Whether you are pouring a backyard patio, setting fence posts, or footing a new addition, every concrete project starts with the same question: how much do I actually need? Get it wrong on the low side and you risk a cold joint, the permanent weak seam that forms when a second batch meets concrete that has already started to set. Get it wrong on the high side and you have paid for material that ends up in a wheelbarrow you have to dispose of.
The good news is that concrete estimating comes down to one formula, a thickness chart, and a single decision: bags or a truck. This guide walks through all three, with worked numbers you can check against your own project.
The only formula you need
Concrete is sold by volume, measured in cubic yards. Every estimate, no matter the shape, reduces to length times width times thickness, in matching units, converted to cubic yards:
- Measure length and width in feet.
- Convert thickness from inches to feet by dividing by 12 (so 4 inches becomes 0.33 ft).
- Multiply the three numbers together to get cubic feet.
- Divide by 27 to get cubic yards — because a cubic yard is 3 ft x 3 ft x 3 ft, which is 27 cubic feet.
Take a 12 ft x 12 ft patio at 4 inches thick: 12 x 12 x 0.33 = about 48 cubic feet, and 48 ÷ 27 = roughly 1.78 cubic yards. Hold onto that number — we will turn it into an order later.
Step by step for any pour
- Break the shape into rectangles. An L-shaped patio is just two rectangles; measure each one separately and add the volumes at the end.
- Choose your thickness based on what the slab will carry (see the next section).
- Convert and total using the formula above, then add a waste margin before you buy.
How thick should it be?
Thickness controls both strength and cost. Going from 4 to 6 inches on that same 12 x 12 pad is 50% more concrete, so it pays to match the thickness to the job rather than over-building everything.
- Patios, walkways, sidewalks, shed floors: 4 inches.
- Driveways (passenger vehicles): 4 inches minimum; 5 to 6 inches for heavier trucks, RVs, or soft soil.
- Garage floors: 4 to 6 inches.
- Footings and foundations: 8 to 10 inches, following local code.
Under every slab, plan a compacted 2 to 4 inch gravel base. It is not concrete and it does not show up in your volume number, but it is the single biggest factor in whether the slab cracks. Sizing that base is the same area-times-depth math — the gravel calculator does it in yards or tons.
What strength (PSI) to order
PSI is the pounds per square inch the fully cured concrete can take. Higher is not automatically better — you are matching strength to the load and the climate.
- 2,500–3,000 PSI: sidewalks, patios, steps, and light residential flatwork.
- 3,000–4,000 PSI: driveways, garage floors, and standard footings and slabs.
- 4,000 PSI and up: RV pads, workshops, heavy traffic, and anything exposed to de-icing salt or repeated freeze-thaw. In those regions, also ask for an air-entrained mix, which builds in tiny air pockets that let the concrete shrug off freezing.
Bags or a ready-mix truck?
There are two ways to buy concrete: dry bags you mix on site, or wet ready-mix delivered by truck. The deciding factor is volume.
Bag yields are fixed by size:
- 80 lb bag → about 0.60 cu ft → 45 bags per cubic yard
- 60 lb bag → about 0.45 cu ft → 60 bags per cubic yard
- 40 lb bag → about 0.30 cu ft → 90 bags per cubic yard
As a rule of thumb, under about half a cubic yard, bags win — no delivery to schedule, no minimum order. Above one to two cubic yards, a truck is both cheaper and far less work; two cubic yards is roughly 90 eighty-pound bags to haul, open, and mix by hand. At roughly $5–$8 per 80 lb bag, bagged concrete works out to about $270–$300 per cubic yard of material, while ready-mix is often $125–$200 — though suppliers usually charge a short-load fee for anything under a full truck (around 10 cubic yards).
Odd shapes: circles, L-shapes, and footings
Real projects are rarely a clean rectangle. The trick is to break them down:
- L-shapes and T-shapes: split into rectangles, find each volume, and add them.
- Round pads and columns: area = 3.1416 x radius², where the radius is half the diameter, then multiply by the thickness. A round column or sonotube is just a cylinder.
- Post holes: each hole is a small cylinder; a common guideline is to bury one-third of the post's height, and the calculator's column mode handles the volume.
Always add a waste margin
Order 5% to 10% more than the bare calculation. Subgrade is never perfectly flat, forms bow a little, and some concrete is always lost to the wheelbarrow. Coming up short mid-pour is the worst outcome, so a small leftover is cheap insurance.
For our 12 x 12 patio: 1.78 cubic yards + 10% ≈ 1.96, so you would order 2 cubic yards.
A full worked example
A 12 ft x 12 ft patio, 4 inches thick, over a 3-inch compacted gravel base:
- Volume: 12 x 12 x 0.33 = ~48 cu ft → ~1.78 cubic yards
- With 10% waste: order 2 cubic yards of ready-mix at about 3,000 PSI
- If you mixed it from bags instead, that is about 88 eighty-pound bags — which is exactly why a small truck makes more sense at this size.
How long does concrete take to cure?
Pouring is the fast part — strength comes later, and rushing it is how slabs crack. At about 70°F, a standard residential pour follows a predictable timeline:
- 24–48 hours: safe for light foot traffic, once the surface is firm enough not to dent.
- 7 days: safe for passenger cars and light trucks — concrete reaches roughly 70% of its rated strength (the "70 in 7" rule).
- 28 days: full design strength. Heavy trucks, RVs, and concentrated point loads should wait the full 28 days.
The single most important thing you can do is keep the surface moist for the first 7 days — mist it, cover it with plastic, or spray on a curing compound. "Drying" and "curing" are not the same thing: concrete gains strength through a chemical reaction with water, not by drying out, so letting it dry too fast actually weakens it.
Pouring in cold or hot weather
Temperature changes the whole timeline.
- Cold (below 50°F): curing slows sharply, and below 40°F it nearly stops. The real danger is freezing in the first 24 hours, before the concrete reaches about 500 PSI — the water inside expands and can permanently cut strength by half or more. Use an air-entrained mix, insulating blankets or a heated enclosure, and consider a high-early (Type III) cement or an accelerator. Never pour onto frozen ground.
- Hot (above 85°F): concrete sets faster but can dry too quickly, causing surface (plastic-shrinkage) cracking. Pour in the early morning or evening, shade the slab, and keep it wet.
Common mistakes to avoid
- Forgetting to convert thickness to feet — using 4 instead of 0.33 inflates your order twelvefold.
- Skipping the waste margin and ordering exactly to the number.
- Buying the wrong strength — overpaying for 4,000 PSI on a garden path, or under-building a driveway.
- Ignoring the gravel base, then wondering why the slab cracked.
- Hand-mixing a volume that really wants a truck.
Once you have your yardage and a 10% cushion, you are ready to order with confidence. Measure twice, add the margin, and match the strength to the job — that is the whole game.