What Do the Figures on a Coal Bag Mean?

Every legally sold bag of coal carries a declaration table. Most buyers never look at it — yet those few figures tell you in advance exactly how the coal will burn.

Calorific value (kcal/kg)

This is the key line: the heat released by one kilogram of coal. A higher figure means less coal for the same warmth.

It may appear in two forms. Dry basis is measured after the moisture is removed — a theoretical ceiling. Net calorific value is what the coal delivers as sold, moisture included. What you feel in the stove is closer to the second.

A rough scale: 4000-5000 kcal is economical domestic coal, 5000-6500 mid-range, 7000 and above is the imported bracket.

Ash content (%)

What remains after burning. It affects two things directly: how often you clear the ash, and how much of your money burns away to nothing.

A 25 kg bag at 15% ash leaves about 3.75 kg behind. At 25% that becomes 6.25 kg — an extra 2.5 kg of ash per bag and noticeably more cleaning.

Sulphur (%)

Burning sulphur produces sulphur dioxide. Two consequences: it corrodes the flue and stove pipe, and it contributes to air pollution. That is why a legal ceiling exists.

Below 1% is considered good. Very low sulphur extends the life of your chimney and produces less soot.

Moisture (%)

Water inside the coal. High moisture hurts twice: energy is spent boiling that water off, wasting part of the calorific value, and the coal becomes harder to light.

The table may list "moisture on dry basis" and "total moisture" separately. The one that concerns you is total moisture — the coal as it arrives.

Granulation / size (mm)

The size of the pieces. Matching it to your stove directly affects efficiency.

• 10-18 mm (nut): Lights easily, burns steadily. Suits small stoves and automatic feed systems.
• 22-65 mm (walnut / lump): Burns longer, good for overnight. Ideal for classic stoves and central heating boilers.
• +18 mm: Means "larger than 18 mm" — mixed sizes.

Oversized pieces jam automatic feeders; undersized pieces fall through the grate of a classic stove. Your stove manual states the recommended size.

Conformity certificate number

Issued by the Ministry of Environment, Urbanisation and Climate Change. Coal without this number cannot be sold legally. Do not buy from a seller who has no number or avoids the question.

In short

Turn the bag over, find the table, read three lines: calorific value, ash, certificate number. Those three tell you more than any sales pitch. All our products list these figures on their product pages, next to a photograph of the actual bag.