Concrete mix calculator
Enter quantities from a confirmed recipe and its known or measured finished yield. The calculator scales that recipe; it does not design a strength class.
Optional manufacturer reference: 256.5 kg cement, 0.67 m³ coarse sand and 0.67 m³ stone for approximately 1 m³. Specific to AfriSam in South Africa; not a universal recipe or a strength guarantee. Water is not specified. AfriSam manufacturer brochure (PDF, concrete table)
Leave blank if unknown. Use the recipe's specified water for the actual aggregate moisture; no automatic water-cement ratio is assumed.
Confirm the recipe, yield and suitability for your project. Bag surplus is unused cement, not extra cement to add to the mix.
How much concrete do I need?
Measure the concrete portion of the project using your specified dimensions. A rectangular slab, patio or walkway uses length × width × thickness. A straight footing uses its net length × width × height. A solid round column uses π × (diameter ÷ 2)² × height. Multiply by the number of identical elements, add the non-overlapping sections, then apply your chosen ordering allowance.
This calculator measures quantity. It does not choose slab thickness, footing depth, post embedment, reinforcement, concrete strength or a suitable mix. Enter dimensions from your project specification. The starting 10 × 10 ft and 4 in values are an arithmetic example, not a recommended design.
Slabs, patios, footings and multiple sections
Use one slab section for a constant thickness. Add separate sections for different dimensions or thicknesses. Divide an L-shaped area into rectangles without counting the shared area twice. The calculator adds the entered volumes; it does not detect overlaps in a site plan.
For a footing layout, measure net straight lengths. At corners and T-junctions, counting the full length of every overlapping rectangle can overestimate the volume. Choose a consistent measurement method and exclude the overlap from one section. A closed footing ring can be measured as the outer rectangular volume minus the empty inner volume and checked against your net sections.
Concrete around fence posts
A round hole is a cylinder. When a post occupies part of the concrete-filled space, subtract that part: hole volume − post cross-section area × embedded length. A round post uses π × (diameter ÷ 2)²; a square post uses side × side. Enter the actual post dimensions, not a nominal lumber size. A square post must fit inside the round hole including its corners.
Concrete fill depth excludes a gravel bed or unfilled space. Post length within concrete cannot exceed that fill depth. If your post stops above the bottom of the concrete, enter only its immersed length. Irregular excavations and soil conditions can change the quantity; there is no automatic excavation-loss factor.
How many bags of concrete do I need?
Use the finished mixed-volume yield per bag printed on the selected product. Bags needed = order volume ÷ mixed yield per bag, rounded up to a whole bag. A bag of cement is not a bag of premixed concrete: cement alone does not include the required aggregates, and its weight does not tell you the volume of finished concrete.
The optional named reference is QUIKRETE Concrete Mix 1101, 80 lb, with an approximate yield of 0.60 ft³ per bag in its manufacturer sheet. It is not a universal yield for every 80 lb product. Confirm your current label. Editing the yield changes the selection to a custom product. You can enter a yield in liters, cubic feet, cubic yards or cubic meters; changing its unit preserves the physical quantity.
WORKED EXAMPLE / ILLUSTRATIVE QUOTES
A 4 × 3 m slab with a specified 100 mm thickness
Measured concrete = 4 × 3 × 0.1 = 1.2 m³. A chosen 10% allowance gives 1.32 m³, about 1.726 yd³. If your hypothetical product yields 20 L (0.02 m³) per bag, buy 66 bags. At an illustrative $8 per bag plus $30 delivery, that alternative costs $558. A separate ready-mix quote at $150/m³ plus $40 delivery costs $238. These prices, thickness and yield are examples, not market rates or product recommendations.
For a post-hole check, a 0.4 m diameter hole with 1 m concrete fill depth has a volume of approximately 0.125664 m³. A measured 0.1 m square post occupying the full depth displaces 0.01 m³. Net concrete is approximately 0.115664 m³ per hole before allowance.
Ready-mix and bagged concrete cost
Ready-mix material cost uses the order volume and your price per cubic yard or cubic meter. Bagged material cost uses the rounded whole-bag quantity. Each alternative adds its own quoted delivery or other entered fees once. Compare the alternatives rather than adding them together. Supplier minimum orders, short-load fees, pump charges, waiting time, taxes and labor are not automatically inferred.
A blank price means an unknown quote. An explicit zero price still includes the entered fees. Switching currency changes the denomination without an exchange-rate conversion. Unit changes preserve dimensions, but changing a price basis does not convert the price: enter the supplier's rate for that basis.
Sources and limits
Packaging reference checked October 20, 2026: QUIKRETE Concrete Mix 1101 technical data sheet (PDF, revised October 2022). The manufacturer's quantity calculator also distinguishes slab, post-setting and other product uses. Product data support a quantity estimate, not the selection of a structural design. No brand affiliation is implied.
Conversions follow NIST conversion factors: 1 ft = 0.3048 m, 1 in = 0.0254 m, and 1 yd = 0.9144 m. No universal fresh-concrete density, mix ratio, water addition, curing time or compaction factor is assumed. Follow the selected product's current application instructions and your project specification.
Frequently asked questions
How do I calculate concrete in cubic yards?
Calculate the volume in cubic feet and divide by 27. For a slab measured in feet with thickness in inches, multiply length × width × thickness ÷ 12 ÷ 27, then add your chosen allowance.
Can I calculate more than one slab or footing?
Yes. Add up to 20 non-overlapping section groups. Use the number field for repeated identical sections. Different dimensions need separate groups.
How much does an 80 lb concrete bag cover?
Coverage depends on the exact product's finished yield and the specified thickness. The optional QUIKRETE 1101 reference is approximately 0.60 ft³ per 80 lb bag. Check your own product label instead of assuming this yield for all bags.
Is concrete bag weight the same as concrete yield?
No. Weight is the dry contents' mass; yield is the volume of finished concrete after correct mixing. Use mixed-volume yield to calculate bags.
Does a fence post reduce the concrete needed?
Yes. The post occupies space within the concrete. The post-hole mode subtracts the volume of the measured round or square post only over the length embedded in the concrete.
Does this calculate cement, sand and gravel proportions?
Cement, sand, gravel and optional water for mixing your own concrete. Enter quantities from a confirmed recipe and its known or measured finished yield. The calculator scales that recipe; it does not design a strength class.
Is the allowance a compaction factor?
No. It is an optional ordering allowance for the supplied dimensions. Bag rounding adds a separate package surplus. Neither sets design dimensions or predicts excavation conditions.
Should I add the ready-mix and bagged costs?
No. They are two alternative ways of buying the concrete. Each uses its own price and delivery fees. Select the option that matches your actual supplier quote.