Interactive lesson

How much firewood you need, step by step

Interactive lesson: firewood measures, species and moisture, house and stove, your own sums and buying without being short-changed.

Free sample Chapters 1 and 2 (8 steps) are free. The full lesson comes with the How much firewood you need module. Already bought? Unlock

This lesson is general guidance, not professional training. Local regulations and expert advice always take precedence. Read our disclaimer.

The whole lesson as text

1. Measures: stacked, solid and loose

Before every purchase, be clear which measure you're talking about.

Stacked cubic metre — A stacked m³ (stere) is 1 × 1 × 1 m of stacked 1 m lengths. It holds about 70 % wood and 30 % air. A US or Canadian cord is 3.62 stacked m³.
Solid wood — A solid m³ is wood with no gaps, as the forester measures it. 1 solid m³ gives about 1.4 stacked m³.
Loose in a trailer — A loose m³ (bulk bag, tipped load) holds only about 50 % wood. A trailer with 3 loose m³ of split logs is about 2 stacked m³, not 3.
Stack, then pay — Unload, stack and measure before you pay. Shorter logs stack 5–15 % lower, and that's normal.

Why: “A load”, “a cubic metre”, “a bulk bag” and “a cord” don't always mean the same thing. In an advert the same word can hide three very different amounts of wood.

The conversion you'll use all the time: 1 solid m³ ≈ 1.4 stacked m³ ≈ 2 loose m³.
For 1 stacked m³ of split, loose wood you need about 1.4 m³ of trailer or bulk bag.

2. Wood species and heat

Hardwood is heavier, so more heat fits into the same cubic metre.

The kilogram is the same — A kilogram of dry wood gives roughly 5 kWh. Heavier wood packs more kilograms into a cubic metre, so more heat.
Hardwood — Hornbeam, beech and oak give about 2,000 kWh per stacked m³. Oak burns slowly with strong embers, but needs longer seasoning.
Softwood isn't bad — Spruce and fir give about 1,400 kWh, poplar about 1,200. They're excellent for kindling, spring, autumn and the range in summer.
What not to burn — Varnished, painted and treated wood, chipboard, old window frames and railway sleepers. They give off toxic smoke and ruin the stove and the chimney.

Why: A kilogram of completely dry wood gives roughly 5 kWh, whatever the species. The difference is how many kilograms fit into a cubic metre, which is why softwood costs less and you need more of it.

Black locust gives as much as hornbeam, about 2,100 kWh per stacked m³, but it is hard to split when dry.
Values are approximate: the same species from a hill and from a valley can differ by 10 %.