Concrete Calculator — How Many Bags of Concrete Do I Need?
This concrete calculator estimates how much concrete you need for slabs, footings, and any rectangular poured-concrete element. Enter the dimensions and quantity of your elements, and the calculator will tell you the volume (in cubic yards or cubic meters), total weight, the number of bags to buy, and the optional material cost in your currency.
What Is Concrete?
Concrete is one of the most widely used construction materials in the world. It is a mixture of cement (typically Portland cement), water, fine aggregate (sand), and coarse aggregate (gravel or crushed stone). When water reacts with cement through a process called hydration, the paste hardens and binds the aggregates into a rock-like mass. After 28 days, concrete reaches approximately 99% of its design compressive strength — though the hardening process continues for years, making concrete stronger over time.
Why Use the Concrete Calculator?
Estimating concrete by eye is a common source of waste and frustration. You either run out mid-pour and wait for a second delivery, or you overbuy and store bags that harden in the shed. The concrete calculator removes the guesswork:
- Enter your slab or footing dimensions and the number of elements.
- Choose your unit system — Imperial (feet, pounds, cubic yards) or Metric (meters, kilograms, cubic meters).
- The calculator computes exact volume, weight, and bag count, including a customizable waste allowance (typically 5–10%).
- Add a price per bag to get an instant cost estimate in any of 20 world currencies, including USD and RUB.
How Much Concrete Do I Need?
The calculation follows three steps:
- Volume — Multiply the quantity of elements by length × width × depth.
Imperial: result in cubic feet, then divided by 27 to get cubic yards (the standard sales unit in the US).
Metric: result directly in cubic meters. - Weight — Multiply volume (in ft³ or m³) by the concrete density.
Standard density: 150 lb/ft³ (Imperial) or 2,400 kg/m³ (Metric). Check the bag for the exact value. - Bags — Divide the adjusted weight (after adding the waste factor) by the bag weight, then round up.
Common bag sizes: 60 lb or 80 lb (Imperial), 25 kg or 40 kg (Metric).
Example Calculation
Suppose you need 4 concrete slabs, each 3 ft × 3 ft × 0.33 ft (≈ 4 inches thick):
- Volume: 4 × 3 × 3 × 0.33 = 11.88 ft³ = 0.44 cu yd
- Weight: 11.88 × 150 = 1,782 lb
- With 5% waste: 1,782 × 1.05 = 1,871 lb
- Bags (60 lb each): ⌈1,871 ÷ 60⌉ = 32 bags
Concrete Slab Cost
Once you know the number of bags, multiply by the price per bag from your local supplier. The calculator supports 20 currencies:
- USD (US Dollar), RUB (Russian Ruble), EUR (Euro), GBP (British Pound)
- CAD, AUD, BRL, JPY, CNY, INR, MXN, CHF, PLN, TRY, SEK, NOK, DKK, ARS, KRW, CZK
The cost estimate covers material only (bags of ready-mix concrete). Labor, forms, rebar, and delivery are separate.
How to Make Concrete
Bagged ready-mix concrete already contains the right proportions of cement, sand, and gravel — just add water and mix:
- Pour the dry mix into a wheelbarrow or mixer.
- Add the amount of water specified on the bag (usually about 3 quarts per 80 lb bag).
- Mix until the concrete is smooth and uniformly wet, with no dry pockets.
- Pour immediately and consolidate with a screed or float.
- Cure for at least 7 days, keeping the surface moist.
Concrete vs. Cement
Cement is an ingredient; concrete is the finished product. Specifically, Portland cement is the binding agent that, when mixed with water, sand, and gravel, creates concrete. You cannot pour "cement" — the correct term is always "concrete." Portland cement is a generic material type, not a brand name.
Types of Concrete
- Normal-strength — 3,000–4,000 psi; standard residential slabs, walkways, footings.
- High-strength — 6,000+ psi; bridges, high-rise columns, industrial floors.
- Lightweight — Density ≈ 115 lb/ft³ (1,840 kg/m³); roofing, precast panels.
- Fiber-reinforced — Steel or polypropylene fibers added to reduce cracking.
- Self-compacting (SCC) — Highly fluid, no vibration needed; complex formwork.
- Shotcrete — Applied pneumatically; tunnels, swimming pools, retaining walls.
Frequently Asked Questions
How many bags of concrete do I need for a 10×10 slab?
For a 10 ft × 10 ft × 4 in (0.33 ft) slab: volume ≈ 33 ft³ ≈ 1.22 cu yd; weight ≈ 4,950 lb; with 5% waste ≈ 5,200 lb. Using 60 lb bags: ⌈5,200 ÷ 60⌉ = 87 bags. Using 80 lb bags: 65 bags.
Can I use cubic yards directly?
Pre-mixed concrete ordered by truck is sold by the cubic yard. The calculator shows cubic yards for Imperial inputs. For orders under 1 cu yd, bagged mix is usually more practical.
What waste factor should I use?
Professional contractors typically add 5–10% for spillage, uneven subgrade, and over-pouring. Use 5% for smooth, well-prepared surfaces and 10% for rough terrain or complex shapes.
Does concrete density vary?
Yes. Normal concrete is 140–160 lb/ft³ (2,240–2,560 kg/m³). Check the bag label for the exact value, or use the default 150 lb/ft³ (2,400 kg/m³) as a reliable estimate for standard ready-mix.
How long does concrete take to cure?
Concrete gains about 70% of its strength in the first 7 days and ~99% by 28 days. Do not apply heavy loads before 28 days. Keep the surface moist (cover with plastic or burlap) for at least the first week.