Garden guide
Concrete Mix Ratios Explained: Cement, Sand & Ballast
The short version
The ratio is cement to all-in ballast (sand and gravel mixed). 1:6 is the general-purpose mix for paths and shed bases, 1:5 suits patios and garage floors, and 1:4 is the strong stuff for driveways and structural work. More cement means more strength — but water matters just as much, and too much of it ruins a rich mix.
The ratios by job
1:4 (four parts ballast to one cement) gives roughly C20 structural concrete — driveways, garage bases and anything carrying vehicles. 1:5 is the all-rounder for patio bases and workshop floors. 1:6 suits garden paths, shed bases and non-structural pads. 1:8 is a weak mix used for haunching fence posts and blinding over hardcore — never for a wearing surface.
How the mix affects strength
Cement is the glue; ballast is the skeleton. Skimp on cement and the surface dusts, crumbles at the edges and breaks up in frost. But the silent killer is water: a sloppy, over-wet mix can halve the strength of good proportions. Aim for a mix that holds together when squeezed but doesn't glisten — workable, not pourable.
Mortar and screed ratios are different
Mortar uses building (soft) sand, not ballast: 4:1 for bricklaying, 5:1 for blockwork and pointing, with plasticiser or a dash of washing-up liquid for workability. Floor screed uses sharp sand at 3–4:1. And render is 4–5:1 with lime or plasticiser. Same principle throughout — the first number is always the cement.
Measuring it out
Use a bucket, not a shovelful — shovels lie. One bucket of cement to five level buckets of ballast gives the 1:5 general mix. Per cubic metre of concrete that's about 13 bags of 25kg cement and 1,100 litres of ballast. Our concrete calculator's 'mix your own' mode works out both quantities and prices them for your slab size.