Home & Construction⏱ 5 min read
Concrete Mix Ratios: What They Mean and How to Get Them Right
The wrong concrete mix is one of the most expensive DIY mistakes. Here's how the cement:sand:aggregate ratio system works, which mix to use for which job, and how much to order.
Concrete mix ratios describe the proportions of cement, sand, and aggregate (gravel) by volume or weight. Getting the ratio wrong produces concrete that's either too weak to do its job or unnecessarily expensive.
How the Ratio System Works
Mix ratio = Cement : Sand : Aggregate (by volume)
1:2:4 means:
1 part cement + 2 parts sand + 4 parts aggregate
Total: 7 parts
Water is added separately — typically water:cement ratio of 0.4–0.6
Less water = stronger concrete (but harder to work with)
Standard Mix Designations
Mix (C:S:A)Strength ClassApplications
1:3:6C10 / ST2Blinding, mass fill, non-structural
1:2:4C15 / ST3Paths, steps, general bases
1:1.5:3C25 / ST5Driveways, foundations, garage floors
1:1:2C35+Structural, reinforced concrete
Mortar 1:3–5 (C:S only)M4–M6Bricklaying, pointing, rendering
Calculating How Much You Need
Volume needed = Length x Width x Depth (all in metres)
Driveway: 5m x 3m x 0.1m (100mm depth)
Volume = 1.5 m3
Add 10% for waste and compaction:
Order: 1.5 x 1.1 = 1.65 m3 → order 1.7 m3
Bags vs Ready-Mix: The Threshold
Bagged concrete (25kg bags):
One 25kg bag yields approx 0.012 m3 of mixed concrete
For 1.5 m3: 1.5 / 0.012 = 125 bags
At ~£5/bag = £625 in materials
Plus labour mixing by hand or mixer hire
Ready-mix delivered:
~£90-110 per m3 (2025 prices, minimum order ~0.5 m3)
1.7 m3 = ~£170 delivered, poured in one go
Break-even: above ~0.5 m3, ready-mix is almost always
more cost-effective and produces a better result.
Water:Cement Ratio — the Key Variable
The water:cement (w/c) ratio is the single biggest determinant of concrete strength. Lower w/c = higher strength, but less workability.
w/c 0.40: very strong but stiff, needs plasticiser for flow
w/c 0.50: good strength, workable — ideal for most DIY
w/c 0.60: easy to work, lower strength, more shrinkage
w/c 0.70+: avoid — significantly weakens concrete
For 50kg cement bag (typical strength mix):
At w/c 0.50: use 25 litres of water
Do NOT judge by appearance alone — "sloppy" mixes are weaker.
Curing: Where Most DIYers Go Wrong
Concrete gains strength through a chemical reaction (hydration), not by drying out. Covering fresh concrete with polythene sheeting for 7 days doubles its final strength compared to letting it dry in the open air. In hot weather, spraying with water prevents premature drying. This step costs nothing and makes an enormous difference.