pH is minus the log of the hydrogen-ion concentration in mol/L, so each unit is a factor of ten: pH 3 is ten times more acidic than pH 4 and a thousand times more than pH 6, and 0.1 mol/L of a strong acid is pH 1.
A strong acid ionises completely, so its hydrogen-ion concentration is its concentration, and the pH is the negative of the power of ten: 0.1 mol/L is 10 to the minus 1, pH 1; 0.001 mol/L is pH 3. Strong bases are the mirror image, with pH + pOH = 14 at 25 °C, so 0.1 mol/L of sodium hydroxide is pH 13.
It holds for strong acids from about 10 to the minus 6 up to about 0.1 mol/L. Much more dilute, the water’s own ions take over (10 to the minus 8 mol/L of hydrochloric acid is pH 6.98, not 8); more concentrated, the ions do not behave ideally and a meter reads differently; and a weak acid has far fewer hydrogen ions than its concentration suggests.
Estimate with the rule, then check it against the calculator that models it properly.
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