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Mg Number (Mg#) Calculator

Calculate the magnesium number Mg# (and Fe#) on a molar basis from MgO and FeO, or directly from Mg and Fe cations, with the iron basis stated explicitly.

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How to use this calculator

Inputs

Mg# uses molar proportions — you cannot ratio wt% MgO and FeO directly because their molar masses differ.

Result

Runs entirely in your browser. Your inputs are not uploaded or stored by Zerdly.

More needed
  • Enter Mg and Fe values.
Assumptions & basis
  • Iron basis: FeO-equivalent (Fe²⁺). State the assumption if only total Fe is known. (user)

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Choose oxide inputs (MgO, FeO wt%) or cation inputs (molar Mg, Fe).
  2. Read Mg# and Fe#, plus the molar Mg and Fe used.
  3. Confirm the iron basis — Mg# is sensitive to how iron is reported.

How it works

Mg# = Mg / (Mg + Fe) × 100 on a MOLAR (cation) basis. From oxides, the molar amounts are MgO wt% ÷ M(MgO) and FeO wt% ÷ M(FeO), which is why you cannot simply ratio the weight percents — MgO and FeO have different molar masses (40.30 vs 71.84 g/mol), so equal wt% does not mean equal moles.

The result depends on the iron basis. Mg# is conventionally computed with ferrous iron (Fe²⁺/FeO); if you only have total iron, that is an assumption you should state. The tool names the basis it used and never silently treats Fe₂O₃ as FeO or vice versa.

Worked example

MgO 8% and FeO 8%. Molar Mg = 8 ÷ 40.304 = 0.19849; molar Fe = 8 ÷ 71.844 = 0.11135. Mg# = 0.19849 ÷ (0.19849 + 0.11135) × 100 = 64.06 — well above 50 despite equal weight percents, because MgO is lighter per mole.

Common mistakes

  • Ratioing wt% MgO and FeO directly instead of converting to moles.
  • Using total iron as Fe²⁺ without stating the assumption.
  • Confusing Mg# (molar) with an MgO/FeO weight ratio.

Frequently asked questions

Why isn't equal MgO and FeO an Mg# of 50?

Because Mg# is molar. MgO (40.30 g/mol) is lighter than FeO (71.84 g/mol), so equal weight percents mean more moles of Mg, giving an Mg# above 50.

Which iron should I use?

Ferrous iron (FeO / Fe²⁺) by convention. If only total Fe is available, state that you are treating it as Fe²⁺ — the value will differ if some iron is ferric.

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Cite this tool

Zerdly. (2026). Mg Number (Mg#) Calculator. Retrieved from https://www.zerdly.com/tools/mg-number-calculator/

Tip: Enter any known values to calculate the remaining results.

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