pKb Calculator. The pKb (negative log of the base dissociation constant) measures how strongly a base ionizes in water — the lower the pKb, the stronger the base. Select your calculation type — convert Kb to pKb, pKb to Kb, or between all four constants — then enter your known values (Kb, pKb, Ka, or pKa) and temperature. The pKb Calculator returns your primary pKb result alongside secondary outputs for Kb, pKa, and Ka. Also try the Hydrogen Ion Concentration Calculator.
At 25°C, pKa + pKb = 14 for any conjugate acid-base pair. This relationship comes from the water ionization constant (Kw = 1.0 × 10⁻¹⁴). See also our Isoelectric Point Calculator.
pKb is calculated as the negative logarithm of Kb: pKb = -log(Kb). For example, if Kb = 1.8 × 10⁻⁵, then pKb = -log(1.8 × 10⁻⁵) = 4.74.
A higher pKb value indicates a weaker base. Since pKb = -log(Kb), a larger pKb means a smaller Kb, which corresponds to less base dissociation.
For a conjugate acid-base pair, Ka × Kb = Kw = 1.0 × 10⁻¹⁴ at 25°C. This means that knowing one constant allows you to calculate the other.
Yes, pKb can be negative for very strong bases where Kb > 1. However, most weak bases have positive pKb values between 1 and 14.
Temperature affects the water ionization constant (Kw). At 25°C, Kw = 1.0 × 10⁻¹⁴, but it changes with temperature, affecting the pKa + pKb = 14 relationship. You might also find our Tris Base Required — Tris Buffer useful.