pKa and Ka Calculator
This calculator converts between Ka, the acid dissociation constant, and pKa, its negative base ten logarithm, so you can move between the two ways chemists describe how strongly an acid ionises in water. Ka values are often awkward, tiny numbers such as 1.8 times 10 to the minus 5, and pKa turns these into a compact scale that is far easier to compare and remember, in the same way pH does for hydrogen ion concentration. Enter your known value, then choose whether you have Ka or pKa from the dropdown, and the calculator instantly returns the other figure. It also displays your original input alongside a plain-language acid strength rating of very strong, moderately strong, weak or very weak, worked out from where the pKa sits on the scale. The underlying maths is straightforward: pKa equals minus the base ten logarithm of Ka, and Ka equals ten to the power of minus pKa, so a lower pKa always signals a stronger, more fully dissociating acid. This makes the tool useful for checking homework, comparing the relative strength of different acids, selecting a buffer system, or quickly sense-checking a lab result without reaching for a calculator and log tables. Results are rounded for display and assume standard conditions, so use it as a fast reference alongside your course material or lab manual rather than a substitute for it.
The formula
pKa = minus the base ten logarithm of Ka, and conversely Ka = 10 to the power minus pKa. A smaller pKa (larger Ka) means a stronger acid that dissociates more fully.
Worked example
Acetic acid has Ka about 1.8 times 10 to the minus 5, so pKa = minus log of that, about 4.74. Enter 1.8e-5 with Ka selected to confirm.
Frequently asked questions
What is pKa?
The negative base ten logarithm of the acid dissociation constant Ka. It measures acid strength on a convenient scale.
Does a low pKa mean a strong acid?
Yes. A lower pKa corresponds to a larger Ka and a stronger acid that ionises more completely.
How are Ka and pKa related?
pKa = minus log Ka, and Ka = 10 to the minus pKa. They carry the same information in different forms.
Who this calculator is for
This calculator is for chemistry students, teachers and lab workers.
What this calculator assumes
- You enter values in the units shown.
- Standard conditions unless stated.
- Results are rounded for display.
Formula and sources
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