Concrete Calculator: Ballast & Cement Mix

This concrete calculator works out how much ballast and cement you need for a given volume of concrete. You enter the slab length, width, and depth (or the total volume), choose your mix ratio, and it returns the required weight of cement and ballast, plus the number of standard bags. It uses the common 1:4 and 1:5 mixes and converts everything to bags and tonnes so you can order accurately.

Formula

The standard method for concrete mix design uses a dry volume factor of 1.54. This accounts for the fact that dry materials (cement, sand, and aggregate) occupy more volume than the wet concrete they produce—typically about 54% more. The formulas are:

• Wet concrete volume (V): V = L × W × D (in meters, with depth converted from cm to m)
• Dry volume (Dv): Dv = V × 1.54
• Cement volume: Cv = Dv / (ratio + 1) (where ratio is the parts of ballast per part of cement)
• Cement weight: Cw = Cv × 1440 kg/m³ (bulk density of cement)
• Ballast volume: Bv = Dv × ratio / (ratio + 1)
• Ballast weight: Bw = Bv × 1600 kg/m³ (bulk density of ballast)

Variables: L = length (m), W = width (m), D = depth (cm), ratio = parts of ballast per part of cement (e.g., 4 for a 1:4 mix). The result for cement is given in 25 kg bags (rounded up to the nearest whole bag), and ballast in tonnes.

Worked Example

Using the default values: length = 3 m, width = 2 m, depth = 10 cm, and a 1:4 mix ratio.

1. Volume: V = 3 × 2 × (10/100) = 3 × 2 × 0.1 = 0.6 m³
2. Dry volume: Dv = 0.6 × 1.54 = 0.924 m³
3. Cement volume: Cv = 0.924 / (4 + 1) = 0.924 / 5 = 0.1848 m³
4. Cement weight: Cw = 0.1848 × 1440 = 266.1 kg → 11 bags of 25 kg (rounded up)
5. Ballast volume: Bv = 0.924 × 4 / 5 = 0.7392 m³
6. Ballast weight: Bw = 0.7392 × 1600 = 1182.7 kg = 1.18 tonnes

The calculator gives the same: 0.60 m³, 11 bags of cement, and 1.18 t of ballast.

What Is Ballast and Why Does the Mix Ratio Matter?

Ballast is a pre-mixed aggregate of sand and gravel (or crushed stone) typically in a 2:1 ratio of coarse to fine. It’s the “skeleton� of concrete, providing strength and reducing shrinkage. The mix ratio (cement:ballast) determines the concrete’s compressive strength:

• 1:4 mix – produces roughly C20 concrete (20 N/mm²), suitable for general purpose, footings, and shed bases.
• 1:5 mix – about C15, used for paths, patios, and lightly loaded slabs.
• 1:6 mix – about C10, for non-structural work like bedding posts or temporary fill.

Using a weaker mix (more ballast) saves money but risks cracking under load. Stronger mixes (more cement) are more expensive and may be overkill for simple projects.

How to Use the Calculator

Measure the area where you’ll pour concrete in meters. If you have a rectangular slab, multiply length × width. For irregular shapes, break them into rectangles and add the volumes. Depth is usually in centimeters—divide by 100 to get meters. Choose your mix ratio based on the project (see above). The calculator then gives you:

• Concrete volume in cubic meters (m³).
• Cement bags (25 kg each) – always round up to avoid running short.
• Ballast weight in tonnes – most suppliers sell ballast in bulk bags (typically 0.8 m³ or ~1 t).

For a small job, you might buy ballast in 25 kg bags instead; there are about 64 bags per tonne.

Common Mistakes and How to Avoid Them

• Forgetting the dry volume factor: Many people simply multiply length × width × depth and then add cement and ballast by volume. That gives a mix that’s too dry and weak. Always use 1.54.
• Mixing up units: Depth in cm must be converted to meters. A 10 cm slab is 0.1 m, not 10 m.
• Using the wrong ratio: A 1:4 mix is not the same as a 1:5. Check the strength you need.
• Not accounting for waste: Order 5–10% extra to cover spillage, uneven ground, or miscalculation.
• Rounding down bags: Always round up the number of cement bags. You can’t buy a fraction of a bag.

Cost and Ordering Considerations

Prices vary by region and supplier, but ballast is generally sold by the tonne or in bulk bags (often 0.8 m³ ≈ 1.2 t). Cement is sold in 25 kg or 50 lb bags. To estimate cost, multiply the number of bags by the local price and the ballast tonnes by the per-tonne price. For a typical 1:4 mix, you’ll need about 11 bags of cement and 1.2 t of ballast per cubic meter of concrete. Buying in bulk (e.g., a full pallet of cement) often reduces cost per bag.

Safety and Practical Tips

• Wear gloves, safety glasses, and a dust mask when handling cement—it’s caustic.
• Mix concrete on a hard, level surface or in a mixer; never mix by hand for large volumes.
• Add water gradually—too much water weakens concrete. Aim for a workable but stiff consistency.
• Use a wheelbarrow and shovel; wet concrete is heavy (about 2.4 t per m³).
• Dispose of wash water properly—do not pour it down drains.

Frequently Asked Questions

How many 25 kg bags of cement are in a cubic meter of concrete?

For a 1:4 mix, you need about 11 bags (275 kg). For a 1:5 mix, about 9 bags (225 kg). The exact number depends on the dry volume factor and the density of cement.

What is the difference between ballast and aggregate?

Ballast is a specific pre-mixed combination of sand and gravel, typically in a 2:1 ratio. Aggregate is a broader term that includes any granular material—sand, gravel, crushed stone—used in concrete. You can mix your own aggregate, but using ready-mixed ballast is more convenient and consistent.

Can I use this calculator for a concrete slab that is not rectangular?

Yes, but you need to calculate the volume yourself first. Break the shape into rectangles, compute each volume, and add them together. Then enter the total volume as a single “length × width × depth� combination that gives that volume (e.g., length = total volume / (width × depth)).

How much water do I add?

Water is not included in the calculator because it depends on the moisture content of the ballast and the desired workability. A general guideline is about 0.5 parts water per part cement by weight, but adjust to achieve a consistency that holds its shape when squeezed.

What is the strongest mix I can make with ballast?

The strongest common mix is 1:3 (cement:ballast), which yields about C25 concrete. However, for very high strength, you’d use specific aggregates and additives, not standard ballast. For most DIY projects, 1:4 is sufficient.

Why does the calculator use a dry volume factor of 1.54?

Because dry materials have air voids that get filled with water during mixing. The wet concrete occupies less volume than the sum of dry ingredients. The 1.54 factor is a well-established average for typical mixes; it may vary slightly with aggregate shape and moisture.

Related

• Board & Batten Calculator: Measure & Estimate
• Board and Batten Calculator: Inches & Spacing
• Board and Batten Distance Calculator

in l | Length | m | 3 | 0
in w | Width | m | 2 | 0
in d | Depth | cm | 10 | 0
sel mix | Mix ratio | 1:4 (C20)=4, 1:5 (C15)=5, 1:6 (C10)=6
out vol | Concrete volume | m³ | l * w * (d / 100) | 2
out cem | Cement needed | 25kg bags | (vol * 1.54) / (mix + 1) * 1440 / 25 | 1
out bal | Ballast needed | tonnes | (vol * 1.54) * mix / (mix + 1) * 1.6 | 2