How do I calculate pH?

Take the negative base-10 logarithm of the hydrogen ion concentration in moles per litre. A concentration of 1 × 10⁻⁴ gives pH 4.

How much more acidic is pH 3 than pH 5?

One hundred times. The scale is logarithmic, so each whole step is a tenfold change in hydrogen ion concentration.

Is pH 7 always neutral?

Only at 25°C. Neutral pH shifts with temperature because water ionises more readily when warm, reaching about 6.6 at 50°C.

Can pH go below 0 or above 14?

Yes. Very concentrated strong acids and alkalis exceed those values. The 0 to 14 range is conventional rather than a hard limit.

pH Calculator - Acidity from Hydrogen Ion Concentration

pH Calculator

pH measures how acidic or alkaline a solution is, on a logarithmic scale where each step represents a tenfold change.

Formula

pH = −log₁₀[H⁺] · pOH = 14 − pH

[H⁺] is the hydrogen ion concentration in mol/L
The 14 holds at 25 °C; it shifts with temperature

[H⁺] = 1 × 10⁻³ gives pH 3, and pOH 11

The Formula

pH = −log₁₀[H⁺]

Where [H⁺] is the hydrogen ion concentration in moles per litre.

A concentration of 1 × 10⁻⁴ mol/L gives a pH of 4.

Reversed: [H⁺] = 10^−pH. pOH works the same way for hydroxide ions, and at 25°C pH + pOH = 14.

Each Step Is Tenfold

Because the scale is logarithmic, the gaps are far larger than they look. A pH of 3 is ten times more acidic than 4, and a hundred times more acidic than 5.

For reference: stomach acid sits near pH 1.5, lemon juice around 2, black coffee near 5, pure water at 7, seawater about 8, and household bleach around 13.

Neutral Is Not Always 7

pH 7 is neutral only at 25°C. Water's self-ionisation increases with temperature, so neutral pure water at 50°C sits nearer 6.6 — still neutral, because hydrogen and hydroxide concentrations remain equal, but no longer at 7.

The scale is also not strictly capped at 0 and 14. Concentrated strong acids can go below 0 and concentrated alkalis above 14; those bounds are just the usual working range.

Quick Reference

Each whole pH unit is a tenfold change in acidity.
pH	[H⁺] mol/L	pOH	Example
0	1	14	Battery acid
2	0.01	12	Lemon juice
4	0.0001	10	Tomato juice
7	10⁻⁷	7	Pure water, neutral
9	10⁻⁹	5	Baking soda
12	10⁻¹²	2	Bleach
14	10⁻¹⁴	0	Drain cleaner

Frequently Asked Questions

How do I calculate pH?: Take the negative base-10 logarithm of the hydrogen ion concentration in moles per litre. A concentration of 1 × 10⁻⁴ gives pH 4.
How much more acidic is pH 3 than pH 5?: One hundred times. The scale is logarithmic, so each whole step is a tenfold change in hydrogen ion concentration.
Is pH 7 always neutral?: Only at 25°C. Neutral pH shifts with temperature because water ionises more readily when warm, reaching about 6.6 at 50°C.
Can pH go below 0 or above 14?: Yes. Very concentrated strong acids and alkalis exceed those values. The 0 to 14 range is conventional rather than a hard limit.