# The Science of Concrete and Mortar Mix Ratios
Whether you are laying a garden slab, building a block wall, or pouring a floor screed, getting the concrete mix ratio right is the single most important factor for structural integrity. This calculator converts professional engineering dosages — expressed in kg per cubic metre — into the practical units you actually work with on site: 25kg bags, shovelfuls, and litres.Why Water Ratio Determines Concrete Strength
The water-to-cement ratio (w/c ratio) is the key variable controlling concrete strength. Every extra litre of water you add beyond the minimum makes the mix more workable — but permanently weakens the cured concrete by leaving voids as it evaporates.
The correct consistency resembles a stiff, cohesive paste, not a pourable liquid. If your mix slumps off a trowel too easily, you have added too much water.
# Standard Mix Proportions by Application
The three mixes in this calculator cover the most common DIY and trade applications. H-200 concrete (1:2.5:3 ratio) is suitable for foundations, slabs, and structural elements. M-40 masonry mortar (1:4 ratio) is ideal for laying bricks and blocks. M-80 screed mortar (1:3 ratio) gives a strong, dense finish for floor levelling and external rendering. Always choose the mix appropriate to the load the structure will bear.# Calculating Cubic Metres for Your Project
Worked Example for a Garden Slab
For a garden patio 4m long, 3m wide and 15cm deep: 4 × 3 × 0.15 = 1.80 m³. Enter these dimensions directly into the calculator and it handles the rest automatically.
Always order 10% more material than the calculated figure to account for spillage, uneven sub-base and the fact that wet aggregates contain less dry material by volume than you expect.
# Mixing Order and the Bag Dosage System
In professional construction, mixes are specified as "300kg concrete", meaning 300kg of cement per finished cubic metre. To achieve this with a mixer: add water first, then coarse aggregate (gravel and sand), and finally add cement gradually to prevent it balling on the drum walls. This sequence ensures even distribution throughout the mix.Always Buy 10% Extra to Cover Wastage
Material waste on a concrete job is not optional — it is unavoidable. Sub-base irregularities, form overfill, mixer residue and the compaction factor all consume material beyond the theoretical volume. A 10% surplus prevents the costly and time-critical problem of running short mid-pour with no way to restart the job.