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Concrete calculator: volume, bags and mix

Units

1. Concrete volume

Describe each part you are pouring. Several parts add up: a slab, its footings and a few posts, for example.

    The allowance covers concrete you lose (uneven subgrade, forms that bow, leftovers in the mixer). Use 5% for tight forms and up to 10% on rough ground.

    Concrete needed, waste included —

    Diagram of the selected part

    Rough sketch, not to scale. Click inside a part to show it.

    2. Premixed concrete in bags

    Premixed bags already contain cement, sand and gravel: you only add the water printed on the bag.

    Typical value only: the volume one bag makes depends on the product. Check the yield printed on the bag and type it here.

    Number of bags (rounded up) —

      3. Mix your own: proportions

      Cement, sand, gravel and water for the volume above, waste included. Pick a use, then adjust the figures if needed.

      Common mix figures, for guidance only, per cubic yard or cubic meter of placed mix. They vary with the cement, how damp the sand is and the gravel size: follow the instructions on the cement bag.

      Quantities for the volume above
      MaterialQuantityBags or loose volume

        These figures are estimates for planning a job, not a structural design. For anything load-bearing (house foundations, a suspended slab, a retaining wall), follow local building codes and have the design checked by a qualified professional.

        Your parts and settings stay on this device and come back on your next visit. The copied link reopens the calculation with the same values.

        How much concrete do I need?

        This concrete calculator works out the volume for a slab or screed, a strip footing, a wall, a square or round column, a post hole and a set of solid steps. Add several parts together, include a waste allowance, then see how many bags of premixed concrete to buy, or how much cement, sand, gravel and water to order if you mix it yourself.

        How to use it

        1. Imperial units are selected by default: lengths in feet, thicknesses and diameters in inches. Switch to "Metric (m, cm)" if your plans are in meters.
        2. In "1. Concrete volume", pick a "Shape" and enter its measurements. "Quantity" repeats the part, handy for 12 identical post holes.
        3. Use "Add part" for footings, columns or steps: every part adds to the total.
        4. Set the "Waste allowance (%)": 5% by default, with 8% and 10% buttons.
        5. In "2. Premixed concrete in bags", choose the "Bag size" (40, 60 or 80 lb) and check the "Yield per bag" against the bag.
        6. In "3. Mix your own: proportions", choose a "Use" and a "Cement bag", and tick "Convert to wheelbarrows and buckets" if you batch by the bucket.

        The formulas

        • slab, footing or wall: length × width × thickness, all in feet;
        • round column: π × diameter² ÷ 4 × height;
        • post hole: the hole cylinder minus the post cross-section × depth;
        • solid steps: width × run × rise × n × (n + 1) ÷ 2, since step one is one rise tall, step two is two rises tall, and so on.

        A cubic yard holds 27 cubic feet. Example: a 10 × 10 ft patio slab, 4 in thick, is 10 × 10 × 4 ÷ 12 = 33.33 ft³, which is 1.235 yd³. With a 5% allowance you need 35 ft³, or 1.296 yd³.

        A fence post hole 10 in across and 36 in deep holds π × (10 ÷ 12)² ÷ 4 × 3 ≈ 1.636 ft³. With a 4 in square post standing in it, take away (4 ÷ 12)² × 3 ≈ 0.333 ft³: about 1.303 ft³ of concrete per hole, or roughly three 60 lb bags at 0.45 ft³ each.

        Why add a waste allowance?

        Subgrade is never perfectly flat, forms bow a little and some concrete stays in the mixer or wheelbarrow. Plan 5% for tidy formwork and up to 10% for hand-dug holes or rough ground.

        Bags or ready-mix truck?

        Premixed bags suit small jobs: add water and mix. The yield depends on the product, so read it on the bag. At 0.60 ft³ per 80 lb bag, the patio above needs 35 ÷ 0.6 = 58.3, so 59 bags, about 4,700 lb to lift. Beyond a cubic yard or so, a ready-mix truck is usually easier; ask the supplier about the minimum order and access for the truck.

        Typical mix proportions (guidance only)

        UseCement per yd³Cement per m³
        General concrete≈ 590 lb≈ 350 kg
        Lean concrete, blinding≈ 337 lb≈ 200 kg
        Cement screed≈ 506 lb≈ 300 kg
        Masonry mortar≈ 590 lb≈ 350 kg

        The tool fills in sand, gravel and water too, and every figure can be changed. For the 1.296 yd³ patio: 1.296 × 590 ≈ 765 lb of cement, 8.14 bags of 94 lb, so 9 bags.

        Common slab thicknesses

        As a rough guide, walkways and light shed bases are often 4 in, patios and garage floors 4 to 6 in, and a screed about 2 in. Soil, loads, reinforcement and local codes decide the real figure.

        Water and curing

        Extra water makes concrete easier to place but weaker and more prone to cracking, so add it gradually. Keep the surface damp for several days and protect it from sun, wind and frost. Concrete usually takes foot traffic after a few days and reaches its design strength at around 28 days.

        Safety

        Wet cement is caustic and can burn skin. Wear waterproof gloves, safety glasses, long sleeves and boots, and rinse any splashes with clean water straight away.

        FAQ

        How many 80 lb bags make a cubic yard?

        At 0.60 ft³ per bag, 27 ÷ 0.6 = 45 bags, before any waste allowance.

        Can I use this to size a foundation?

        No. It estimates how much to order. Footing size and rebar for a load-bearing structure must follow local codes and be checked by a professional.

        How do I calculate concrete steps?

        Choose "Solid steps" and enter the number of steps, the rise, the run and the width. The tool assumes solid steps sitting on the ground.

        Is my data sent anywhere?

        No. Everything is calculated in your browser and your values stay on your device.