Coal holds water, and how much depends on rain, washing, storage and how long it has been lying. So a lab has to say which condition its numbers refer to — the 'basis'.
As received (ARB, or 'ar'): the coal as it arrived, with all its moisture (total moisture). Air-dried (ADB, or 'ad'): after the sample has been dried in air to steady conditions — only the inherent moisture is left. Dry basis (DB): as if all moisture were removed. Dry ash-free (DAF) goes further and takes out the ash too.
Because moisture adds weight but no heat, the same coal shows a higher GCV and ash percentage on air-dried basis than as received. Converting between bases uses the moisture figures: for example, a value on air-dried basis is multiplied by (100 − total moisture) ÷ (100 − air-dried moisture) to express it as received. The international method is set out in ISO 1170.
What it means if you trade coal
Before you compare a supplier's report with your own lab's, check that both are on the same basis — and write the basis into purchase and sale terms. An 'ADB' GCV against an 'ARB' GCV is not a quality difference, just a different yardstick.
Sources
- StandardISO 1170:2020 — Coal and coke: calculation of analyses to different bases
- StandardBureau of Indian Standards — IS 1350 Part 1 (Proximate analysis)
Coal Boss explanation, based on the sources above. Last checked: 22 September 2026.
