Fixed Carbon

What It Measures

Fixed carbon is the solid, non-volatile carbonaceous residue that remains after the volatile matter has been driven off. In the standard proximate method it is not measured directly; it is calculated by difference from the other three determinations, conventionally as fixed carbon = 100% − moisture − volatile matter − ash, with all terms expressed on the same basis.

Because it is derived, fixed carbon is an inference from three separate measurements rather than an independent result in its own right.

Why It Matters

Fixed carbon is widely used as an indicator of the carbon-rich, slow-burning fraction of the fuel — the part associated with glowing, flameless combustion rather than the early flaming phase driven by volatiles.

It is also a convenient single summary of the proximate analysis, which is why it appears on many certificates. Its usefulness, however, is bounded by the accuracy of the three measurements it is calculated from.

Typical Measurement Method

Fixed carbon is obtained by calculation. The underlying determinations follow the methods for moisture, volatile matter and ash — for example ASTM D1762 for wood charcoal, ASTM D3172 in combination with ASTM D3173, D3174 and D3175 for coal and coke, and the ISO 18134, ISO 18123 and ISO 18122 series for solid biofuels.

The applicable standard defines the reporting basis (dry or as-received) and the convention for the subtraction. Critically, the three inputs must be measured on the same basis and by compatible methods for the difference to be valid; mixing bases or method families can produce a meaningless figure.

Buyer Relevance

Because fixed carbon is derived, buyers should treat it as a secondary indicator and inspect the underlying moisture, volatile matter and ash figures — and their basis — rather than relying on the fixed-carbon number alone.

Two charcoals can report the same fixed carbon for different reasons: one because volatile matter is low (well carbonised), another because ash is low. For procurement decisions, the individual proximate parameters carry more information than the calculated summary.

Limitations

A calculated value inherits the combined uncertainty of every input: if moisture, volatile matter and ash each carry error, those errors propagate into fixed carbon, and if the three are measured on different bases the subtraction can be invalid.

Fixed carbon says nothing about reactivity, burn rate or calorific value — usable heat is a separate determination (typically a calorific-value method). This page describes methodology only and publishes no results.

Data Source

Public standard methodology (general); not supplier-specific test data.

Last updated: 2026-10-07

Public methods commonly referenced

• ASTM D1762 — Chemical analysis of wood charcoal (fixed carbon by difference)
• ASTM D3172 — Proximate analysis of coal and coke
• ISO 18134 / ISO 18123 / ISO 18122 — Moisture, volatile matter and ash (underlying inputs)

Standard references are given as general, publicly documented method families. Applicability, current edition and reporting basis must be confirmed with the testing laboratory for any specific report.