft³) ÷ 27A 10 × 10 ft slab at 4 inches is 10 × 10 × (4 ÷ 12) = 33.3 ft³. Concrete is
priced by the cubic yard, and there are 27 cubic feet in a yard, so that’s
33.3 ÷ 27 = 1.23 cubic yards.
Measure the actual forms, not the plan. A slab that drifts an inch out of square over 12 feet is still going to get filled with concrete.
Step 2 — convert to bags or cubic yards
Every bag of standard concrete mix lists its yield — the cubic feet of wet concrete one bag makes. Divide the volume by the yield:
| Bag size | Yield | Bags per cubic yard | Cost per bag | Cost per yd³ |
|---|---|---|---|---|
| 40 lb | 0.30 ft³ | 90 | ~$4.00 | ~$360 |
| 50 lb | 0.375 ft³ | 72 | ~$5.00 | ~$360 |
| 60 lb | 0.45 ft³ | 60 | ~$5.50 | ~$330 |
| 80 lb | 0.60 ft³ | 45 | ~$6.50 | ~$293 |
| 90 lb | 0.675 ft³ | 40 | ~$7.50 | ~$300 |
So our 33.3 ft³ slab needs 33.3 ÷ 0.60 = 56 bags of 80 lb mix before waste.
Those yields come straight from the QUIKRETE #1101 and Sakrete data sheets and
are the same across standard mixes — but fast-setting and specialty mixes yield
differently, so read the YIELD line on the bag you’re actually buying.
Two things fall out of that table immediately. Bigger bags are cheaper per cubic foot — 80 lb mix works out to about $10.83/ft³ against $13.33/ft³ for 40 lb bags, so you pay roughly 23% more for the privilege of lighter lifts. And delivered ready-mix at ~$165/yd³ is about $6.11/ft³ — a little over half the price of the cheapest bag. Prices are US national-average estimates; your region will differ, sometimes a lot.
Always add a waste factor — here’s why
Add 10% to the calculated volume (5–15% is the normal range) and round up. This isn’t padding. Do the arithmetic on a 12 × 12 ft slab at 4 inches, which is 48 ft³ on paper:
- The subgrade dips an average of ½ inch below your form bottoms. That’s
144 ft² × (0.5 ÷ 12) = 6 ft³— 12.5% more concrete, on its own. - A ¼-inch average over-dig is still
3 ft³, or 6.3%. - Wet concrete pushes form boards outward between stakes. Every bulge is volume.
- Some mix ends up in the wheelbarrow, on the ground, and stuck in the mixer.
That’s the whole justification for the 10%. Use 5% only if you’ve screeded and compacted a genuinely flat base and your forms are staked every two feet. Use 15% on rough ground, on a hand-dug hole, or for footings, where the excavation is never the shape you drew.
Round up, always. Running short mid-pour costs you a cold joint — a plane of weakness where fresh concrete meets concrete that has already started to set — or an emergency store run with the truck on standby. Leftover bags go back to the store; a cold joint stays in your slab.
With 10% waste our 10 × 10 slab needs 62 bags of 80 lb mix, or 1.5 cubic yards of ready-mix (trucks dispatch in 0.25 yd³ increments, so the order rounds up to the next quarter yard).
Worked examples — 4-inch slab, 10% waste
| Slab | Volume | 80 lb bags | Bag cost | Ready-mix | Ready-mix cost |
|---|---|---|---|---|---|
| 8 × 8 | 21.3 ft³ (0.79 yd³) | 40 | ~$260 | 1.00 yd³ | ~$165 + short-load fee |
| 10 × 10 | 33.3 ft³ (1.23 yd³) | 62 | ~$403 | 1.50 yd³ | ~$248 |
| 12 × 12 | 48.0 ft³ (1.78 yd³) | 88 | ~$572 | 2.00 yd³ | ~$330 |
| 12 × 16 | 64.0 ft³ (2.37 yd³) | 118 | ~$767 | 2.75 yd³ | ~$454 |
| 16 × 20 | 106.7 ft³ (3.95 yd³) | 196 | ~$1,274 | 4.50 yd³ | ~$743 |
| 20 × 20 | 133.3 ft³ (4.94 yd³) | 245 | ~$1,593 | 5.50 yd³ | ~$908 |
Ready-mix costs assume $165/yd³ and exclude delivery and standby charges. Add a $50–$150 short-load fee on anything under about 4 cubic yards, which is nearly every slab on this list. The gap still widens with every square foot — see bags vs ready-mix for where the crossover actually falls.
Can you place it before it sets?
This is the constraint most people skip, and it’s the one that ruins slabs.
Standard mix gives you roughly an hour of working time before it stiffens past the point of screeding. That means every batch for a single slab has to go in, get screeded, and get floated inside a window that starts when the first batch hits the forms.
An 80 lb bag takes about 3 quarts of water and fills a wheelbarrow two bags at a time. Mixing, hauling, and dumping 62 bags is roughly 5,000 lb of material moved by hand. One person with a hoe and a barrow will not do it in an hour. The practical rules:
- Up to about 10–15 bags, one person can pour continuously.
- 20–40 bags needs two or three people and a rented mixer, with someone screeding while others mix.
- Beyond about 40 bags, the honest answer is ready-mix or a planned pour in sections separated by bulkhead forms and keyed joints.
If you truly have to split a pour, plan the joint before you start — a clean construction joint with dowels is fine, a random cold joint in the middle of an open slab is not.
What you’re buying besides concrete
The concrete is often less than half the shopping list. For a 12 × 12 slab at 4 inches over a 4-inch compacted base:
- Base gravel: 48 ft³ = 1.78 yd³ ≈ 2.5 tons of crusher run or #57 stone. Size it with the gravel calculator.
- Excavated spoil: digging 8 inches over 144 ft² produces
144 × (8 ÷ 12) = 96 ft³, which is 3.6 cubic yards of dirt — and it swells when loose. Decide where it goes before you dig. - Forms: 2×4 or 2×6 lumber plus stakes every 2–3 ft, plus form-release oil or scrap poly.
- Reinforcement: 6×6 welded wire mesh on chairs, or #3/#4 rebar on 18–24 inch centers for anything carrying vehicles.
- Vapor barrier: 6-mil poly under any slab that will be inside or heated.
- Tools: screed board, bull float, edger, groover, and a control-joint saw or hand groover.
Slope the finished surface about ⅛ to ¼ inch per foot away from the house so water leaves. Flat is not level, and level is not what you want.
Weather and curing
- Below 50°F concrete gains strength slowly; below freezing it can be permanently damaged. Use warm mix water, keep it blanketed, and never pour on frozen ground.
- Above 90°F the surface flash-sets and plastic shrinkage cracks appear. Pour early, dampen the subgrade, and keep the surface misted.
- Rain within a few hours of finishing wrecks the surface. Check the forecast the morning of, not the week before.
- Curing is not drying. Keep the slab damp or covered for the first 3–7 days. Concrete reaches roughly 70% of design strength at 7 days and its nominal 28-day strength at four weeks.
Rough timeline for a standard 3,500–4,000 PSI mix: forms off after 24–48 hours, foot traffic at 24–48 hours, vehicles at 7 days, full strength at 28 days.
Frequently asked questions
How many 80 lb bags of concrete are in a cubic yard? 45. A cubic yard is 27 ft³ and an 80 lb bag yields 0.60 ft³ (27 ÷ 0.60 = 45). It’s 60 bags of 60 lb, 72 of 50 lb, 90 of 40 lb, or 40 of 90 lb.
How much does a cubic yard of concrete weigh? Cured concrete runs about 140 lb/ft³, so a cubic yard weighs roughly 3,780 lb — nearly two tons. In bag terms that’s 45 × 80 lb bags, or 3,600 lb of dry mix you have to carry from the car before you add any water.
How thick should the slab be? 4 inches for patios, sheds, and walkways; 5–6 inches for driveways and anything holding vehicles. See concrete slab thickness for what each extra inch costs.
Can I add extra water to stretch the mix? No. Extra water increases volume and destroys strength — it’s the single most common way DIY slabs end up soft, dusty, and cracked. Buy another bag instead.
Is it cheaper to hire someone? A pro-installed basic 4-inch residential slab runs about $4–$8 per sq ft including labor — roughly $576–$1,152 for a 12 × 12. Once you price the base, forms, mesh, and tool rental, the DIY saving is smaller than it looks. We ran the comparison in DIY vs hiring a pro.
The short answer
Volume = L × W × (thickness ÷ 12), add 10%, then divide by 0.60 for 80 lb
bags or by 27 for cubic yards. Under about half a yard, buy bags. Around a
yard and up, order ready-mix — it’s roughly half the price per cubic foot and
saves you from mixing a ton of material against the clock.
Get the exact bag and cubic-yard count for your slab with the Concrete Calculator, or browse the other material guides.