Advanced options
Buy by the cubic metre, not the "load": a builders bag is ~0.7 m3 loose-filled. Wood must be "Ready to Burn" certified (under 20% moisture, the legal standard for small sales in England): wet wood halves heat output and tars the flue. A GBP 15 moisture meter checks deliveries in seconds: split a log and test the fresh face.
What The Firewood Calculator Works Out
- Wood burned: fires a week x hours per fire x weeks of season x the stove's burn rate. A typical 5kW woodburner ticking along consumes around 2.5 kg an hour, so four evenings a week through a twenty-week season burns about 800 kg.
- Seasoned hardwood to buy: the kilograms divided by a stacked density of about 400 kg per cubic metre for seasoned hardwood, then split into builders bags of roughly 0.7 m3. The honest trade sells by the cubic metre, loose-fill or stacked, stated; the folkloric "load" means whatever the seller's trailer means.
- Storage: how to keep it. Stacked off the ground, top covered, sides open to the wind, and buying green at half price needs 12 to 18 months of exactly this. The log store is a machine for making cheap wood into good wood.
- Wood burned per hour (advanced): a small 4kW stove run low uses 1.5 to 2 kg an hour, a big 8kW stove run hard 4 kg or more. Burn hot and bright rather than slumbering: the overnight shut-down burn is the tar factory.
- Stacked density (advanced): oak, ash and beech stack at 400 to 450 kg per m3 seasoned; birch around 350; softwoods such as spruce 300 or less, which is why a cube of hardwood carries half again the heat of a cube of softwood.
- Bag or load size and price per m3 (advanced): enter the volume and price your supplier actually quotes. Kiln-dried hardwood costs GBP 130 to GBP 200 per m3 delivered; seasoned is a little less and green half that.
- Firewood cost for the season and per fire (advanced): the cubic metres multiplied by the price, and that total spread across each evening's fire.
- Useful heat delivered (advanced): wood at 20% moisture gives about 4 kWh per kg, multiplied by the stove's efficiency (modern Ecodesign stoves 75% to 80%, open fires 20% or less). The sub-line shows the resulting pence per useful kWh. Wet wood burns its water first, halving output and painting the flue with the tars chimney fires are made of.
- Same heat from gas central heating (advanced): the useful heat bought as gas at your unit rate through a 90% efficient boiler. Stove wood usually lands between mains gas and electric heating per kWh, though the fire's real dividend is the one the spreadsheet never captures. Sweep the chimney annually and fit a CO alarm in any solid-fuel room.