How we calculate
This page lists the formula and the figures behind each calculator. It's built from the calculators' own code and data, so it can't drift out of step with them.
Where the figures come from
- Your own figures: stove output and efficiency from the spec plate or listing, your log supplier's bag sizes and your prices. We never fill these in for you.
- Reference tables: rules of thumb, energy in firewood, species, volumes and the rules by nation. Each row names its source and the date we checked it.
- Our assumptions: a starting point we chose, such as logs at 20% moisture. Change it to match your meter reading.
Anything marked "Not yet checked against the source" is still waiting for our second check. Figures we haven't sourced yet are left blank rather than guessed: the calculator asks you for them, or says the figure isn't confirmed.
Rules of thumb, not designs
Stove sizing here is by room-volume rules of thumb, shown side by side and labelled as such. It isn't a heat-loss calculation and it isn't a design: your HETAS-registered installer sizes and fits the stove. See how we size stoves.
Rounding
Sums are done in metric units and rounded once, at the end. Anything you buy whole, such as bulk bags, is rounded up.
We don't test products
We don't test stoves or kit. Comparisons use makers' published specifications and say so.
Formulas and figures by calculator
CO alarm checklist
- One check each: an alarm in the stove room, positioned as the maker says, test button pressed recently, and within its replace-by age
- Years left = the replace-by age on the label − the alarm's age
A CO alarm is a back-up, not a substitute for a swept chimney, a properly installed stove and dry fuel. The rules on fitting an alarm with a new stove differ between England, Wales, Scotland and Northern Ireland: your installer will know which apply.
This calculator has no built-in defaults: every figure comes from you or your product's data.
Ecodesign and exempt appliance explainer
Open the ecodesign and exempt appliance explainer
- Nation → the official pages on smoke control, Ecodesign and fuel sales (data/tables/rules.json, each to be confirmed against gov.uk, gov.wales, gov.scot or nidirect)
- In a smoke control area (or not sure) → check with your council and the Defra exempt-appliance list
- Logs → buy dry wood (Ready to Burn where it applies); smokeless fuel → buy an approved fuel
A guide to the rules, not legal advice. Rules differ between nations and from one council to another: always check the official pages and with your council.
We never say an appliance or fuel is allowed where you live. Check with your council and the Defra exempt-appliance list.
This calculator has no built-in defaults: every figure comes from you or your product's data.
| Table | Clause | Nations | Status |
|---|---|---|---|
| Rules by nation: smoke control, Ecodesign, fuel sales, CO alarms, Building Regulations gov.uk, gov.wales, gov.scot, nidirect, Defra, HETAS and Woodsure (Researcher to add each page) | Brief §0 and §3 rules.json. We explain and link the rules; we never say an appliance or fuel is legal in an area. | UK-wide | 1/22 values entered Not yet checked against the source |
Firewood calculator
- 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.
| 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 |
Kiln-dried vs seasoned value
Open the kiln-dried vs seasoned value
- Energy per kg (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; per load = per kg × the load's weight
- Useful kWh for each pound = useful heat per load ÷ the price of the load (your prices)
Compare loads by weight, not volume: a loose load and a stacked load of the same size hold different amounts of wood. Ask each supplier what a load weighs.
Moisture matters more than the label. Measure both with a meter on a freshly split face: logs at or below 20% give more heat per kg and a cleaner fire.
This calculator has no built-in defaults: every figure comes from you or your product's data.
| 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 |
Log moisture checker
- Average = the sum of your readings ÷ the number of readings
- Ready when the average is at or below the Ready to Burn threshold (20%, source to be confirmed)
- Above it, keep seasoning and measure again in a few weeks
Measure properly: split a log from the middle of the stack, let it reach room temperature, and press the pins into the fresh face along the grain. Readings from the end grain or the outside read drier than the wood really is.
Pin meters differ: check your meter's manual for the wood setting and how it reports moisture.
This calculator has no built-in defaults: every figure comes from you or your product's data.
| Table | Clause | Nations | Status |
|---|---|---|---|
| Rules by nation: smoke control, Ecodesign, fuel sales, CO alarms, Building Regulations gov.uk, gov.wales, gov.scot, nidirect, Defra, HETAS and Woodsure (Researcher to add each page) | Brief §0 and §3 rules.json. We explain and link the rules; we never say an appliance or fuel is legal in an area. | UK-wide | 1/22 values entered Not yet checked against the source |
Seasoning time estimator
Open the seasoning time estimator
- Seasoning range (months) = the range for your species when split and covered (species table, source to be confirmed), or your own
- Start measuring = the month you stacked it + the shortest time; keep measuring until the month + the longest time
- Ready when a moisture meter reads at or below 20% on a freshly split face
Measure, don't guess. Weather, log size and where the stack stands all change how fast wood dries. The range tells you when to start checking with a moisture meter, not when the logs are dry.
Whole rounds and uncovered stacks dry more slowly than the range shows. Split them, stack them off the ground, cover the top and leave the sides open.
This calculator has no built-in defaults: every figure comes from you or your product's data.
| Table | Clause | Nations | Status |
|---|---|---|---|
| UK firewood species Researcher to cite (e.g. Forest Research, Woodsure or a published species table) | Brief §3 species.json: species → density, seasoning time range, burn notes (RtV). No figures entered. | UK-wide | 0/18 values entered Not yet checked against the source |
Stove chooser quiz
- Fuel: logs only → a wood-only stove (wood burner); logs and smokeless fuel → a multi-fuel stove; not sure → both
- Look: freestanding, inset or both; heating water or radiators adds a boiler stove
- Smoke control area (or not sure) → check with your council and the Defra exempt-appliance list
- No chimney (or not sure) → ask your installer about a flue system; every route ends with a HETAS-registered installer's survey
A shortlist of stove types, not a recommendation of a model. Your HETAS-registered installer checks the room, hearth, chimney and ventilation before you choose.
We explain the rules and link the official pages; we never say an appliance is allowed where you live. Check with your council and the Defra exempt-appliance list.
This calculator has no built-in defaults: every figure comes from you or your product's data.
Stove size calculator
Open the stove size calculator
- Volume (m³) = length × width × ceiling height (plus the other space when the room is open plan)
- Rule of thumb A: kW = volume ÷ 14 (brief §4; source to be confirmed)
- Rule of thumb B: kW = volume ÷ a second divisor (not yet confirmed, so not shown)
- Each is multiplied by an insulation adjustment for the house's age once that adjustment is confirmed; until then the figures are unadjusted
These are rules of thumb, not a heat-loss calculation. A bigger stove isn't better: an oversized stove gets run turned down, burns cool and smoky, and blackens the glass and the flue. Your HETAS-registered installer will size the stove for your room, chimney and ventilation.
For a proper figure, a room-by-room heat-loss calculation counts walls, windows, insulation and air changes.
This calculator has no built-in defaults: every figure comes from you or your product's data.
| Table | Clause | Nations | Status |
|---|---|---|---|
| Room-volume rules of thumb for stove size brief §4 sizing.js (Researcher to cite a published source for each rule, e.g. HETAS, a stove maker or a trade body) | Rules of thumb only, not a heat-loss calculation. Rule A is the brief's example (m³ ÷ 14). Rule B and the insulation-era adjustments are not in the brief and stay null until the Researcher cites them. | UK-wide | 1/6 values entered Not yet checked against the source |