If your pre-boil or post-boil gravity came in higher than the recipe called for, adding water is usually the fix — but the right amount is a calculation, not a splash. Reading 19 L at 1.070 against a 1.060 target needs 3.17 L of water added, bringing the batch to 22.17 L total at the corrected 1.060. Reproduce this with your own reading.
The arithmetic behind the number
Dilution works in gravity points ((SG - 1) x 1000 — so 1.070 is 70 points, 1.060 is 60). Think of the total gravity points in your kettle as a fixed quantity of dissolved stuff that plain water can spread out but never add to or take away from — pour in more water and the same quantity of dissolved extract just occupies a bigger volume, which is exactly the arithmetic finalVolume = currentVolume x currentPoints / targetPoints captures: 19 x 70 / 60 = 22.17 L. Subtract the starting 19 L and the difference, 3.17 L, is exactly how much water to add. This is the same gravity points concept used everywhere else on this site, just run forward from a real reading instead of stated as an abstract definition.
Why this happens
A wort usually comes in stronger than planned for one of a short list of reasons: less pre-boil or sparge water collected than the recipe assumed (a common brew-in-a-bag or partial-mash issue), a longer boil than planned driving off more water and concentrating the wort further, or a grain bill that simply extracted more efficiently than the recipe's assumed mash efficiency. None of these are mistakes exactly — they're just deviations from the recipe's planning assumptions — and diluting back to target is the standard, well-understood fix rather than a workaround.
When to dilute, and when not to
Diluting with plain water works cleanly before or shortly after the boil, while the wort hasn't fermented yet — water added at this stage just lowers concentration uniformly across sugar, colour and bitterness alike. See when to dilute vs. when to blend for the cases where blending with a second batch, rather than plain water, is the better tool — mainly when you don't want to lower colour and bitterness proportionally along with gravity, since a second, differently-flavoured batch changes those independently while water only ever dilutes everything at once.
If the wort is already through active fermentation and you're looking at a finished beer that came out stronger than expected rather than a too-strong wort, the same gravity-points logic still applies but the practical considerations are different — see can I add water after fermentation for what changes once alcohol, not just sugar, is part of what's being diluted.
Don't overdo the correction
Dilution only ever adds water, and adding too much overshoots your target below what the recipe intended — there's no clean way to remove water again afterward short of blending in a stronger batch. Measure the reading carefully, run the exact number through the dilution & blending calculator, and add the water in stages if you're at all unsure of your volume measurement, checking gravity again before adding more.