Strength falls roughly exponentially with the water-cement ratio. Going from 0.50 to 0.60 is about two extra litres in a 25 kg bag — invisible in a mixer — and costs around a quarter of the final strength; 0.70 costs nearly half.
The mix ratio sets how much cement is present, but the water-cement ratio sets how good that cement ends up being, and it is the one thing a person on site actually changes. Water beyond what hydration consumes eventually leaves, and the capillary voids it leaves behind are exactly where the strength was going to be. A wetter mix is easier to place and worse forever afterwards.
Only downward, and only to a point. Below about 0.40 the mix is too stiff to compact by hand, and the voids left by poor compaction cost more strength than the extra water would have. A dry mix is stronger only if it is actually consolidated — otherwise the curve inverts and you get the worst of both.
Estimate with the rule, then check it against the calculator that models it properly.
Open Concrete Mix Calculator →Strength falls roughly exponentially with the water-cement ratio. Going from 0.50 to 0.60 is about two extra litres in a 25 kg bag — invisible in a mixer — and costs around a quarter of the final strength; 0.70 costs nearly half. The mix ratio sets how much cement is present, but the water-cement ratio sets how good that cement ends up being, and it is the one thing a person on site actually changes.