You will need approximately 12-25 kg of firewood to heat up the hot tub once. Prepare different sizes of firewood in advance: make some kindling, finely chopped firewood, and then have your larger logs ready. Make sure you’re using kiln dried firewood to avoid smoke and ensure quick heating. Use hardwood (oak, beech, etc.) only or combine softwood (e.g. pine, fir) with hardwood in equal amounts.