Firewood calculator — stere/m³ converter and heating season

A stere (Polish: metr przestrzenny, mp) is a unit of firewood stacked in a 1×1×1 m pile. It contains about 65% solid wood and 35% air. 1 stere ≈ 0.65 m³ of solid wood. It is the most common unit used in Poland when buying firewood.

m³ (cubic metre) refers to a solid block of wood with no air gaps. A stere is a 1×1×1 m stack that includes air between the logs — 1 stere ≈ 0.65 m³ of solid wood. A loose stere (mp.s.) is wood poured in bulk — 1 loose stere ≈ 0.40 m³ of solid wood.

For a 100 m² house with standard insulation (150 kWh/m²/year) and a 5-month heating season: demand ≈ 100 × 150 × 5/12 = 6,250 kWh. Hardwood (beech, oak) gives about 2,200 kWh/m³ → you need about 2.84 m³ = 4.4 steres. The calculator works this out automatically.

The highest-energy species are hardwoods: hornbeam (3,150 kWh/m³), oak (2,900 kWh/m³), beech (2,800 kWh/m³), acacia (2,850 kWh/m³). Softwoods have lower values: pine (2,000 kWh/m³), spruce (1,900 kWh/m³). The calculator uses 2,200 kWh/m³ as an average for dry mixed hardwood.

1 stere (mp) = 0.65 m³ of solid wood (hardwood, log length 25–33 cm). 1 m³ = 1.538 steres. 1 loose stere (mp.s.) = 0.40 m³. The calculator converts automatically between all units. Coefficients may vary slightly depending on log length and wood species.

Optimal moisture for firewood is 15–20%. Fresh wood has 40–60% moisture, burns poorly, produces a lot of smoke and damages the chimney. Seasoning takes 1–2 years. The calculator uses values for dry wood (approx. 15% moisture).

A typical single-axle trailer (1.0×2.1×0.4 m) holds about 0.84 m³ ≈ 2 loose steres. A twin-axle agricultural trailer (2.5×4.0×0.4 m) holds about 4 m³ ≈ 10 loose steres. When buying firewood, always confirm which unit the supplier is using.

Calculate the annual heat demand of the house (kWh = area × unit demand), then divide by the calorific value of the wood (approx. 2,200 kWh/m³ for dry hardwood) and by the stove efficiency (0.7–0.9). The seasonal calculator includes these factors.

The calculator currently uses approximate calorific values for dry hardwood (about 2,200 kWh/m³) without a stove-efficiency correction. Open fireplaces have an efficiency of about 30–60%, heat-storage stoves 70–85%, stoves with a water jacket 75–90%.

Unit converter mode: select the input unit (stere, m³ or loose stere), enter a value and the calculator converts to all other units and estimates the calorific value. Heating season mode: enter the heated floor area, the insulation standard and the number of heating months to get the wood you'll need.