What it is
The water-cement ratio (w/c) is the weight of water divided by the weight of cement in the mix. A mix using 25 kg of cement and 11 kg (11 litres) of water has a w/c of 11 ÷ 25 = 0.44. It is a ratio by weight, not by volume, and it counts all the water — including the water already in damp sand.
Why it matters more than the mix ratio
Cement needs only about 0.25 of its own weight in water to hydrate fully. Everything beyond that is there purely to make the mix workable, and when it eventually evaporates it leaves capillary voids behind. Those voids are what strength is lost to. Lower the w/c and you get denser, stronger, less permeable concrete; raise it and strength falls away fast — roughly halving for every 0.20 you add.
This is why a soupy mix disappoints. You can pour a 1 : 1.5 : 3 mix that should reach around 2,900 psi and get well under half that, simply by adding water to make it easier to place.
Typical water-cement ratios
Planning reference. These are typical values for general work. Durability-driven maximum w/c ratios for exposure classes are set by code — structural work must follow your local building code and an engineer's mix design.
How much water does a bag need?
Bagged premix already contains its cement, sand and aggregate in proportion, so the only variable you control is water. As a rule of thumb an 80 lb bag takes roughly 3 quarts (about 2.8 litres), a 60 lb bag about 2.5 quarts and a 40 lb bag about 1.5 quarts — but the bag itself is the authority, and it accounts for the aggregate already being damp.
Add most of the water, mix, then bring it to consistency with the last of it. You are aiming for a mix that holds a shape when you furrow it with a trowel and does not run back — not one that pours like batter.
Working out quantities? The volume calculators give the bag count for your pour, and the mix ratio reference covers the proportions for site-batched mixes.