What is the formula this pH calculator uses?

The core relation is pH equals the negative base-ten logarithm of the hydrogen-ion concentration in mol/L, written pH = -log10[H+]. Going the other way, [H+] = 10^(-pH). The same pattern holds for the base side: pOH = -log10[OH-] and [OH-] = 10^(-pOH). For example, a hydrogen-ion concentration of 1e-3 mol/L gives -log10(0.001) = 3, so the pH is 3 and the solution is a moderately strong acid.

Why does [H+] = 1e-7 give a pH of 7 and count as neutral?

At 25 degrees Celsius pure water self-ionizes just enough that both [H+] and [OH-] sit at 1e-7 mol/L. Plugging in, pH = -log10(1e-7) = 7, and pOH works out to 7 as well. Because the hydrogen and hydroxide concentrations are equal, the solution is neutral. Push [H+] above 1e-7 and the pH drops below 7 (acidic); drop it below 1e-7 and the pH climbs above 7 (basic).

How are pH and pOH related?

At 25 degrees Celsius they always add up to 14, because the water ionization constant Kw is 1e-14 and pH + pOH = -log10(Kw) = 14. So if you measure a pH of 4.2, the pOH is 14 - 4.2 = 9.8 without any further work. This calculator fills in pOH automatically, which is handy when a problem hands you one quantity but the answer sheet wants the other.

How do I tell if a solution is acidic, neutral or basic?

Compare the pH to 7. Below 7 the solution is acidic, meaning [H+] is greater than [OH-]; vinegar at about pH 3 is a familiar example. Exactly 7 is neutral, the balance point of pure water. Above 7 the solution is basic (alkaline), where [OH-] dominates; household ammonia sits near pH 11. This tool prints the verdict and marks the spot on the 0-14 scale so the result is obvious at a glance.

Why is each pH unit a tenfold change in concentration?

Because pH is a base-ten logarithm, every whole unit corresponds to a factor of ten in [H+]. A pH 3 solution has ten times the hydrogen-ion concentration of pH 4 and a hundred times that of pH 5. That is why a small-looking pH shift matters: dropping from pH 7 to pH 5 is not a tiny change, it is a hundredfold rise in acidity. Watching [H+] update as you nudge the pH makes that scale visible.

Can the pH be below 0 or above 14?

Yes. The 0-to-14 range covers ordinary aqueous solutions, but it is not a hard limit. A concentrated strong acid such as 10 mol/L HCl gives [H+] near 10, so pH = -log10(10) = -1. Likewise a very concentrated strong base can push pH above 14. This calculator accepts those values and reports them honestly rather than clamping, while the colour scale simply pins its marker at the nearer end.

Check a titration or buffer answer in gen chem

Your worksheet hands you a hydrogen-ion concentration of 2.5e-4 mol/L and asks for the pH, pOH and whether the solution is acidic. Type the value in [H+] mode and the tool returns pH 3.60, pOH 10.40 and an acidic verdict in one step, so you can check your own working instead of re-running the log on a calculator and second-guessing a sign.

Convert a measured pH into [H+] for a rate equation

A kinetics problem gives the pH of the reaction mixture but the rate law needs the actual hydrogen-ion concentration. Switch to pH mode, enter the reading, and copy the [H+] in mol/L straight into your expression. No more typing 10^(-pH) into a phone and fumbling the negative exponent.

Build intuition for the logarithmic scale

Teaching why pH 4 is ten times more acidic than pH 5 lands better when students watch it happen. Step the pH from 7 down to 4 and the [H+] readout climbs from 1e-7 to 1e-4, a thousandfold jump, while the marker slides across the colour strip. The abstract log becomes something you can see move.

Sanity-check a hydroponics or aquarium reading

Your nutrient solution reads pH 6.2 and you want the matching hydroxide concentration before dosing a pH-up additive. Enter 6.2, read [OH-] at about 1.6e-8 mol/L, and confirm the solution is mildly acidic so you know which way to adjust rather than guessing from the number alone.

pH Calculator with pOH, [H+] and [OH-]

pH, pOH, [H+] and [OH-] from any one value, with an acid/base verdict and a 0-14 colour scale, all in your browser

Runs locally
Category Calculator
Best for Getting a realistic range before a purchase, plan, workout, or schedule decision.

Known value · pH

Solution is

Neutral

Results at 25 °C

pOH

[H⁺]

1.000e-7

[OH⁻]

1.000e-7

pH scaleAcid 0 · 14 Base

02468101214

Everyday reference

What this tool does

A pH calculator that turns any one of pH, pOH, hydrogen-ion concentration or hydroxide-ion concentration into the other three, then tells you whether the solution is acidic, neutral or basic. The chemistry is the standard set of relations at 25 degrees Celsius: pH equals minus the base-ten log of the hydrogen-ion concentration, pOH equals minus the log of the hydroxide-ion concentration, and pH plus pOH always sums to 14. Type a measured [H+] like 1e-3 and read back pH 3; type pH 9 and get the matching [OH-]. Concentration inputs accept scientific notation the way acid-base problems are actually written. A coloured 0-14 strip marks where your value falls, and chips for everyday liquids like lemon juice and baking soda give context. Everything runs as plain JavaScript in the page, nothing is uploaded, and the share link reopens your exact calculation.

Tool details

Input: Numbers; The page exposes text boxes, numeric controls, file pickers, or structured inputs depending on the tool.
Output: Live result + Copy; The result area focuses on usable output, with copy, download, or preview actions when supported.
Privacy: Browser-side processing; The main tool logic does not call an external API, so inputs normally stay in the current tab.
Save / share: Shareable URL state; Key settings are encoded in the URL so another person can reopen the same setup.
Performance budget: Initial JS <= 9 KB; No WASM budget is declared, keeping the tool quick to open on mobile.
Best fit: Calculator · Student; Category and role tags drive related tools, internal links, and quick fit checks.

How to use

1. Input

Paste or drop your content into the tool panel.
2. Process

Click the button. All processing is local in your browser.
3. Copy / Download

Copy the result or download to disk in one click.

How pH Calculator fits into your work

Use it for fast estimates, comparisons, and planning numbers before you make the final call.

Calculation jobs

Getting a realistic range before a purchase, plan, workout, or schedule decision.
Comparing scenarios by changing one input at a time.
Turning rough assumptions into a number you can discuss.

Calculation checks

Double-check units, dates, rates, and rounding assumptions.
Treat health, finance, tax, and legal outputs as planning aids, not professional advice.
Save the inputs that produced an important result so you can reproduce it later.

Good next steps

These links move the current task into a more complete workflow.

Real-world use cases

Check a titration or buffer answer in gen chem
Your worksheet hands you a hydrogen-ion concentration of 2.5e-4 mol/L and asks for the pH, pOH and whether the solution is acidic. Type the value in [H+] mode and the tool returns pH 3.60, pOH 10.40 and an acidic verdict in one step, so you can check your own working instead of re-running the log on a calculator and second-guessing a sign.
Convert a measured pH into [H+] for a rate equation
A kinetics problem gives the pH of the reaction mixture but the rate law needs the actual hydrogen-ion concentration. Switch to pH mode, enter the reading, and copy the [H+] in mol/L straight into your expression. No more typing 10^(-pH) into a phone and fumbling the negative exponent.
Build intuition for the logarithmic scale
Teaching why pH 4 is ten times more acidic than pH 5 lands better when students watch it happen. Step the pH from 7 down to 4 and the [H+] readout climbs from 1e-7 to 1e-4, a thousandfold jump, while the marker slides across the colour strip. The abstract log becomes something you can see move.
Sanity-check a hydroponics or aquarium reading
Your nutrient solution reads pH 6.2 and you want the matching hydroxide concentration before dosing a pH-up additive. Enter 6.2, read [OH-] at about 1.6e-8 mol/L, and confirm the solution is mildly acidic so you know which way to adjust rather than guessing from the number alone.

Common pitfalls

Confusing [H+] with pH directly. A concentration of 1e-3 mol/L is not a pH of 1e-3; the pH is -log10(1e-3) = 3. Feed the concentration in the [H+] mode and let the tool take the log, rather than typing the raw exponent into the pH field.
Dropping the negative sign in the log. pH = -log10[H+], not +log10[H+]. Forgetting the minus turns an acidic pH 3 into -3. The tool always applies the negative for you, which is the most common place a hand calculation goes wrong.
Assuming pH + pOH = 14 holds at every temperature. That sum equals 14 only at 25 degrees Celsius, because Kw changes with temperature. This calculator works at the standard 25 degrees; for hot or cold solutions the neutral point shifts away from 7.

Privacy

Every step here is plain JavaScript running in your browser tab: the logarithms, the pH-pOH sum, the concentration conversions and the acid-base verdict. No value you enter is sent anywhere and nothing is logged. The one thing to know is that the share link encodes your input and mode in the query string, so a link pasted into chat will record those numbers in the recipient server's access log. For anything you would rather keep to yourself, use the copy button and paste the text instead of sharing the URL.

FAQ

Tool combos

Folks in your role tend to reach for these alongside this tool.

Browse all tools for this role

pH Calculator with pOH, [H+] and [OH-]

Results at 25 °C

Everyday reference

What this tool does

Tool details

How to use

1. Input

2. Process

3. Copy / Download

How pH Calculator fits into your work

Calculation jobs

Calculation checks

Good next steps

Real-world use cases

Check a titration or buffer answer in gen chem

Convert a measured pH into [H+] for a rate equation

Build intuition for the logarithmic scale

Sanity-check a hydroponics or aquarium reading

Common pitfalls

Privacy

FAQ

Molar Mass Calculator

Scientific Calculator

Unit Converter

Percentage Calculator

24-Point Solver & Game

Add Days to Date Calculator

Age Calculator

Age Difference Calculator

AI Token Counter

Airline Luggage Allowance Checker

Annuity Calculator

APR to APY Calculator