Wood Gasifier Calculator
Design a TLUD wood gasifier stove: calculate thermal output, burn duration, fuel load, air inlet sizing, and biochar yield from reactor dimensions.
About this calculator
A TLUD (top-lit up-draft) gasifier burns wood in two stages: pyrolysis gases rise off the fuel bed and burn cleanly above it, leaving behind biochar rather than ash. This calculator sizes a reactor from its inner diameter and fuel-chamber height, computing the cross-sectional area and chamber volume, then multiplying by your fuel's bulk density to get the fuel load per batch. Burn duration and thermal output both come from a fixed specific gasification rate of 150 kg per square meter per hour — a typical figure for a well-built TLUD — combined with wood's energy content (about 15 MJ/kg) and an assumed 35% of that energy reaching the pot. Primary air holes (bottom, feeding the pyrolysis front) are sized at roughly 7% of the reactor's cross-section, and secondary air holes (upper, burning the rising gas) at about 3.5%, both standard TLUD proportions.
The calculator also reverse-solves for the diameter needed to hit a target cooking power, and estimates biochar yield at 22% of the original fuel mass by weight. Keep in mind the 150 kg/(m²·hr) gasification rate and 35% cooking efficiency are single fixed assumptions, not adjustable inputs — actual performance varies with fuel moisture, chip size, and how well you manage the air ratio in practice. Diameter and height are the two levers that actually change your results; fuel density mainly shifts burn duration and total fuel needed per batch, not power output.
Inputs
Results
Estimated thermal output
3.87 kW
≈ 4 microwaves
How to Use This Calculator
- Enter the reactor inner diameter and fuel chamber height in cm.
- Set fuel bulk density and your target cooking power in kW.
- The calculator returns estimated thermal output, burn duration, fuel load per batch, biochar yield, and required air inlet areas.
- Compare estimated thermal output to your target; adjust reactor diameter if needed.
- Use the primary and secondary air areas to size inlet holes during construction.
How the result changes with Reactor inner diameter
| Reactor inner diameter | Estimated thermal output |
|---|---|
| 7.5 | 0.97 kW |
| 11 | 2.08 kW |
| 23 | 9.09 kW |
| 38 | 24.81 kW |
What each input means
- Reactor inner diameter
- Inner diameter of the gasifier combustion chamber. Camp stove ≈ 10–15 cm, cookstove ≈ 15–30 cm.
- Fuel chamber height
- Height of the fuel loading zone. Taller = longer burn but heavier.
- Fuel bulk density
- Bulk density of your fuel. Wood chips ≈ 200–300, pellets ≈ 550–650, twigs ≈ 150–250 kg/m³.
- Target cooking power
- Desired thermal output at the pot. 1–2 kW for simmering, 3–5 kW for vigorous boiling.
What each result means
- Estimated thermal output
- Thermal power output at the pot from these reactor dimensions.
- Burn duration
- How long a full fuel load will burn before needing a refill.
- Fuel load per batch
- Weight of fuel that fits in the reactor chamber.
- Biochar yield
- Approximate biochar produced per batch (~22% of fuel mass). Can be used as soil amendment.
- Primary air inlet area
- Total area of bottom air holes (~7% of reactor cross-section).
- Secondary air holes area
- Total area of upper secondary air holes for clean combustion (~3.5% of cross-section).
- Recommended diameter for target power
- Reactor diameter needed to achieve your target cooking power.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersReactor inner diameter = 15, Fuel chamber height = 25, Fuel bulk density = 300, Target cooking power = 3 = 4 input(s) provided
- Calculate Estimated thermal outputEstimated thermal output = (fuelConsumptionKgHr * 15 * 0.35 * 1000) / 36003.87 = 3.87
- Calculate Burn durationBurn duration = fuelMassKg / fuelConsumptionKgHr30 = 30
- Calculate Fuel load per batchFuel load per batch = fuelVolumeM3 * fuelDensityKgM31.33 = 1.33
Engine last updated . Checked against 1 independently-derived test — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does changing the reactor diameter affect power so much more than changing the height?
Thermal output is driven entirely by cross-sectional area (diameter squared), since the fixed specific gasification rate of 150 kg/(m²·hr) is applied per unit of that area — doubling diameter roughly quadruples the area and therefore the power. Height only sets how much fuel volume sits above that cross-section, which changes burn duration and total fuel per batch but doesn't appear anywhere in the thermal output or air-inlet formulas.
Why does the calculator give me a 'recommended diameter' output separate from the diameter I entered?
It runs the power formula in reverse: starting from your target cooking power, it solves for the cross-sectional area needed at the fixed 150 kg/(m²·hr) gasification rate and 35% cooking efficiency, then converts that area back to a diameter. If this recommended value differs a lot from your entered diameter, your current reactor size won't actually deliver your target power at the assumed burn rate.
Does fuel bulk density change how much heat the stove puts out?
No — fuel density only changes how much mass of fuel fits in the fixed chamber volume, which drives fuel load and burn duration. Thermal output depends solely on reactor cross-section and the fixed specific gasification rate, so a denser fuel gives a longer burn at the same power rather than a hotter one.
How is the primary air inlet area different from the secondary air holes?
Primary air (sized at 7% of cross-section) feeds the pyrolysis front at the bottom of the fuel bed, where wood is converted into combustible syngas. Secondary air (3.5% of cross-section) is introduced higher up to burn that syngas cleanly once it rises, which is what gives a TLUD its low-smoke flame — get the ratio between them wrong and you'll either starve the pyrolysis or leave unburned gas escaping.
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