Firewood calculator
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check the figures against your stove's manual, your supplier and your installer.
Short answer
Enter how many evenings a week you light the stove, for how long, its output in kW, its efficiency, your season length and your logs' moisture. The calculator works out the heat you need and the kilos of logs that supply it, with every step shown. Every figure is editable, so you can try wetter or drier logs and see the difference.
Worked example
| You enter | Value |
|---|---|
| Evenings a week you light the stove | 4 |
| Hours it burns each evening | 4 h |
| Heat output you run it at | 5 kW |
| Weeks in your burning season | 26 |
| Moisture content of your logs | 20% |
Heat needed: 2,080 kWh from the stove over the season
| Result | Value |
|---|---|
| Heat from the stove over the season | 2,080 kWh |
Show the working
- Heat: 4 evenings × 4 h × 5 kW × 26 weeks = 2,080 kWh
How it works
- Heat (kWh) = evenings a week × hours each evening × stove output (kW) × weeks
- Energy per kg of your logs (kWh/kg) = energy in oven-dry wood × (1 − moisture) − energy to dry off the water × moisture
- Useful heat per kg = energy per kg × stove efficiency
- Logs (kg) = heat ÷ useful heat per kg; stacked m³ = kg ÷ your logs' kg per stacked m³; bulk bags = m³ ÷ bag size, rounded up
Wetter logs give less heat: some of the energy goes on boiling off the water. Logs at or below 20% moisture give more heat per kg and a cleaner fire.
Real use varies with the weather, how you run the stove and the log size. Treat this as a planning figure and keep a log-use tracker for a season.
Assumptions and sources
The figures this calculator starts from, and where each comes from. Your stove's spec plate, your supplier and your own meter readings come first: change any figure to match.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Moisture content of your logs | 20 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job Our starting point: logs at the Ready to Burn threshold (brief §4, 20%, RtV). Enter your own meter reading. |
| Table | Clause | Nations | Status |
|---|---|---|---|
| Energy in firewood by moisture content Researcher to cite (e.g. Forest Research, the Biomass Energy Centre or Woodsure) | Net calorific value by moisture content: NCV = dry-wood NCV × (1 − w) − latent heat of water × w, with w the moisture content (wet basis). Brief §4 wood.js: standard formula, RtV. | UK-wide | 0/2 values entered Not yet checked against the source |
Rules of thumb and reference figures are checked against their sources before launch; anything not yet checked is marked.
FAQs
How much firewood do I need for a winter?
Multiply evenings a week × hours × stove output (kW) × weeks to get the heat you need in kWh, then divide by the useful heat in a kilo of your logs.
Where do I find my stove's output and efficiency?
On the spec plate, in the manual or on the maker's product page. Use the output you actually run it at, not the maximum.
Why does moisture change the answer so much?
Water in a log adds weight but no heat, and some of the fire's heat goes on boiling it off. Drier logs give more heat per kilo, so you need fewer. See log moisture content explained.
Can I convert the answer into cubic metres or bags?
You need your logs' weight per stacked cubic metre, which depends on species and dryness. Ask your supplier. See how much a cubic metre of logs is.
Guides
Buying logs in the UK: a practical guide
How to buy logs: dry enough, sold by volume or weight, loose or stacked, and how to compare suppliers on heat per pound.
How much firewood do I need for a winter?
How much firewood a winter takes, worked out from your evenings, stove output and log moisture, in kg, cubic metres and bags.
When to buy logs: order in September
Why September is the time to order and stack your winter logs, and how much to order.