Concrete Calculator
GroundworkMeasurements in, purchasing amount out
How the Concrete Calculator works
The calculator finds volume first, then applies the selected material’s estimated bulk-density range to show weight. Your suggested order adds 10% for settling, uneven ground, and small measurement errors.
Using your measurements
12 ft × 8 ft × (4 in ÷ 12) ÷ 27 = 1.19 cu yd
Density assumption: Normal-weight concrete · 3,780–4,050 lb/yd³
The full breakdown
How much concrete a slab, footing, or post actually takes
The concrete calculator above turns slab or footing dimensions into cubic yards, cubic meters (m³), and bag counts in one pass. Ready-mix is ordered by the cubic yard or cubic meter; smaller jobs use pre-mixed 40, 50, 60, or 80 lb bags. For round piers and deck footings, the sonotube calculator adds belled bases, pads, and rebar; for fence posts, the fence calculator works out concrete per hole alongside the posts and rails.
The formula is length (ft) × width (ft) × depth (in ÷ 12) ÷ 27 = cubic yards, or length × width × thickness in metres for cubic meters. Real job sites add volume the plan does not show: an uneven subgrade, forms that bow under wet concrete, spillage, and footings dug deeper than drawn. Plan on about 10% extra — 5–10% on a large slab, 15% on deep piers or rough trench footings — so you never run short mid-pour; a cold joint in fresh concrete weakens the finished work.
Mix grade is a separate decision from volume. Nominal mixes are written as cement : sand : coarse aggregate by volume, from a lean M10 (1:3:6) through M20 (1:1.5:3, about 20 MPa or 2,900 psi) to M25 (1:1:2, about 25 MPa or 3,600 psi); the FAQ below lists the common grades. This calculator gives the volume and bag count for whatever grade you order — it does not split that volume into cement, sand, and stone. Crushed concrete is a recycled base gravel rather than concrete, and the paver base calculator carries its weight. For a driveway priced the other way, the asphalt calculator works in tons of compacted mix rather than cubic yards of pour.
Concrete type and density variations
Standard normal-weight concrete weighs roughly 140–150 lb/ft³ (approx 3,780–4,050 lb per cubic yard, or 2,400 kg/m³). Structural lightweight concrete uses shale, clay, or slate aggregates to reduce dead load (90–115 lb/ft³), while fiber-reinforced mix adds synthetic micro-fibers to arrest early shrinkage cracking without altering overall density.
| Concrete | Weight |
|---|---|
| Structural lightweight | 2,400–3,100 lb/yd³ |
| Fiber-reinforced mix | 3,780–4,050 lb/yd³ |
Recommended concrete thickness by project type
Thickness determines structural load-bearing capacity, resistance to cracking, and reinforcement depth. Always verify local municipal building codes and frost line depths before excavating.
| Walkways, sidewalks & residential paths | 4 in (10 cm) |
|---|---|
| Patios, sheds & hot tub pads | 4–5 in (10–12.5 cm) |
| Driveways & light vehicle parking | 5–6 in (12.5–15 cm) |
| Heavy trucks, RV pads & commercial slabs | 6–8 in (15–20 cm) |
| Sonotube footings & deck piers | Full depth below frost line (typically 30–48 in) |
Ready-mix delivery vs. pre-mixed bags & costs
A cubic yard equals 27 cubic feet. A common misunderstanding with bag concrete calculators is bag yield: an 80 lb bag does not yield 80 lb of volume; it yields approximately 0.60 cubic feet of wet concrete once mixed with water. It takes 45 bags of 80 lb mix, 60 bags of 60 lb mix, or 90 bags of 40 lb mix to equal one cubic yard. Ready-mix concrete costs roughly $125–$175 per yard delivered (plus short-load fees under 4–5 yards), making ready-mix significantly cheaper and faster once a project exceeds 1 to 1.5 cubic yards.
- 45 × 80 lb bags ≈ 1 cubic yard
- 60 × 60 lb bags ≈ 1 cubic yard
- 72 × 50 lb bags ≈ 1 cubic yard
- 90 × 40 lb bags ≈ 1 cubic yard
A standard wooden pallet holds 42 to 48 bags of 80 lb concrete mix (~3,360–3,840 lb).
Common concrete mistakes to avoid
Adding too much water to the mix
Excess water makes wet concrete easier to trowel and place, but drastically raises the water-cement ratio. This weakens compressive strength, causes excessive bleed water, leads to surface dusting, and promotes shrinkage cracks.
Pouring on uncompacted or dry subgrade
A soft subgrade causes slab settling and uneven stress cracks. Pouring concrete directly onto dry, thirsty soil sucks moisture out of the bottom of the fresh slab before it can hydrate, causing severe curling and bottom-up cracking.
Frequently Asked Questions
Concrete calculations, mixes & grades
Why add 10% to the calculated amount?
Extra material covers small dips, settling, spillage, and measurements that are not perfectly square. The calculated amount remains visible so you can compare both figures.
Are bag counts based on the same amount?
Both are shown. “Calculated” bags cover the measured volume; “Order” bags include the 10% allowance and round partial bags up to whole bags.
How accurate is the weight estimate?
Volume comes directly from your dimensions. Weight is a range because moisture, particle size, composition, and compaction change bulk density.
What is concrete made of?
Concrete is a composite material made of Portland cement (10–15%), fine aggregate (sand, 25–30%), coarse aggregate (gravel or crushed stone, 40–45%), and clean water (15–20%), plus optional chemical admixtures. When mixed, water chemically hydrates the cement to form a hardened crystalline binder locking the aggregates into artificial stone.
How much is a yard of concrete?
Delivered ready-mix typically costs $125 to $175 per cubic yard in the US, depending on the supplier, the strength (3,000 vs. 4,000 psi), and the delivery distance; orders under 4 to 5 yards usually add a $50 to $150 short-load fee. Mixing bags yourself works out to $225 to $360 per yard at $5 to $8 per 80 lb bag. A professionally poured and finished slab runs about $6 to $12 per square foot, or $450 to $900 per yard installed.
How much concrete do I need?
Multiply length (ft) × width (ft) × depth (in ÷ 12) ÷ 27 to get cubic yards. For round footings and sonotubes, use π × radius² (ft) × depth (ft) ÷ 27, or the sonotube calculator, which also adds the footing under the tube. Always add about 10% to cover uneven excavation, form bowing, and waste.
How many bags of concrete do I need?
Divide total project volume in cubic feet by the bag yield: an 80 lb bag yields 0.60 ft³, a 60 lb bag yields 0.45 ft³, a 50 lb bag yields 0.375 ft³, and a 40 lb bag yields 0.30 ft³. For 1 cubic yard (27 ft³), you need 45 bags of 80 lb, 60 bags of 60 lb, 72 bags of 50 lb, or 90 bags of 40 lb.
How long does concrete take to dry?
Concrete sets within 24 to 48 hours for light foot traffic. It reaches about 70% of its compressive strength in 7 days, and reaches full 28-day cure strength. Concrete does not simply dry — it cures through chemical hydration, requiring moisture retention during the first week.
How to pour a concrete slab?
Excavate and compact a 4-inch gravel base, build rigid wood forms with a 1/4-inch per foot slope for drainage, place vapor barrier and rebar or wire mesh, pour concrete evenly, screed flat with a straightedge, float with a bull float, tool edges and control joints, apply a broom finish, and wet cure for at least 7 days.
How to make concrete?
Mix concrete by volume using the standard 1:2:3 rule — 1 part Portland cement, 2 parts clean concrete sand, and 3 parts gravel or crushed stone. Gradually add 0.4 to 0.5 parts clean water until you reach a workable, plastic consistency without making it watery.
How many bags of concrete in a yard?
One cubic yard (27 cu ft) requires 45 bags of 80 lb mix (0.60 cu ft yield), 60 bags of 60 lb mix (0.45 cu ft yield), 72 bags of 50 lb mix (0.375 cu ft yield), or 90 bags of 40 lb mix (0.30 cu ft yield).
How much does a yard of concrete weigh?
A cubic yard of standard normal-weight concrete weighs approximately 4,000 pounds (2 short tons, or ~2,400 kg/m³). Lightweight concrete weighs 2,400 to 3,100 lb/yd³, while heavyweight aggregate concrete can exceed 5,000 lb/yd³.
What is m5, m10, m15, M20, M25?
These designate standard concrete mix grades where "M" stands for Mix and the number represents the characteristic compressive strength in Megapascals (MPa or N/mm²) at 28 days: M5 (5 MPa / 725 psi, lean concrete), M10 (10 MPa / 1450 psi, leveling beds), M15 (15 MPa / 2175 psi, mass concrete & paths), M20 (20 MPa / 2900 psi, standard reinforced slabs & beams), and M25 (25 MPa / 3625 psi, heavy structural columns & footings).
What is type 3 concrete?
Type 3 (Type III) concrete is made with high-early-strength Portland cement (ASTM C150 Type III). It is ground finer to accelerate hydration, reaching standard 7-day or 28-day design strength in just 1 to 3 days. It is widely used in precast manufacturing, fast-track road repairs, and cold-weather construction.
What is the M30 ratio for concrete?
The nominal mix ratio for M30 grade concrete is approximately 1 : 0.75 : 1.5 (1 part cement : 0.75 parts sand : 1.5 parts coarse aggregate) with a water-cement ratio around 0.40, though in practice M30 is designed as a laboratory mix design per ACI / IS codes to guarantee 30 MPa (4,350 psi) compressive strength.
Which is better, M30 or M25?
Neither is universally better; the right choice depends on the structural application. M30 provides higher compressive strength (30 MPa / 4,350 psi) and durability for commercial slabs, water-retaining structures, and severe weather exposure. M25 (25 MPa / 3,625 psi) is more cost-effective and standard for residential foundations, slabs, and columns.
What are the 7 grades of concrete mix ratio?
The seven standard nominal concrete mix grades and their Cement : Sand : Aggregate (C:S:A) volume ratios are: 1) M5 (1:5:10), 2) M7.5 (1:4:8), 3) M10 (1:3:6), 4) M15 (1:2:4), 5) M20 (1:1.5:3), 6) M25 (1:1:2), and 7) M30 (Mix design, ~1:0.75:1.5).
What is type 1, 2, 3, and 4 cement?
These are ASTM C150 standard Portland cement types: Type I is general-purpose cement for everyday construction; Type II offers moderate sulfate resistance and moderate heat of hydration for drainage and piers; Type III provides high early strength for fast construction and cold weather; Type IV generates very low heat of hydration for massive pours like dams to prevent thermal cracking.
What is PPC and OPC cement?
OPC (Ordinary Portland Cement) is manufactured from clinker and gypsum; it sets quickly and achieves high early strength, making it ideal for structural framing and fast formwork removal. PPC (Portland Pozzolana Cement) blends OPC with pozzolanic materials like fly ash; it hydrates slower, produces lower heat, resists chemical/sulfate attacks, and offers greater long-term durability for marine and foundation work.