Concrete Calculator FAQs
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Multiply your slab’s length × width × depth, all measured in feet, then divide by 27 to convert cubic feet into cubic yards. For example, a 10×10 slab at 4 inches (0.333 ft) deep = 10 × 10 × 0.333 = 33.3 cubic feet ÷ 27 = 1.23 cubic yards. Always order 10% extra to account for spillage, uneven sub-base, and finishing waste.

Measure all three dimensions in metres, then multiply: length (m) × width (m) × depth (m) = volume in m³. A slab 3m × 4m × 0.1m deep = 1.2 m³. One cubic metre equals 1.308 cubic yards or 35.31 cubic feet. Remember that 4 inches converts to 0.1016 metres — a common depth for residential concrete work.

Use the formula: Volume (m³) = L × W × D with all dimensions in metres. For circular slabs, use π × r² × depth. Break irregular shapes into rectangles and add sub-totals together. Always add 5–10% to your calculated figure to cover wastage. Convert inches to metres by dividing by 39.37 (e.g., 6 inches = 0.152 m).

Measure the pour area in metres and apply: m³ = length × width × depth. Example: 5m long, 3m wide, 100mm (0.1m) deep = 5 × 3 × 0.1 = 1.5 m³. Add waste buffer: 1.5 × 1.10 = 1.65 m³ to order. For odd shapes, divide the area into simple rectangles, calculate each, and sum the results.

In a standard M20 mix (1:1.5:3 — cement:sand:aggregate), cement accounts for roughly 1/5.5 of the total dry volume. For 1 m³ of finished concrete, you need approximately 8 bags of 50kg cement. Multiply your total concrete volume by 8 to estimate cement bag requirements. Weaker mixes like M15 use around 6.5 bags per m³; higher-strength M25 mixes use 9–10 bags per m³.

A 20kg premixed concrete bag yields approximately 0.009–0.01 m³ (9–10 litres) of mixed concrete. At 100mm (4-inch) depth, that covers roughly 0.09 m² — about a 30×30cm square. At a shallower 50mm depth, one bag covers 0.18 m². For large pours, bagged concrete becomes impractical quickly — a 10 m² slab at 100mm depth needs about 110 bags.

One cubic metre (1 m³) of standard structural concrete weighs approximately 2,300–2,400 kg (roughly 2.4 metric tonnes). It contains around 300–350 kg of cement, 650–700 kg of sand, 1,200 kg of coarse aggregate, and 150–180 litres of water. In imperial terms, 1 m³ equals 1.308 cubic yards or 35.31 cubic feet.

One 20kg bag produces approximately 0.009–0.01 m³ of finished concrete. This means you need 100–110 bags of 20kg premix to produce 1 cubic metre of concrete. At $8–$12 per bag, that makes bagged concrete significantly more expensive per m³ than ordering ready-mix ($120–$180/m³ delivered) for any pour over about 0.5 m³.

Divide your total volume (in m³) by 0.009 (the yield per 20kg bag). Example: a 1m × 2m path at 75mm deep = 0.15 m³ ÷ 0.009 = about 17 bags. For anything over 0.5 m³, ready-mix is more economical and saves significant labor. Always buy 1–2 extra bags as insurance against short mixes or spills.

A 25kg bag of cement (not premixed concrete) is a binder ingredient. Mixed in a standard 1:2:3 ratio with sand and aggregate, one 25kg cement bag produces approximately 0.018–0.02 m³ of finished concrete, covering about 0.18 m² at 100mm thickness. To get the equivalent of one 20kg premix bag output, you’d still need sand and aggregate totaling about 45–50 kg.

Coverage per 20kg bag by depth: at 25mm = 0.36 m²; at 50mm = 0.18 m²; at 75mm = 0.12 m²; at 100mm = 0.09 m². To find how many bags you need: divide your total surface area (m²) by the coverage at your chosen depth. For example, 5 m² at 100mm depth: 5 ÷ 0.09 = 56 bags.

At 100mm (4-inch) depth: one 20kg bag covers 0.09 m² — roughly the size of a 30×30cm tile. At 50mm depth, coverage doubles to 0.18 m². One standard contractor wheelbarrow (approximately 65 litres capacity) requires about 7 bags of 20kg premix to fill. For small jobs under a square metre, bagged premix is by far the most convenient option.

One cubic yard = 27 cubic feet. A large contractor wheelbarrow holds about 5–6 cubic feet — so one yard fills approximately 5–6 large wheelbarrow loads. Mid-size 3.5 cu ft barrows require 7–8 trips; standard homeowner 2 cu ft barrows need 13–14 trips. Keep in mind concrete sets in 30–60 minutes, so wheelbarrow transport works only for small local pours.

Using a 6 cu ft contractor barrow: about 4–5 loads. Using a 3.5 cu ft mid-size barrow: 7–8 loads. Using a 2 cu ft homeowner barrow: 13–14 loads. Each load requires a helper to shovel and you to wheel — factor this into your team size. With concrete setting in under an hour, having multiple barrows working in parallel is strongly recommended.

For a 10×10 foot (100 sq ft) slab: at 3.5 inches = 1.07 cubic yards; at 4 inches = 1.23 cubic yards; at 6 inches = 1.85 cubic yards. Most residential jobs order 1.5 yards to comfortably cover a 4-inch pour with the required 10% waste buffer. In 80-pound bags, a 4-inch 10×10 slab needs approximately 34–36 bags.

Exact calculation: 10 × 10 × (4÷12) = 33.33 cubic feet ÷ 27 = 1.23 cubic yards. Add 10% waste: 1.35 cubic yards. Order 1.5 yards from a ready-mix supplier (the standard minimum truck delivery). In bagged concrete: approximately 45 sixty-pound bags, or 34 eighty-pound bags. Wire mesh or rebar is recommended for this size slab.

A 4×4 slab at 4 inches deep = 4 × 4 × 0.333 = 5.33 cubic feet. Using 60-pound bags (yield ≈ 0.45 cu ft): 12 bags needed. Using 80-pound bags (yield ≈ 0.60 cu ft): 9 bags needed. Always round up by at least one bag. This is a manageable DIY pour — mix in a small drum mixer or large tub, working in batches.

A 12×12 foot slab (144 sq ft) at 4 inches thick requires about 1.8 cubic yards. Concrete material alone: $150–$250. Professional installation — site prep, forming, pouring, screeding, and finishing — typically totals $700–$1,500 depending on region, access, and finish type. Decorative finishes like stamped or stained concrete add $2–$8 per square foot.

A 10×12 foot slab (120 sq ft) at 4 inches deep requires approximately 1.5 cubic yards of concrete. Materials run $120–$200. Full professional installation — excavation, gravel base, forming, pouring, and broom finishing — typically runs $600–$1,200. If you handle your own labor, materials including forms and gravel base bring the cost to $300–$450.

A 30×30 foot slab (900 sq ft): at 4 inches thick = 11.1 cubic yards (order 12.2); at 6 inches thick = 16.7 cubic yards (order 18.3). This volume requires a ready-mix truck delivery — bagged concrete is not practical here. Plan for 2–3 workers to spread and screed efficiently during the pour window.

A 30×30 foot slab (900 sq ft) at 4 inches thick requires about 11.1 cubic yards of concrete. Material costs: $900–$1,500. Professional installed total — including grading, compacted gravel base, rebar, forming, pouring, and finishing — typically runs $3,600–$9,000. Regional labor rates and site accessibility are the biggest cost variables.

A 20×20 foot slab (400 sq ft): at 4 inches = 4.94 cubic yards; at 5 inches = 6.17 cubic yards; at 6 inches = 7.41 cubic yards. Include 10% waste: order approximately 5.4 yards at 4 inches. Most suppliers round to the nearest half-yard, so order 5.5 yards to be safe. Wire mesh reinforcement is highly recommended for a slab this size.

A 20×20 slab (400 sq ft) at 4 inches needs roughly 5 cubic yards. Concrete materials: $400–$700. Professional installation including site prep, forming, pouring, and finishing: $1,600–$4,000. If you want exposed aggregate, broom finish, or sealing, budget an additional $400–$1,200. DIY material-only cost runs $800–$1,200 including gravel, forms, and mesh.

A 30×40 foot slab (1,200 sq ft) at 4 inches requires approximately 14.8 cubic yards. Concrete materials: $1,200–$2,000. Full installation costs with labor typically run $4,800–$12,000. This is a significant pour — rebar grid reinforcement and control joints are essential. Factor in pump truck rental ($500–$800) if the site has limited ready-mix truck access.

A 30×40 foot slab (1,200 sq ft): at 4 inches = 14.8 cubic yards + 10% = 16.3 yards to order; at 6 inches = 22.2 cubic yards + 10% = 24.4 yards. Schedule a ready-mix truck — likely 2 loads for the 4-inch pour, 3 loads for 6-inch. Coordinate workers and pump truck in advance for efficient placement.

A 12×12 foot slab (144 sq ft) at 4 inches = 12 × 12 × 0.333 = 47.95 cu ft ÷ 27 = 1.78 cubic yards. Order 2 cubic yards to cover waste. In bagged concrete: about 80 sixty-pound bags or 60 eighty-pound bags. At this quantity, renting a half-yard electric mixer or ordering a mini-mix delivery is more efficient than hand-mixing bags individually.

For slabs under 1 cubic yard, bagged concrete mixed yourself is cheapest. For larger pours, ready-mix becomes more economical once you exceed about 30 bags. To minimize total cost: do your own excavation and forming, use basic broom or float finish instead of decorative options, skip the concrete pump if the truck can access your site directly, and pour during shoulder seasons when contractor rates are lower.

Ten cubic yards at 4 inches thick covers 810 square feet (10 × 27 ÷ 0.333 = 811 sq ft). In practical terms, that’s a 27×30 foot area — a large two-car driveway, a substantial patio, or a garage floor. This is a single-truck ready-mix order for most suppliers (typical trucks carry 8–10 yards).

One cubic yard covers these areas depending on depth: at 3.5 inches = 92 sq ft; at 4 inches = 81 sq ft; at 5 inches = 65 sq ft; at 6 inches = 54 sq ft. The 81 sq ft coverage at 4 inches is the most useful benchmark — roughly a 9×9 foot area per yard ordered.

At exactly 4 inches (0.333 ft) thick, 1 cubic yard covers 81 square feet. This is the single most useful number in concrete estimating — multiply it by the number of yards you plan to order to quickly estimate your total coverage area. Divide your total square footage by 81 to find how many yards you need at 4-inch depth.

Two cubic yards at 4 inches thick covers 162 square feet — roughly a 10×16 foot area, a compact patio, or a shed foundation. Coverage at other depths: at 3.5 inches = 185 sq ft; at 5 inches = 130 sq ft; at 6 inches = 108 sq ft. Two yards is a common order size for small residential pours that are too large for bags but below minimum truck delivery thresholds.

Twenty cubic yards covers: at 4 inches = 1,620 sq ft; at 5 inches = 1,296 sq ft; at 6 inches = 1,080 sq ft. At 4 inches, 20 yards is enough for a 30×54 foot area — a large three-car garage floor, a full parking court, or an expansive patio. This requires 2 ready-mix trucks and a dedicated crew for efficient placement.

One cubic metre of concrete covers: at 50mm = 20 m²; at 75mm = 13.3 m²; at 100mm (4 inches) = 10 m²; at 150mm (6 inches) = 6.7 m². The quick formula: 1 ÷ depth in metres = coverage in m². Use this to rapidly estimate volume for any area once you’ve settled on a pour depth.

Two cubic yards covers: at 3.5 inches = 185 sq ft; at 4 inches = 162 sq ft; at 5 inches = 130 sq ft; at 6 inches = 108 sq ft. At 4-inch depth, this is comfortably enough for a 10×16 foot patio, a standard single-car parking pad, or a small workshop floor. For most suburban residential projects, 2–3 yards covers the typical pour.

Thickness is project-specific: Walkways = 3.5–4 inches; Patios = 4 inches; Residential driveways = 4 inches; Heavy-load driveways = 5–6 inches; Garage floors = 4–6 inches; Footings and foundations = 6–12+ inches (engineer-specified). Cold climates experiencing freeze-thaw cycles generally require an additional half-inch to one inch beyond these minimums. Always check local building codes before pouring.

Standard thickness recommendations: sidewalks and paths = 3.5 inches; residential patios and driveways = 4 inches; heavy-use driveways = 5–6 inches with rebar; garage floors = 4–6 inches; structural footings = 6–12 inches minimum. Always verify with local building codes. A thicker slab with rebar costs more upfront but dramatically reduces cracking and extends the concrete’s service life by decades.

Standard residential driveways are poured at 4 inches thick, which is code-compliant in most jurisdictions and adequate for passenger vehicles. In cold-climate areas with significant frost heave, 5 inches is the recommended minimum. For driveways that regularly bear heavy trucks, trailers, or RVs, pour at 6 inches with rebar or welded wire mesh for maximum durability.

Choose your depth based on use: foot traffic only = 3.5–4 inches; light vehicles = 4 inches; heavy vehicles = 5–6 inches; structural = 6+ inches. If your soil is soft, expansive clay, or poorly compacted fill, increase the slab thickness by 1–2 inches and add rebar. A stable, compacted gravel base of 4–6 inches is equally important for preventing cracking.

Yes — 4 inches meets the standard for residential driveways supporting passenger cars, SUVs, and standard pickup trucks. It becomes inadequate for repeated heavy loads like concrete mixer trucks, dump trucks, or heavy construction equipment. For mixed-use driveways that occasionally see heavy vehicles, 5 inches strikes a good balance between cost and longevity.

Yes — 4 inches is the standard and sufficient for residential patios supporting foot traffic, patio furniture, grills, and light appliances. If you plan to park a vehicle, position a heavy hot tub, or install a permanently anchored structure, increase to 5–6 inches with wire mesh or rebar reinforcement. Control joints at 8–10 foot intervals prevent cracking at any thickness.

Two inches is viable only as a resurfacing layer poured over an existing, structurally sound concrete slab. As a new stand-alone pour, 2-inch concrete lacks the mass to resist cracking from freeze-thaw cycles, ground movement, and normal loading. The absolute minimum for new concrete work is 3.5 inches for lightly loaded walkways, and 4 inches for anything structural.

For a typical garden or storage shed, pour the slab at 3.5–4 inches thick. This handles the weight of the shed structure plus typical contents including lawnmowers, hand tools, bicycles, and shelving. For sheds storing vehicles, ATVs, or heavy equipment like generators or compressors, pour at 5–6 inches with wire mesh reinforcement.

Standard residential concrete is mixed to achieve 3,000–4,000 PSI compressive strength at 28-day full cure. At this strength, 4-inch concrete comfortably supports the weight of passenger vehicles and all standard residential applications. Unreinforced 4-inch slabs can handle roughly 3,000 lbs per square foot in direct compression — significantly more than any typical residential load scenario.

At 3,000–4,000 PSI compressive strength, 4-inch concrete can bear approximately 3,000–4,000 pounds per square foot in pure compression. Practically, passenger cars (2,000–5,500 lbs total) spread their weight across four contact patches — each patch bearing only a few hundred pounds per square foot. Adding rebar or welded wire mesh substantially improves resistance to flexural (bending) loads, which is the primary failure mode in flat slabs.