Rocket Stove Design Calculator
Calculate combustion chamber dimensions, chimney height, pot gap, and fuel consumption for a rocket stove from target BTU output.
About this calculator
A rocket stove burns small-diameter sticks in an insulated, L-shaped chamber topped by a vertical chimney (riser), driving near-complete combustion through strong draft. This calculator starts from your target heat output in BTU/hr, converts it to kilowatts, and sizes a square combustion chamber using the Aprovecho Research Center rule of thumb of roughly 1 kW of output per 10 cm² of chamber cross-section. It then rounds the chamber's side length up to the nearest whole number of fuel sticks (based on your stick diameter), so the design is buildable rather than an odd fractional size — this rounding means your actual output usually comes in slightly above target, which is why "actual output" is reported separately from your target.
From the rounded chamber size, the calculator derives chimney height (2.5× chamber width, for adequate draft), feed tube length (1.5× chamber width), pot gap (clamped between 6–9 mm, the range known to transfer heat efficiently without choking flow), skirt height, and outer diameter once insulation thickness is added. Fuel consumption factors in moisture content: wetter wood has less usable energy per kilogram (some combustion heat evaporates the trapped water instead of reaching your pot) and the model applies a fixed 40% stove-to-pot efficiency on top of that. Keep fuel moisture realistic — air-dried wood runs 12–20%, green wood 30% or higher — since it directly drives how much wood you'll burn per hour to hit your target heat.
Inputs
Results
Chamber cross-section side
7.5 cm
How to Use This Calculator
- Enter your target heat output in BTU/hr based on the cooking task.
- Set fuel stick diameter, moisture content, and insulation thickness.
- The calculator outputs chamber cross-section, chimney height, feed tube length, pot gap, and fuel consumption rate.
- Use these dimensions as a blueprint for building the combustion chamber and riser.
- Verify that actual BTU output meets your target and adjust stick diameter if needed.
How the result changes with Fuel stick diameter
| Fuel stick diameter | Chamber cross-section side |
|---|---|
| 1.25 | 7.5 cm |
| 1.88 | 7.5 cm |
| 3.75 | 7.5 cm |
| 6.25 | 12.5 cm |
What each input means
- Target heat output
- Desired heat output. 10,000–15,000 BTU for household cooking, 20,000+ for large pots.
- Fuel stick diameter
- Diameter of typical fuel sticks. Rocket stoves work best with 1–4 cm diameter wood.
- Fuel moisture content
- Moisture content of fuel wood. Air-dried ≈ 12–20%, green wood ≈ 30–50%.
- Insulation thickness
- Thickness of insulation around the combustion chamber (perlite, vermiculite, or wood ash).
What each result means
- Chamber cross-section side
- Inner side length of the square combustion chamber opening.
- Chimney (riser) height
- Vertical insulated chimney above the elbow for draft and combustion.
- Feed tube length
- Horizontal fuel feed opening length.
- Pot gap (channel gap)
- Gap between pot bottom and stove top for optimal heat transfer (6–9 mm).
- Pot skirt height
- Height of metal skirt around the pot to capture exhaust heat.
- Outer body diameter
- Total outer diameter including insulation.
- Actual output
- Actual heat output from the rounded chamber dimensions.
- Fuel consumption
- Wood fuel consumed per hour at full output.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersTarget heat output = 15000, Fuel stick diameter = 2.5, Fuel moisture content = 15, Insulation thickness = 3 = 4 input(s) provided
- Calculate Chamber cross-section sideChamber cross-section side = sticksWide * stickDiamCm7.5 = 7.5
- Calculate Chimney (riser) heightChimney (riser) height = actualSideCm * 2.519 = 19
- Calculate Feed tube lengthFeed tube length = actualSideCm * 1.511 = 11
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why is the actual BTU output different from the target I entered?
The calculator first computes an ideal chamber side length from your target output, then rounds that up to the nearest whole number of fuel sticks so the chamber is actually buildable. Because that rounding almost always increases the chamber size slightly, the resulting 'actual output' typically comes in a bit above your target rather than matching it exactly.
Why does fuel moisture content change fuel consumption but not chamber size?
Chamber dimensions are sized purely from your target heat output and stick diameter using the fixed 1 kW per 10 cm² rule of thumb — moisture never enters that geometry calculation. Fuel consumption is a separate step: wetter wood has less usable energy per kilogram (net energy = 15 × (1 − moisture%) − 2.44 × moisture%, since some heat evaporates trapped water instead of reaching the pot), so at the same chamber output you burn more kilograms per hour of wetter wood.
Why does the chimney height scale directly with chamber size rather than target BTU?
Chimney (riser) height is calculated as 2.5 times the actual chamber side length, which is itself derived from your target output rounded to a whole number of sticks — so it's really the built chamber size, not the raw BTU target, driving chimney height. Two different targets that round to the same stick count will produce identical chimney heights.
Why is the pot gap clamped between 6 and 9 mm instead of scaling freely with stove size?
The formula scales pot gap as 0.6 times the chamber side length, but clamps the result to a 6–9 mm range because that band is the range known to transfer heat efficiently between the stove top and pot bottom without choking exhaust flow. A very small or very large stove would otherwise compute a gap far outside the range that actually works for combustion draft.
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