Concrete and mortar mix ratios explained

Reading a mix ratio

A ratio such as 1:5 means one part cement to five parts aggregate by volume, measured with the same bucket or shovel each time. Consistency matters more than precision: measure every batch the same way so the strength and colour stay even.

Common mixes

These are widely used starting points. Structural work should follow the mix or strength class given in the design or by building control.

• General concrete (paths, bases, footings): about 1 part cement to 5 or 6 parts all-in ballast.
• Stronger concrete (driveways, garage floors): a richer mix, or a ready-mixed concrete of the specified strength class.
• Bricklaying mortar: about 1 part cement to 4 to 6 parts building sand, with plasticiser or lime for workability.
• Floor screed: about 1 part cement to 3 to 5 parts sharp sand, mixed fairly dry.
• Sand and cement render: about 1 part cement to 4 to 5 parts plastering sand, often with lime or plasticiser.

Water and workability

Add water gradually. Too much water makes a mix easier to place but weaker and more likely to crack or shrink. Plasticiser improves workability without adding extra water, and lime makes mortar and render softer and more forgiving of slight movement.

When to use ready-mixed

For larger pours, ready-mixed concrete delivered to site is usually quicker and more consistent, and it lets you specify the exact strength class. Mixing on site suits small bases, post holes and repairs.