How Much Ballast and Cement Do I Need?
Enter project dimensions, a known concrete volume, or ballast already on hand. The calculator finds finished concrete volume, applies the selected waste allowance, and scales the published cement and ballast quantities for the chosen ratio.
Ballast is an all-in aggregate containing sand and stone. Cement must be added before mixing with water. The live result shows material mass separately from the whole bags you need to purchase.
Common Cement-to-Ballast Ratios
The presets use 1:6 for foundations, blinding and fence posts, 1:5 for shed or garage bases and domestic floors, and 1:4 for paths, drives and steps. Ratios are measured by loose volume. They are practical estimating labels, not promises that a nominal mix will achieve a specific concrete class.
Reference quantities follow The Concrete Society's batching-by-volume guidance: per 1 m³, 1:6 uses 125 kg cement and 1,300 kg ballast; 1:5 uses 150 kg and 1,300 kg; and 1:4 uses 200 kg and 1,200 kg. Custom ratios from 1:4 to 1:6 are interpolated between these reference points.
Bulk Bags, Handy Bags and Cement Bags
UK merchant bag names do not guarantee one universal packed weight. The calculator defaults to an 800 kg bulk bag and 25 kg cement and handy bags, while allowing common alternatives. Use the packed weight stated by your supplier before ordering.
10 mm vs 20 mm Ballast
This estimate assumes 20 mm ballast for general domestic concrete. 10 mm ballast is also sold and may suit thinner sections or tighter placement. The calculator does not silently change density for aggregate size; follow the project specification for structural or specified work.
Ballast vs MOT Type 1
Ballast is an aggregate blend intended for concrete when combined with cement and water. MOT Type 1 is a compacted sub-base material and is not a substitute for concrete ballast.
Worked Shed Base Example
A 3 m by 2.4 m shed base at 100 mm deep has a net volume of 0.72 m³. With 10% waste, estimate 0.79 m³. At 1:5, that is about 119 kg of cement and 1.03 tonnes of ballast: buy five 25 kg cement bags and two 800 kg bulk bags.
12 × 12 ft Slab Example
A 12 ft by 12 ft slab at 4 in thick with 10% waste needs about 1.50 m³ of finished concrete. Using 1:5 and the default packs gives approximately 224 kg of cement, 1.94 tonnes of ballast, nine 25 kg cement bags and three 800 kg bulk bags.
Accuracy and Safe Mixing
The material quantities are scaled from the Concrete Society batching-by-volume guidance, while water and waste remain estimating inputs. Moisture, grading, compaction and site conditions affect actual yield. Add water gradually, follow cement and admixture instructions, and do not use household detergent as an improvised admixture. Seek professional advice for structural, regulated or safety-critical work.
Frequently Asked Questions
How do I calculate how much ballast and cement I need?
Find the finished concrete volume and add waste. The calculator then scales the Concrete Society published quantities per cubic metre for the selected 1:4, 1:5 or 1:6 site mix before rounding purchase quantities.
How many 25 kg cement bags do I need for 1 m³ of concrete?
The Concrete Society table estimates 5 bags for a 1:6 mix, 6 bags for a 1:5 mix, or 8 bags for a 1:4 mix. The ratio is estimating guidance, not a guaranteed structural grade.
How many bags of cement do I need for one bulk bag of ballast?
Scaling the Concrete Society quantities, an 800 kg bulk bag requires about 77 kg of cement at 1:6 or 1:5, and about 133 kg at 1:4, before rounding to the selected cement bag size.
What area will one tonne of ballast cover?
Coverage depends on the cement-to-ballast ratio and finished depth. Enter one tonne in Ballast on hand and add the target depth to calculate approximate finished-concrete coverage.
Is 10 mm or 20 mm ballast better for concrete?
20 mm ballast is the default estimating assumption for general domestic concrete. 10 mm may suit thinner sections or tighter placement, but specified or structural work should follow the project specification.